Feline Panleukopenia Virus

Overview and Taxonomy of Feline Panleukopenia Virus

Feline panleukopenia virus (FPV) stands as one of the most significant and historically impactful pathogens affecting both domestic and wild felids worldwide. As the etiological agent of feline panleukopenia (FPL), a disease characterized by profound leukopenia, severe gastroenteritis, and high mortality, FPV has been recognized for nearly a century and continues to pose substantial challenges to veterinary medicine, conservation biology, and the companion animal industry [17, 28]. The virus is classified within the species Protoparvovirus carnivoran 1, genus Protoparvovirus, subfamily Parvovirinae, family Parvoviridae [17, 19, 26]. This taxonomic placement reflects its close evolutionary relationship with canine parvovirus type 2 (CPV-2), mink enteritis virus (MEV), and other carnivore parvoviruses, all of which share a common ancestor and exhibit varying degrees of cross-species infectivity [11, 24, 26]. The family Parvoviridae itself is divided into three subfamilies: Parvovirinae (infecting vertebrates), Densovirinae (infecting arthropods), and Hamaparvovirinae (infecting both vertebrate and invertebrate hosts), with FPV firmly situated within the Parvovirinae [17]. The World Organisation for Animal Health (WOAH) recognizes FPV as a pathogen of significant concern, particularly in the context of endangered felid conservation and international animal movement, underscoring its global veterinary importance.

Genomic Organization and Structural Biology

The FPV genome is a linear, single-stranded DNA molecule of approximately 5,000 nucleotides in length, with a GC content of approximately 36–37% [10, 20, 29]. The genome is encapsidated within a non-enveloped, icosahedral capsid approximately 20–26 nm in diameter, as visualized by transmission electron microscopy [15, 25]. The viral genome contains two major open reading frames (ORFs). The 5′ ORF encodes the non-structural proteins NS1 and NS2, which are essential for viral DNA replication, transcriptional regulation, and cytotoxicity [18, 24, 29]. The 3′ ORF encodes the structural proteins VP1 and VP2, which constitute the viral capsid [29]. VP2 is the major capsid protein and the primary determinant of host range, tissue tropism, and antigenicity [1, 3, 7, 29]. VP1 is a minor capsid protein that arises from alternative splicing and contains an N-terminal extension (the VP1 unique region) that is critical for nuclear localization and genome packaging [29]. The NS1 protein, a multifunctional phosphoprotein, possesses helicase and nickase activities required for rolling-circle replication and has been implicated in host cell cycle arrest and apoptosis [18, 24, 31]. Recent transcriptomic analyses have revealed that FPV infection profoundly alters host gene expression, with 3,116 differentially expressed genes identified in feline kidney cells, including upregulation of Toll-like receptor, JAK-STAT, IL-17, and TNF signaling pathways, as well as downregulation of cell cycle and cell growth pathways [4]. These findings highlight the intricate host–virus interactions that underpin FPV pathogenesis.

The VP2 Capsid Protein: A Nexus of Host Range and Antigenic Variation

The VP2 protein, comprising approximately 584 amino acids, is the principal target of the host neutralizing antibody response and the primary determinant of viral attachment to the host transferrin receptor (TfR) [1, 3, 24, 29]. The three-dimensional structure of the VP2 capsid features a β-barrel core motif, with prominent surface loops (loops 1–4) that form the antigenic determinants and receptor-binding interface [1, 25]. Amino acid substitutions within these loops, particularly at residues 80, 93, 101, 232, 299, 300, and 426, have been repeatedly associated with alterations in host range, antigenicity, and virulence [1, 3, 6, 7, 18, 24, 25, 27]. For instance, the substitution of alanine with serine at position 91 (A91S) in VP2 has emerged as a hallmark of contemporary FPV strains circulating in Asia, particularly in China, where its prevalence has increased from 15.63% in 2017 to 100% in 2024 [7, 27]. This mutation extends the random coil region from residues 92–95 to 91–95, potentially altering capsid surface topology and receptor interactions [27]. Similarly, the isoleucine-to-threonine substitution at position 101 (I101T) is now highly prevalent globally, with some studies reporting frequencies exceeding 89% [1, 8, 19]. Molecular modeling of the I101T mutation predicts altered surface charge distribution, which may affect host receptor binding and potentially facilitate immune evasion [1]. The G299E substitution, first identified in a giant panda-derived FPV strain, is located on the top region of interconnecting surface loop 3, a region known to control host range and antigenicity [20, 25]. The A300P substitution, detected for the first time in feline-derived FPV, has been shown to confer the ability to replicate efficiently in canine cell lines and cause gastrointestinal disease in dogs, representing a critical step in cross-species transmission [6]. These mutations are not merely random genetic drift; selection pressure analyses have identified residue 91 as a positive selection site with the highest dN/dS value, indicating that adaptive evolution is actively shaping the FPV capsid in response to host immune pressure [7].

Evolutionary Dynamics and Phylogenetic Diversity

FPV exhibits an evolutionary rate approaching that of RNA viruses, estimated at approximately 1.13 × 10⁻⁴ substitutions per site per year, which is remarkably high for a DNA virus [8]. This rapid evolution, combined with the virus’s broad host range and global distribution, has generated substantial genetic diversity. Phylogenetic analyses based on complete VP2 gene sequences consistently delineate FPV strains into distinct clades that often correlate with geographic origin. For example, Asian FPV strains form a separate group from European and American strains, with the Asian group further subdivided into clusters that reflect regional circulation patterns [7, 8, 21]. Within China, FPV strains have been classified into two major groups (FPLV-China groups), with the A91S variant forming a distinct evolutionary branch that has become dominant since 2017 [7, 18, 27]. The global FPV population is characterized by a complex pattern of synonymous and non-synonymous substitutions. Analysis of 947 VP2 sequences revealed 279 non-synonymous substitutions, of which 55% occurred as single events, while six substitutions (A91S, I101T, V232I, K93N, D323N, and V562L) were each found in 20–40 sequences, indicating their widespread fixation [18]. In the NS1 protein, three non-synonymous substitutions (I443V, H595Q, and V596L) were detected in more than half of 157 sequences analyzed, suggesting that adaptive evolution is also shaping the non-structural proteins [18]. The Shannon entropy analysis of VP2 sequences has identified hypervariable regions concentrated in nucleotide positions 111–411, 477–1,038, and 1,500–1,752, with the highest entropy peak at nucleotide 271 (corresponding to amino acid 91), further confirming the intense selective pressure at this site [16].

Host Range and Cross-Species Transmission

Historically, FPV was considered a pathogen of domestic cats and other felids. However, accumulating evidence over the past two decades has dramatically expanded the known host range of FPV to include a remarkable diversity of carnivore and even non-carnivore species. FPV has now been documented in domestic dogs (Canis lupus familiaris), causing enteric disease and shedding in feces [6, 26, 30]; in giant pandas (Ailuropoda melanoleuca), where it can cause fatal hemorrhagic enteritis [14, 20, 25]; in Siberian tigers (Panthera tigris altaica) and other captive large felids, where outbreaks have resulted in mortality [19]; in jaguars (Panthera onca) in Brazil [23]; in Marsican brown bears (Ursus arctos marsicanus) and crested porcupines (Hystrix cristata) in Italy, representing the first detection in ursids and hystricids [11]; in Pallas’s cats (Otocolobus manul) [9]; in Amur leopard cats (Prionailurus bengalensis euptilura) [2]; in banded linsangs (Prionodon linsang) [32]; in raccoon dogs (Nyctereutes procyonoides) in China [13]; and in a puma (Puma concolor) rescued from illegal wildlife trade in Colombia [12]. This extraordinary host range is facilitated by the high degree of conservation of the transferrin receptor (TfR) across mammalian species. The giant panda TfR, for example, shares high homology with feline TfR, enabling FPV to bind and enter panda cells [14]. Importantly, some FPV isolates, such as the pFPV-sc strain from a giant panda, have been shown to infect human cell lines in vitro, raising the specter of zoonotic potential, although no human infections have been documented to date [14]. The World Health Organization (WHO) and the Food and Agriculture Organization (FAO) have highlighted the importance of monitoring such cross-species transmission events, particularly in the context of emerging infectious diseases and wildlife conservation.

The molecular determinants of host range are primarily encoded within the VP2 protein. The key residues that distinguish FPV from CPV-2, such as lysine at position 80, lysine at position 93, and valine at position 103 in FPV versus the corresponding residues in CPV-2, are critical for species-specific TfR binding [21, 24]. However, the emergence of FPV strains with mutations that enhance replication in canine cells (e.g., A300P) or that broaden the host range to include non-carnivore species (e.g., G299E) indicates that the virus is actively evolving to overcome host barriers [6, 25]. The detection of FPV in wastewater in Arizona, USA, with 100% VP2 sequence identity to the 1964 Johnson snow leopard strain present in the Felocell® vaccine, highlights the role of modified-live vaccination in environmental shedding and potential transmission to naive animals [5]. This finding underscores the need for careful vaccination practices and environmental decontamination in multi-cat and wildlife settings.

Taxonomic Considerations and Nomenclature

The taxonomy of FPV has undergone revision in recent years. The species Protoparvovirus carnivoran 1 encompasses FPV, CPV-2 and its variants (2a, 2b, 2c), MEV, and other closely related carnivore parvoviruses [11, 17, 19, 26]. While FPV and CPV-2 are genetically distinct, exhibiting approximately 97–99% nucleotide identity in the VP2 gene, they are classified within the same species due to their shared biological properties and ability to recombine [1, 24]. The distinction between FPV and CPV-2 is based on a combination of genetic markers (specific VP2 amino acid residues at positions 80, 93, 103, 323, 426, and others), host range (FPV replicates efficiently in feline cells but not in canine cells, whereas CPV-2 replicates in both), and antigenic properties [21, 24, 30]. However, the increasing detection of FPV in dogs and CPV-2 in cats, as well as the emergence of recombinant or intermediate strains, has blurred these traditional boundaries [6, 22, 24, 26, 30]. The term “carnivore protoparvovirus 1” is therefore preferred in some contexts to reflect the genetic and ecological continuum among these viruses [11, 19, 26]. The International Committee on Taxonomy of Viruses (ICTV) maintains the current classification, but ongoing genomic surveillance is essential to refine taxonomic boundaries as new variants emerge.

Molecular Pathogenesis: Viral Replication and Host-Virus Interactions

Feline panleukopenia virus (FPV), a member of the species Protoparvovirus carnivoran 1 within the genus Protoparvovirus of the family Parvoviridae, is a non-enveloped, single-stranded DNA virus that exhibits a profound tropism for rapidly dividing cells. Its pathogenesis is a complex, multi-stage process dictated by the viral life cycle, the molecular architecture of its capsid, and a dynamic interplay with host cellular machinery and immune defenses. This section provides an exhaustive analysis of the molecular mechanisms governing FPV replication and the intricate host-virus interactions that underpin the clinical manifestations of panleukopenia.

Genomic Organization and the Molecular Replication Strategy

The FPV genome is approximately 5.0–5.2 kb in length, characterized by terminal palindromic sequences that form hairpin structures essential for viral DNA replication [10, 20]. This compact genome encodes two non-structural proteins (NS1 and NS2), essential for replication, and two structural capsid proteins (VP1 and VP2), the latter of which constitutes the primary antigenic determinant and host range determinant [29, 38]. The replication strategy of FPV is inherently dependent on the host cell’s S-phase machinery, as the virus lacks its own DNA polymerase. Upon cellular entry and uncoating, the single-stranded viral DNA is translocated to the nucleus, where it relies on host DNA polymerases and replication factors to convert the viral genome into a double-stranded replicative form. This replicative form then serves as a template for transcription and further replication via a rolling hairpin mechanism [41].

The NS1 protein, a multifunctional helicase and nickase, is the master regulator of this process. It binds to the origin of replication, introduces a site-specific nick, and unwinds the DNA, thereby orchestrating the replication of progeny genomes [24, 29]. The evolutionary dynamics of the NS1 gene are now recognized as critical to viral fitness and host adaptation. Recent comparative genomic analyses of FPV strains have identified a high number of non-synonymous substitutions in NS1, with specific residues such as Ile443Val, His595Gln, and Val596Leu appearing in over half of the sequences analyzed globally [18]. Furthermore, molecular characterization of a canine-derived FPV strain that caused only mild disease in cats suggested that specific NS1 mutations, including Val630Ile and Asp60Glu, were positively selected during adaptation to the canine host, likely modulating replication efficiency and host tropism [24, 36]. This underscores that NS1 is not merely a static replication enzyme but a dynamic molecular interface undergoing selective pressure.

The VP2 Capsid: A Molecular Determinant of Host Range and Antigenicity

The VP2 capsid protein is the central molecular determinant of FPV pathogenesis, governing host range, cellular tropism, receptor binding, and antigenicity [1, 6, 38]. The primary cellular receptor for FPV is the transferrin receptor (TfR), and the specificity of the VP2-TfR interaction is the key molecular barrier that historically restricted FPV replication primarily to feline cells [14, 30]. The VP2 protein forms an eight-stranded, antiparallel β-barrel core with large surface loops that constitute the primary antigenic regions and receptor-binding interfaces [20, 30].

Antigenic Drift and the Emergence of Dominant Variants. Molecular epidemiological surveillance has revealed that FPV is undergoing continuous, and in some respects rapid, antigenic evolution, with specific codons exhibiting signatures of positive selection [1, 7, 16]. The most notable example is the Ala91Ser (A91S) substitution in VP2. Initially rare, the prevalence of strains carrying A91S has skyrocketed in China from 15.63% in 2017 to virtually 100% in 2024, establishing it as the dominant molecular signature of contemporary FPV strains in that region [7, 27]. This substitution lies within a critical antigenic determinant region on the capsid surface, and structural modeling indicates that it extends the random coil of adjacent amino acid residues, potentially altering surface charge distribution and receptor binding kinetics [1, 27]. Viral strains harboring A91S, often in conjunction with the Ile101Thr (I101T) mutation, exhibit enhanced virulence in both cell culture and animal models, showing a 1.8- to 2.3-fold greater pathogenicity compared to earlier field strains [1, 3, 8].

The I101T mutation is another critical, high-frequency substitution that has stabilized in the global FPV population, suggesting it has reached an adaptive equilibrium [16, 19]. This mutation, located in the GH loop of VP2, is a key residue involved in host range determination. In canine parvovirus (CPV), which evolved from FPV, an analogous residue (Ile101Thr) is critical for the virus’s ability to infect canine cells [30]. The near-fixation of I101T in modern FPV strains suggests a pervasive selective advantage, likely related to improved binding affinity for the feline TfR or the TfRs of other susceptible carnivore hosts [1, 19].

Novel Mutations and the Expansion of Host Tropism. The ongoing molecular evolution of FPV is leading to the emergence of strains with expanded host ranges and increased pathogenic potential. A landmark study isolated an FPV strain (FPV-251) from a cat that carried a novel Ala300Pro (A300P) substitution in VP2 [6]. This single amino acid change, located on the top region of the interconnecting surface loop 3, was sufficient to confer upon FPV the ability to productively infect canine cell lines (MDCK, A72 cells) and, critically, to cause clinical disease (gastroenteritis) in dogs following experimental oral infection [6]. This represents a direct molecular demonstration of a single point mutation driving the cross-species transmission of a carnivore parvovirus.

Similarly, strains isolated from giant pandas have been found to harbor a Gly299Glu (G299E) substitution in VP2, also on loop 3, which is predicted to alter the antigenic surface and potentially facilitate infection of this new host [20, 25]. The threat of cross-species spillover is further evidenced by the isolation of FPV from raccoon dogs, indicating that the virus is actively circulating in wild canid populations and blurring the traditional host boundaries [13]. Whole genome sequencing of a highly virulent FPV strain from a captive giant panda (pFPV-sc) confirmed that it is essentially a feline-derived virus that crossed into a new host, facilitated by the high homology between the feline and giant panda TfR [14]. Alarming data from this study demonstrated that pFPV-sc could infect human cell lines in vitro, raising concerns about the zoonotic potential of emerging parvovirus variants and underscoring the need for surveillance as recommended by global health frameworks [14]. More recently, FPV has been identified in entirely novel hosts such as the Marsican brown bear and the crested porcupine, demonstrating a remarkable capacity for host jumping that extends beyond the traditional order Carnivora [11].

Molecular Markers of Vaccine Escape. The divergence of contemporary field strains from vaccine lineages is a major concern. The A91S and I101T mutations, for instance, are absent from most commercial vaccine strains (e.g., Cu-4, Felocell). Phylogenetic analyses consistently show that emerging Chinese strains (e.g., FPLV-CC19-02, ZZ202303) occupy distinct clades separate from vaccine lineages, differing at multiple antigenic sites [1, 3]. The VP2 of the ZZ202303 strain shares only 97.5–98.2% identity with CPV strains and forms a distinct cluster with a bootstrap value of 94%, diverging from classic FPV vaccine strains [1]. Furthermore, strains from China and Vietnam have been found to carry a T440A substitution, which co-occurs with N564S and A568G, a combination of mutations characteristic of CPV-2c, potentially enhancing immune evasion capabilities of FPV [16, 21]. These genetic shifts necessitate continuous molecular surveillance to ensure that prophylactic strategies remain effective against circulating virulent variants [1, 3, 7].

Host-Virus Interactions: Cellular Signaling, Innate Immunity, and Transcriptional Reprogramming

FPV infection triggers a profound and complex cellular response, primarily characterized by the subversion of host cell cycle machinery and the orchestrated manipulation of innate immune signaling pathways.

Impact on Cell Cycle and Apoptosis. As a virus dependent on host S-phase, FPV forces infected cells into a state permissive for replication. Transcriptomic analysis of FPV-infected feline kidney cells (F81) using RNA-seq has systematically revealed this interaction. A total of 3,116 differentially expressed genes (DEGs) were identified, with the down-regulated DEGs being overwhelmingly enriched in pathways related to the cell cycle, cell growth, and cellular senescence [4]. This indicates a virally directed shutdown of normal cell division processes to sequester resources for viral genome replication.

Concurrently, the host cell initiates a pro-apoptotic response. The same RNA-seq study showed that up-regulated DEGs were significantly enriched in pathways governing extrinsic apoptotic signaling [4]. This is a double-edged sword for the virus: while apoptosis can limit viral spread, FPV has evolved mechanisms to delay or control this process to complete its replication cycle. The virus also manipulates the expression of non-coding RNAs, such as long non-coding RNAs (lncRNAs). Upon FPV infection, significant alterations in lncRNA expression occur, with 291 lncRNAs and 873 mRNAs being differentially expressed. The target genes of up-regulated lncRNAs were enriched in the MAPK signaling pathway, a central regulator of cell proliferation, differentiation, and stress responses, suggesting a complex, multi-layered regulatory network of host cell manipulation [42].

Subversion of the Type I Interferon (IFN-I) Response. The host’s primary antiviral defense is the IFN-I system. FPV has evolved sophisticated strategies to suppress this pathway, primarily through the manipulation of host microRNAs (miRNAs).

One key mechanism involves the miR-1343-5p. FPV infection upregulates the expression of miR-1343-5p in both cell culture and in vivo. This miRNA directly targets and suppresses the expression of interleukin-1 receptor-associated kinase 1 (IRAK1), a critical signaling adaptor downstream of Toll-like receptors (TLRs). By inhibiting IRAK1, miR-1343-5p acts as a potent negative regulator of the NF-κB and IFN-I signaling cascades, thereby facilitating FPV replication [31]. A parallel mechanism involves miR-92a-1-5p. During FPV infection, the host upregulates the transcription factor specificity protein 1 (SP1), which in turn drives the expression of miR-92a-1-5p. This miRNA then targets the suppressor of cytokine signaling 5 (SOCS5), which normally inhibits NF-κB signaling. The net result is an enhancement of IFN-α/β expression, representing a host countermeasure to limit viral replication [37]. The balance between these pro- and anti-viral miRNA responses is a critical determinant of infection outcome.

Further transcriptomic profiling using RNA-seq has provided a comprehensive view of these pathways. Beyond the specific miRNA axes, FPV infection leads to the upregulation of key immune-related genes, including IGSF6, IFI44L, IFI6, IFITM10, IL1R1, and JAK3 [4]. The activation of JAK-STAT signaling, along with TNF and IL-17 pathways, signifies a robust but clearly subverted inflammatory response that contributes to the systemic pathology of the disease, including the characteristic fever and enteritis.

Co-infection Dynamics and Immune Suppression. The pathogenic landscape is further complicated by co-infections. For instance, Canine Circovirus (CanineCV), a cause of immune suppression in dogs, can be detected in cats and synergistically exacerbate FPV infection. The replication (Rep) protein of CanineCV has been shown to significantly enhance the replication of FPV in F81 cells while simultaneously suppressing the host’s IFN-I response, as indicated by reduced transcription of IFN-α, IFN-β, MxA, and ISG15 [33]. This demonstrates that heterologous viral proteins can directly modulate the cellular environment to favor FPV replication. Co-infection with other enteric pathogens, such as Clostridium spp., Giardia spp., or feline coronavirus, is also common and associated with increased FPV DNA shedding and more severe clinical signs, underscoring the role of microbial synergy in driving pathogenesis [34, 35].

Evolutionary Rate and the Challenge of Viral Persistence

The cumulative effect of these molecular interactions is a virus that is not genetically static. A critical recent finding is that FPV exhibits an evolutionary rate approaching that of RNA viruses, calculated at approximately 1.13 × 10⁻⁴ substitutions per site per year [8]. This rate, driven by the continuous replication error of host DNA polymerases and strong selective pressures from host immunity and changing receptor landscapes, explains the rapid emergence and fixation of mutations like A91S [7, 8, 16]. This dynamism challenges the notion of FPV as a slow-evolving DNA virus and highlights the need for periodic re-evaluation of vaccine strains to ensure they provide broad protection against these rapidly diverging field isolates [3, 39, 40]. The interplay between viral replication, host cell manipulation, and immune subversion defines the molecular pathogenesis of FPV, explaining its remarkable contagiousness, high lethality, and ongoing evolution as a pan-carnivore pathogen of global veterinary significance.

Genetic Diversity and Phylogenetic Divergence of the VP2 Gene

The VP2 capsid protein constitutes the primary structural component of the feline panleukopenia virus (FPV) virion, orchestrating host receptor recognition, tissue tropism, and antigenic determination. As the major target of the neutralizing antibody response, VP2 is under continuous selective pressure from both host immune systems and vaccine-induced immunity, rendering it a critical locus for evolutionary adaptation. The genetic diversity and phylogenetic divergence observed within the VP2 gene across global FPV strains have profound implications for vaccine efficacy, cross-species transmission potential, and molecular epidemiological surveillance. The protoparvoviral VP2 gene, encoding approximately 584 amino acids, forms an eight-stranded antiparallel β-barrel motif interspersed with large loop regions that constitute the capsid surface. These surface loops, particularly loop 1 (residues 90–105), loop 2 (residues 222–240), loop 3 (residues 293–300), and loop 4 (residues 426–440), are the principal sites of antigenic variation and host range determination. The genetic plasticity of these regions enables FPV to navigate the evolutionary landscape between maintaining capsid stability and evading host immune recognition. Comprehensive sequence analyses have revealed that the VP2 gene evolves at a rate approaching that of RNA viruses, with estimates placing the substitution rate at approximately 1.13 × 10⁻⁴ substitutions per site per year [8]. This unexpectedly high evolutionary rate for a single-stranded DNA virus underscores the dynamic nature of FPV adaptation and explains the accelerating emergence of novel genetic variants across diverse geographic regions.

Hypervariable Domains and Mutational Hotspots

The VP2 gene is not uniformly conserved; rather, it harbors distinct hypervariable regions that concentrate genetic diversity and serve as focal points for adaptive evolution. Detailed Shannon entropy analyses of global VP2 sequences have identified three principal hypervariable domains concentrated within nucleotide positions 111–411, 477–1,038, and 1,500–1,752 [16]. Among these, the highest entropy peak occurs at nucleotide position 271, corresponding to amino acid residue 91, which represents the most polymorphic site across all analyzed FPV strains. This residue lies within the N-terminal region of VP2, a domain intimately involved in antigenic determinant presentation. The remarkable variability at position 91 has been unequivocally linked to positive selective pressure, with dN/dS ratio analyses confirming that this site undergoes diversifying selection far exceeding neutral expectations [7]. The A91S substitution (alanine to serine) has emerged as the most prevalent mutation among contemporary FPV strains, particularly those circulating in Asian feline populations. Molecular epidemiological surveys have documented a dramatic temporal increase in the frequency of VP2-91S variants, rising from 15.63% of Chinese FPV isolates in 2017 to 100% by 2024, indicating complete fixation within the circulating population [7]. This substitution extends the random coil conformation of the capsid surface loop from residues 92–95 to include residues 91–95, potentially altering the conformational epitopes recognized by neutralizing antibodies [27]. The structural ramifications of this extended loop likely modulate the accessibility of antigenic sites to host immunoglobulins, providing a mechanistic basis for the observed vaccine evasion phenotypes. Complementary analyses of FPV strains from Henan Province have corroborated the dominance of the I101T substitution in the same antigenic determinant region, with a prevalence of 89.13% among contemporary isolates [1]. Molecular modeling of the VP2 capsid incorporating the I101T mutation predicts an altered surface charge distribution that may affect host transferrin receptor (TfR) binding affinity, thereby modulating both tissue tropism and host range specificity.

Temporal Dynamics and Adaptive Equilibrium of Key Substitutions

The evolutionary trajectory of VP2 mutations follows distinct temporal patterns that reflect the interplay between selective advantage, fitness costs, and population immunity. The I101T substitution, while occurring at high frequency, has reached a state of adaptive equilibrium characterized by stabilized substitution frequencies over time [16]. This stabilization suggests that the mutation confers a selective advantage that has been largely integrated into the circulating population without ongoing directional selection. In contrast, the substitution frequency at residue 232 has exhibited a gradual decline over successive sampling periods, indicating either negative selection against this variant or its replacement by more advantageous mutations [16]. The A91S substitution, however, continues to show increasing frequency, suggesting that this mutation may still be under active positive selection and has not yet reached fixation equilibrium in all geographic regions. The presence of multiple concurrent nonsynonymous substitutions across the VP2 gene creates complex epistatic interactions that can either potentiate or constrain further evolutionary change. Recent analyses of 947 VP2 sequences from the NCBI database identified 279 nonsynonymous substitutions, of which 55% (153/279) occurred as single events in individual sequences, while six substitutions (Ala91Ser, Thr101Ile, Val232Ile, Lys93Asn, Asp323Asn, and Val562Leu) were each detected in 20 to 40 sequences [18]. This distribution pattern indicates that while the VP2 gene experiences frequent mutation, only a limited subset of substitutions achieve population-level prevalence, likely because the majority impose structural or functional constraints that preclude fixation.

Phylogenetic Clustering and Geographic Segregation

Phylogenetic reconstruction based on complete VP2 nucleotide sequences reveals a clear pattern of geographic segregation among global FPV isolates, with strains clustering into distinct phylogeographic lineages. Chinese FPV isolates form a cohesive Asian FPV strain group that is phylogenetically distinguishable from European and American reference strains [7]. This geographic clustering likely reflects founder effects, localized selective pressures from regionally specific vaccination programs, and limited contemporary gene flow between continental virus populations. The FPV ZZ202303 strain, isolated from clinically severe panleukopenia cases in Henan Province, occupies a distinct phylogenetic cluster with strong bootstrap support (94%), positioned at the interface between FPV and canine parvovirus (CPV) clades [1]. This strain shares only 97.5–98.2% nucleotide identity with CPV strains compared to 98.8–99.7% identity with reference FPV strains, indicating an evolutionary intermediate that may represent an ongoing host range transition. Such phylogenetic positioning raises the possibility of bidirectional genetic exchange between FPV and CPV, facilitated by the overlapping host ranges of domestic cats and dogs. Cross-species transmission events are further substantiated by the detection of FPV strains in dogs from Italy and Egypt, where genomic sequencing confirmed FPV circulation in canine populations with VP2 sequences clustering within traditional FPV clades [26]. Similarly, FPV-like viruses carrying the Thr101 mutation have been isolated from Vietnamese dogs, providing the first reliable evidence that FPV may present a genuine threat to canine populations, challenging the long-held dogma that FPV replication is restricted to feline hosts [30].

Interspecies Transmission and Host Range Expansion

The genetic diversity of VP2 directly governs the host range specificity of FPV through its interaction with the cellular transferrin receptor (TfR). Mutations that alter VP2 surface topology can expand or restrict the range of TfR orthologs that the virus can utilize for entry, thereby determining cross-species transmission potential. The isolation of FPV from a giant panda (Ailuropoda melanoleuca) in China, designated strain pFPV-sc, exemplifies the consequences of VP2-mediated host range expansion. Phylogenetic analysis positioned pFPV-sc within the Chinese FPV clade, and structural modeling suggested that the giant panda TfR shares high homology with feline TfR, facilitating receptor utilization [14]. Crucially, this strain demonstrated the capacity to infect human cell lines in vitro, raising zoonotic concerns that warrant ongoing surveillance by organizations such as the World Health Organization (WHO) and the World Organisation for Animal Health (WOAH). Additional mutations associated with host range expansion include the G299E substitution, first identified in FPV isolated from a captive giant panda with mild diarrhea. This substitution resides on the top of interconnecting surface loop 3, a region known to control host range and antigenicity in protoparvoviruses [25]. The same G299E mutation was subsequently identified in FPV-am2020 isolated from diarrheic giant pandas, confirming that this residue is a recurrent target for adaptation to novel hosts [20]. In the context of canine adaptation, the A300P substitution in VP2, detected for the first time in feline-derived FPV strain FPV-251, enabled efficient replication in Madin-Darby canine kidney (MDCK) and A72 cells, and oral infection of dogs resulted in gastrointestinal symptoms and intestinal lesions [6]. This single amino acid change at position 300, which distinguishes FPV-251 from the non-canine-adapted FPV-271 strain, underscores the profound phenotypic consequences that minor genetic alterations in VP2 can produce.

Emerging Variants and Vaccine Escape

The phylogenetic divergence of contemporary FPV strains from vaccine lineages represents a critical concern for prophylactic strategies. The Chinese epidemic strain FPLV-CC19-02, which carries the Ala91Ser and Ile101Thr substitutions in VP2, exhibits substantial genetic distance from all major commercial vaccine strains, including those manufactured by Pfizer, MSD, Virbac, Merial, and Nobivac [3]. This divergence raises the possibility that vaccinated populations may harbor incomplete protection against circulating field strains. The FPV ZZ202303 strain similarly forms a distinct cluster diverging from vaccine lineages, and its enhanced virulence (1.8- to 2.3-fold greater pathogenicity compared to contemporary field strains) correlates with its unique VP2 genetic signatures [1]. The T440A substitution, newly identified in FPV-BJ-J2 and FPV-BJ-J3 strains isolated in Beijing during 2024–2025, co-occurs with N564S and A568G, forming a substitution combination characteristic of CPV-2c variants that may be associated with enhanced immune evasion [16]. This mutation pattern suggests convergent evolution between FPV and CPV-2c, potentially driven by similar selective pressures from host immunity. The antigenic consequences of these mutations are underscored by the finding that virus-like particle (VLP) vaccines based on the VP2 of the Chinese epidemic strain (carrying Ala91Ser/Ile101Thr) provided 100% protection against homologous challenge, whereas vaccines derived from older prototype strains may offer reduced cross-protection [43]. The increasing genetic heterogeneity of FPV populations, coupled with ongoing phylogenetic divergence from vaccine strains, necessitates periodic reassessment of vaccine composition to ensure continued efficacy.

Epidemiology: Global Distribution and Transmission Dynamics

Feline panleukopenia virus (FPV), the prototypic member of the species Protoparvovirus carnivoran 1 within the family Parvoviridae, represents one of the most significant and globally pervasive viral pathogens of domestic and wild felids. Its epidemiological profile is characterized by an extraordinary environmental stability, a broad and expanding host range, and a transmission dynamic driven by both direct contact and extensive fomite-mediated spread. The virus is classified by the World Organisation for Animal Health (WOAH) as a pathogen of multispecies significance, and its global distribution is virtually coterminous with the presence of susceptible carnivore populations. The epidemiological landscape of FPV is not static; rather, it is undergoing a dynamic evolution shaped by viral genetic drift, emerging cross-species transmission events, and the selective pressures exerted by vaccination programs.

Global Distribution and Seroprevalence Patterns

FPV is endemic on all continents where felid populations exist, with seroprevalence and infection rates varying significantly based on geographic region, management practices (owned versus free-roaming), vaccination coverage, and the diagnostic modalities employed. Large-scale molecular surveys consistently reveal high circulation rates. In a comprehensive study from China utilizing a quadruplex quantitative PCR on 381 fecal samples, FPV was detected in 13.65% of submissions, a figure that underscores its endemic stability even in populations presenting with other concurrent enteric pathogens [44]. Studies from Bangladesh, a region where FPV is hyperendemic, report a molecular prevalence of 22.9% among suspected cats using VP2-targeted PCR, with a case fatality rate of 45.9% [48]. Similar rates are observed across the Middle East; in Duhok Province, Iraq, a prevalence of 70% was documented using conventional PCR, with significantly higher odds of infection identified in stray, unvaccinated, and young animals [47]. In Egypt, a seminal study combining ELISA and PCR approaches found a prevalence of 45% among 165 clinically suspected cats, with the authors noting a high degree of genomic stasis in the circulating strains [51].

Conversely, lower prevalence rates are reported in well-vaccinated, closed populations. In breeding catteries in Europe, fecal shedding of FPV DNA was observed in only 2.5% of healthy cats, though shedding was significantly associated with diarrheic feces (Odds Ratio: 9.9) [34]. This indicates that subclinical shedding is generally low in managed populations, but the virus can rapidly re-emerge in the presence of clinical disease. The prevalence data align with risk factor analyses demonstrating that age is the most critical intrinsic risk factor. In a study from a Vietnamese province, cats under 12 months of age had a significantly higher infection rate (57.77%) compared to older cats (28.33%), a pattern consistently replicated across geographic boundaries [45, 50, 52]. The biological basis for this age-related susceptibility is the virus’s tropism for rapidly dividing cells; the highly mitotic intestinal crypt epithelium and bone marrow of kittens provide a permissive environment for explosive viral replication. Furthermore, the waning of maternally derived antibodies (MDA) creates a window of vulnerability typically between 4 and 12 weeks of age, a period during which vaccination protocols must be meticulously timed to avoid MDA interference [19].

Transmission Dynamics: Routes, Reservoirs, and Environmental Persistence

The transmission dynamics of FPV are governed by its exceptional biophysical resilience. FPV is a non-enveloped virus, rendering it resistant to lipid solvents, many common disinfectants, and desiccation. It can persist in the environment, on surfaces, bedding, food bowls, and in organic matter, for over one year at room temperature [29]. This environmental hardiness is the primary driver of its epidemiology, as fomite transmission via contaminated objects, clothing, and human hands is arguably more significant than direct animal-to-animal contact in many settings. The primary route of infection is the fecal-oral pathway, following ingestion or inhalation of virus particles from contaminated environments.

Shedding dynamics are a critical component of transmission. Acutely infected cats excrete massive quantities of virus in feces, vomitus, and urine, with viral loads reaching (5.01 \times 10^8) copies per gram of feces in clinical cases [49]. Notably, shedding begins before the onset of severe clinical signs and can persist for up to six weeks post-recovery, although convalescent animals generally do not become long-term carriers. The role of subclinical and pre-clinical shedders in maintaining transmission chains within multi-cat environments is of paramount importance. In a shelter setting, modified-live virus (MLV) vaccination itself was shown to induce detectable FPV DNA shedding in feces in 21.6% of clinically healthy cats, peaking at day 7 post-vaccination, although at significantly lower copy numbers than natural infection (median (1.13 \times 10^7) vs. (5.01 \times 10^8) copies/g) [49]. This finding has profound implications for shelter biosecurity; it suggests that recent vaccination can confound diagnostic PCR results and that vaccine virus itself may transiently circulate, though it does not appear to cause disease in immunocompetent contacts.

The role of wildlife as reservoirs for FPV has been increasingly recognized, altering traditional concepts of transmission dynamics. While domestic cats (Felis catus) are the primary reservoir, FPV has been isolated from an astonishing array of wild carnivores and even non-carnivoran species. A landmark study in Italy documented FPV infection in a Marsican brown bear (Ursus arctos marsicanus) and a crested porcupine (Hystrix cristata), representing a significant expansion of the known host range [11]. Similarly, FPV was isolated for the first time from wild raccoon dogs (Nyctereutes procyonoides) in residential areas of Shanghai, China, with the VP2 genes showing 100% homology to local feline strains [13]. These findings support a spillover-spillback dynamic where FPV circulates between domestic and wild populations. The detection of FPV in free-ranging jaguars (Panthera onca) in Brazil and in a puma (Puma concolor) rescued from the illegal wildlife trade in Colombia further confirms that wild felids act as both sentinels and potential maintenance hosts, particularly in habitats encroached by domestic animals [12, 23]. This interspecies transmission is not merely a curiosity; it poses a direct threat to conservation efforts for endangered species, as demonstrated by fatal outbreaks in captive Siberian tigers in the Republic of Korea and in captive Pallas’s cats (Otocolobus manul) in China [9, 19]. The mechanism underlying this cross-species jump is rooted in the structure of the VP2 capsid protein, which binds to the host transferrin receptor (TfR). The high homology between feline TfR and those of other carnivores, including giant pandas, facilitates viral entry, while specific amino acid substitutions (e.g., A300P) can broaden the host range to include canids [6, 14].

Molecular Epidemiology and the Emergence of Dominant Variants

Contemporary epidemiological studies have shifted focus from simple prevalence to the molecular characterization of circulating strains, as the genetic landscape of FPV is demonstrably changing. The most significant molecular epidemiological trend globally is the near-complete fixation of the A91S substitution in the VP2 protein in many Asian populations. In China, systematic analysis of available VP2 sequences from GenBank revealed that the occurrence rate of the A91S mutant increased from 15.63% in 2017 to 100% in 2024, establishing it as the dominant circulating genotype [7]. This substitution is located in a hypervariable region of VP2 (entropy peak at nt 271) and is under strong positive selection pressure [16, 18]. The A91S mutation extends a random coil in the capsid surface from residues 91-95, potentially altering antigenicity and facilitating immune evasion from vaccine-induced antibodies [27]. Concurrently, the I101T mutation has reached a high and stable frequency, suggesting it has reached an adaptive equilibrium that may confer a fitness advantage without incurring a significant fitness cost [16, 18].

These mutations are not merely markers of genetic drift; they have documented phenotypic consequences. Strains carrying both A91S and I101T have been experimentally confirmed to cause typical severe panleukopenia and acute enteritis in specific-pathogen-free cats [8]. Furthermore, the A91S/I101T variant (exemplified by strain FPLV-CC19-02) shows a substantial genetic distance from all major commercial vaccine strains (Pfizer, MSD, Virbac, Merial, Nobivac) and exhibits enhanced virulence in both cell culture and animal models [3]. This raises the concerning possibility of vaccine breakthrough, where current vaccines may confer suboptimal protection against these divergent field strains. Indeed, a fatal outbreak in vaccinated Siberian tigers was attributed to vaccine failure likely triggered by MDA interference or insufficient protective immunity against the circulating KTPV-2305 strain, which carried I101T, I232V, and L562V mutations [19]. The implications for global vaccine strategy are clear: continuous molecular surveillance is essential to ensure vaccine antigens remain relevant to circulating field strains.

Beyond Asia, regional adaptation is evident. In Portugal, FPV strains circulating between 2006-2008 and 2012-2014 formed distinct phylogenetic clusters related to Italian and Asian isolates, but no CPV spillover was detected in cats, suggesting that FPV remains the dominant parvovirus in that geographic niche [53]. In Vietnam, the first molecular characterization of FPV in diarrheic cats revealed that local strains formed a narrow cluster distinct from vaccine strains, sharing 98.51% nucleotide homology with vaccine references [21]. In India, isolates from Mizoram exhibited unique S179T and I401V mutations not previously reported, highlighting the existence of geographically restricted evolutionary trajectories [46]. Even within China, geographic compartmentalization is observed; strains from Xinjiang province formed distinct clades compared to those from eastern China, and the evolutionary rate of FPV was estimated at (1.13 \times 10^{-4}) substitutions/site/year, approaching that of RNA viruses despite its DNA genome [8]. This relatively high substitution rate is driven by the error-prone nature of host cell DNA polymerases during replication in rapidly dividing cells, combined with the strong selective pressures exerted by host immune systems and cross-species transmission events.

Transmission in Anthropogenic and Conservation Contexts

The transmission dynamics of FPV are profoundly influenced by human activity. The illegal wildlife trade represents a high-risk pathway for introducing FPV into naive wild populations. A case report of FPV in a juvenile puma rescued from illegal trade in Colombia demonstrated that stressed, immunocompromised, and co-housed wild animals are highly susceptible [12]. Similarly, the detection of vaccine-origin FPV in wastewater in Arizona, USA, via long-range PCR and MinION sequencing, provides a novel population-level surveillance tool and reveals that vaccine virus is shed into the municipal sewer system, offering a non-invasive method to monitor parvovirus diversity at the community level [5]. This wastewater-based epidemiology (WBE) approach has the potential to serve as an early warning system for the emergence of novel variants.

In multi-cat environments such as shelters, catteries, and zoos, transmission is amplified by high population density, stress, and the presence of fomites. FPV is a core pathogen for which rigorous disinfection protocols using parvovirus-specific disinfectants (e.g., 4% sodium hypochlorite, accelerated hydrogen peroxide) are mandatory. The virus’s ability to persist in the environment and its high contagiousness mean that even indirect contact via a fomite can establish infection. For captive endangered species, such as the giant panda (Ailuropoda melanoleuca), the introduction of FPV from sympatric domestic cats is a documented conservation threat, with strains like FPVD-am2020 and Giant panda/CD/2018 carrying G299E mutations that may be associated with enhanced binding to the panda TfR [20, 25]. The detection of FPV in a giant panda with mild diarrhea, and the subsequent 100% mortality in experimentally infected kittens, underscores the high stakes of transmission dynamics in conservation settings.

Clinical Features and Pathological Consequences

Pathophysiology and Cellular Tropism

Feline panleukopenia virus (FPV) exhibits a profound tropism for cells with high mitotic activity, a characteristic that dictates its clinical and pathological signature. The virus requires host cellular DNA polymerase for replication, making tissues undergoing rapid turnover the primary targets [4, 17]. This tropism explains the devastating impact on the intestinal crypt epithelium, hematopoietic progenitor cells in the bone marrow, and lymphoid tissues, including the thymus, spleen, and lymph nodes [9, 59, 61]. The non-structural protein NS1 and the capsid protein VP2 orchestrate host cell cycle arrest and apoptosis, driving the extensive tissue damage observed in infected animals [4, 31, 37]. Transcriptomic analyses of FPV-infected feline kidney cells have revealed significant downregulation of genes associated with cell cycle progression and cell growth, coupled with upregulation of genes governing extrinsic apoptotic signaling, toll-like receptor pathways, JAK-STAT signaling, and tumor necrosis factor pathways [4]. This host-virus interaction highlights a sophisticated mechanism where FPV simultaneously suppresses cellular proliferation to create a favorable environment for its own replication while triggering antiviral immune responses that paradoxically contribute to tissue pathology.

The VP2 protein, which constitutes the primary antigenic determinant and host range determinant, interacts with the host transferrin receptor (TfR) to mediate viral entry [14, 24]. This receptor-virus interaction is a critical bottleneck for cross-species transmission. The high homology between feline TfR and that of other carnivores, including giant pandas and raccoon dogs, explains the expanding host range of FPV and the emergence of spillover events into novel species [13, 14, 25]. Mutations in VP2, particularly at residues 91, 101, 232, 299, 300, and 426, have been demonstrated to alter surface charge distribution, receptor binding affinity, and antigenicity, thereby influencing tissue tropism, virulence, and the potential for immune evasion [1, 3, 6, 7, 18, 25, 30].

Clinical Syndromes and Disease Progression

The clinical manifestation of FPV infection ranges from subclinical to peracute, with the severity determined by age, immune status, viral strain virulence, and the presence of concurrent infections [17, 54]. The incubation period is typically 2 to 7 days following oronasal exposure to contaminated feces, vomitus, or fomites [63, 66]. Peracute disease, most frequently observed in kittens under 12 weeks of age, is characterized by sudden death with minimal prodromal signs, often attributable to profound hypoglycemia, endotoxemia, or cardiovascular collapse secondary to severe dehydration and sepsis [63, 69].

In acute cases, the disease follows a predictable, biphasic course. The initial phase, spanning 24 to 48 hours, is marked by lethargy, anorexia, and pyrexia, with rectal temperatures reaching 40°C to 41.7°C [17, 50, 51]. This febrile response reflects systemic viral replication and the release of pro-inflammatory cytokines. Within 48 to 72 hours, gastrointestinal signs become prominent. Vomiting is a near-ubiquitous finding, reported in 87.5% to 100% of clinical cases, and often precedes the onset of diarrhea [52, 56, 68]. The vomitus may initially consist of bile-stained fluid or foam, progressing to frank blood in severe cases due to extensive gastric mucosal erosion [59, 61]. Diarrhea develops in 93.3% to 100% of symptomatic cats and is initially small intestinal in character, becoming profuse, watery, and hemorrhagic as the disease advances [45, 50, 56]. The feces frequently contain blood and mucus, indicative of severe crypt necrosis and fibrinonecrotic enteritis [9, 61, 68]. Severe abdominal pain, evidenced by posture changes (hunched back), vocalization, and reluctance to be handled, is common. Dehydration rapidly ensues, with clinical estimates of 5% to 12% body weight loss within the first 48 hours of gastrointestinal signs [55, 66, 67]. The dehydration is compounded by persistent vomiting and diarrhea and is exacerbated by the cat's refusal to drink.

A distinct neurological form of FPV infection exists, although it is less commonly reported. In neonatal and perinatal infections, the virus exhibits a tropism for the rapidly dividing cells of the cerebellar external germinal layer [2]. This results in cerebellar hypoplasia, a non-progressive, permanent condition. Affected kittens present with intention tremors, a wide-based ataxic gait, hypermetria, and a lack of menace response, typically becoming apparent when the kitten begins to ambulate at 2 to 3 weeks of age [2]. In cases of acute FPV encephalitis in older animals, clinical signs may include seizures, circling, behavioral changes, and depression, often secondary to viral replication within neurons and glial cells [9, 61]. Hydrocephalus has also been documented in conjunction with FPV infection in some case series, suggesting a broader neurotropic potential [61].

Hematological Consequences: The Panleukopenia

The eponymous panleukopenia is the most consistent and diagnostically critical laboratory finding. FPV targets hematopoietic progenitor cells in the bone marrow, particularly those of the myeloid, erythroid, and megakaryocytic lineages, leading to a profound reduction in all circulating leukocyte populations [17, 50]. Lymphocytes are affected first due to additional viral replication within the thymus, spleen, and lymph nodes, leading to lymphopenia before a significant neutropenia is detectable [52, 59, 64]. Total white blood cell counts can plummet below 1,000 cells/μL, with counts as low as 200–450 cells/μL documented in severe cases, making the diagnosis of FPV highly likely [50, 56, 60]. The nadir of leukopenia typically occurs 4 to 6 days post-infection. Early studies have demonstrated that a low leukocyte count on days 3, 4, and 7 of hospitalization is associated with non-survival, whereas the leukocyte count at initial admission may not be prognostic [69]. This suggests that the ability of the bone marrow to recover, rather than the initial severity of leukopenia, is the critical determinant of outcome.

Thrombocytopenia is a frequent concurrent finding, with counts often falling below 100,000/μL, contributing to a bleeding diathesis and exacerbating gastrointestinal hemorrhage [56, 65]. Anemia, ranging from mild to severe, may develop secondary to blood loss from the gastrointestinal tract, bone marrow suppression, and the shortened lifespan of erythrocytes in the face of systemic inflammation [50, 52, 59]. The hematological crisis creates a state of severe immunosuppression, rendering the cat highly susceptible to secondary bacterial infections, including septicemia, bacterial pneumonia, and cellulitis, which are common causes of death in hospitalized animals [17, 61, 62].

Systemic Inflammatory Response and Multi-Organ Dysfunction

Beyond the intestinal and hematological systems, FPV infection precipitates a multi-systemic inflammatory response. Serum biochemistry often reveals marked elevations in blood urea nitrogen and creatinine, reflecting prerenal azotemia from severe dehydration and, in some cases, acute kidney injury [50, 64]. Elevated alanine aminotransferase, lactate dehydrogenase, and total bilirubin are indicative of hepatic degeneration and necrosis, directly attributable to viral replication in hepatocytes or secondary to ischemia and hypoxia [50, 61, 64]. C-reactive protein, an acute-phase protein, is significantly elevated in septic panleukopenic cats and correlates with disease severity [64]. Hyperglycemia or hypoglycemia can occur; hypoglycemia is particularly dangerous in kittens and is a leading cause of sudden death due to impaired gluconeogenesis and glucose depletion in the face of massive cellular damage [69].

The quick Sepsis-related Organ Failure Assessment (qSOFA) has been evaluated as a prognostic tool in FPV-infected cats. Septic panleukopenic cats exhibit significantly higher qSOFA scores than non-septic counterparts, although evidence suggests this scoring system may be less sensitive in cats compared to dogs [64]. Nonetheless, cats with FPV-induced sepsis display remarkably high body temperatures and higher qSOFA scores, confirming the systemic nature of the disease and the frequent progression to multi-organ dysfunction syndrome (MODS) [64, 69].

Gross Pathological Findings

At necropsy, the most striking lesions are confined to the gastrointestinal tract and lymphoid organs. The small intestine, particularly the ileum and jejunum, is characteristically dilated, flaccid, and filled with watery, often bloody or fibrinous contents [9, 59, 61]. The intestinal serosal surface may be congested, and the mucosal surface is erythematous, edematous, and ulcerated. In cases of severe fibrinonecrotic enteritis, the mucosa can be covered by a thick, diphtheritic membrane composed of fibrin, necrotic cellular debris, and infiltrating inflammatory cells [61]. The intestinal wall is often markedly thinned due to crypt necrosis and villous collapse, a finding detectable on abdominal ultrasound [42, 68]. Mesenteric lymph nodes are consistently enlarged, edematous, and hemorrhagic, reflecting profound lymphoid depletion and necrosis [9, 59]. The spleen is typically reduced in size due to lymphoid follicle atrophy, and the thymus in kittens undergoes severe involution [61]. The bone marrow appears pale or gelatinous, reflecting widespread hypoplasia [17].

Atypical gross lesions have been increasingly recognized. Necrotizing bronchopneumonia, characterized by multifocal to coalescing areas of pulmonary consolidation, hemorrhage, and suppuration, has been documented in FPV-positive cats, often with concurrent bacterial co-infections [61]. Hepatic and renal degeneration, manifesting as swollen, pale, and friable organs, is also observed [61]. In some cases, particularly with certain viral strains or in immunocompromised hosts, hydrocephalus and degenerative changes in the central nervous system have been reported, widening the spectrum of recognized pathology [61].

Histopathological Features

Histological examination reveals the hallmark lesion of FPV infection: necrosis of the intestinal crypt epithelium. The crypts are dilated, lined by flattened, attenuated epithelial cells with hypereosinophilic cytoplasm and pyknotic or karyorrhectic nuclei. Intranuclear inclusion bodies may be present in crypt epithelial cells, particularly in early or mild cases [9, 59, 61]. The lamina propria is edematous and infiltrated with a mixture of lymphocytes, plasma cells, and neutrophils; in severe cases, extensive transmural necrosis and hemorrhage are evident. In the mesenteric lymph nodes and spleen, there is marked lymphoid depletion, with loss of germinal centers and paracortical areas, replaced by histiocytes and cellular debris [9, 59, 61]. In the central nervous system, cerebellar hypoplasia is characterized by a marked reduction in the thickness and cellularity of the molecular and granular layers, with a corresponding loss of Purkinje cells [2]. In acute encephalitic cases, perivascular cuffing, gliosis, and neuronal necrosis may be observed [9]. Immunohistochemistry reliably demonstrates viral antigen in the cytoplasm of lymphocytes and macrophages within the lamina propria of the small intestine, confirming the site of extensive viral replication [9, 25].

Impact of Viral Strain Variation on Clinical Features

Emerging evidence indicates that contemporary FPV strains, particularly those harboring the A91S and I101T mutations in VP2, present with distinct clinical characteristics compared to classical strains. Experimental infections with these mutant strains have demonstrated the ability to induce typical leukopenia and acute enteritis, confirming that these variants retain full pathogenicity in cats [1, 3, 8]. However, they also exhibit a 1.8- to 2.3-fold greater pathogenicity compared to earlier field strains, with more rapid replication kinetics and higher peak viral loads in infected animals [1, 3]. This enhanced virulence correlates with the increasing prevalence of these mutations in the global feline population, reaching 100% of Chinese isolates by 2024 [7].

Furthermore, the emergence of FPV strains with substitutions at residue 300 (A300P) has expanded the host range, enabling these feline-derived viruses to infect dogs and cause gastrointestinal disease [6]. This cross-species transmission represents a significant clinical and pathological shift, blurring the traditional boundaries between FPV and canine parvovirus. Similarly, FPV strains with the G299E mutation, first identified in giant pandas, have been linked to severe, fatal disease in this endangered species, underscoring the potential for FPV to cause more severe pathology in novel hosts [20, 25]. The presence of these evolving variants necessitates continuous molecular surveillance as the clinical presentation and pathological consequences of FPV are no longer a fixed entity but a dynamic spectrum influenced by ongoing viral adaptation.

Co-infections and Immune Modulation

FPV-induced immunosuppression frequently predisposes cats to concurrent infections with other enteric pathogens. Co-infections with feline coronavirus, feline calicivirus, feline herpesvirus-1, Giardia spp., Cryptosporidium spp., Clostridium spp., and canine circovirus are common and are associated with more severe clinical signs, prolonged disease duration, and increased mortality [33, 34, 44, 57]. In particular, co-infection with canine circovirus enhances FPV replication by suppressing the type I interferon response, leading to higher viral loads and exacerbated tissue damage [33]. Additionally, secondary bacterial infections with opportunistic organisms such as Stenotrophomonas maltophilia are increasingly recognized as contributors to the multi-systemic pathology observed in fatal cases, including necrotizing bronchopneumonia and septicemia [61]. This pathogen synergy amplifies the clinical severity and complicates therapeutic management.

Prognostic Indicators and Case Fatality

The prognosis for FPV-infected cats remains guarded, with survival rates in shelter settings reported as low as 20.3% without intensive intervention [69]. Multiple factors are associated with poor outcomes: a low leukocyte count on days 3, 4, and 7 of hospitalization; hypothermia (rectal temperature < 37.9°C) at admission; low body weight; the presence of lethargy; and the development of sepsis or multi-organ dysfunction [62, 64, 69]. The systemic inflammatory response index and neutrophil-to-lymphocyte ratio have been identified as valuable prognostic markers, with higher values correlating with a more severe disease course and increased risk of death [62, 64]. In contrast, the presence of hyperechoic mucosal bands on jejunal ultrasonography has been associated with survival, suggesting that ultrasonographic features may reflect underlying mucosal integrity and regenerative capacity [68]. The case fatality rate in kittens under 6 months of age approaches 50–75% in untreated cases, highlighting the critical importance of early, aggressive supportive care [45, 48, 69]. While no specific antiviral therapy exists, immunomodulatory agents such as filgrastim and inactivated parapoxvirus ovis have shown promise in improving white blood cell recovery and clinical outcomes, though large-scale controlled trials remain necessary [58, 62].

Diagnostic Strategies: From Conventional Assays to Multiplex qPCR

The accurate and timely diagnosis of feline panleukopenia virus (FPV) infection is paramount for effective clinical management, outbreak containment, and epidemiological surveillance. The diagnostic landscape for FPV has evolved dramatically over the past two decades, transitioning from reliance on clinical and hematological assessments and simple antigen detection to sophisticated nucleic acid amplification technologies capable of simultaneous, high-throughput pathogen detection. This section provides a comprehensive, mechanism-based analysis of the diagnostic armamentarium available for FPV, critically evaluating the strengths, limitations, and appropriate applications of each methodology, from traditional techniques to state-of-the-art multiplex quantitative polymerase chain reaction (qPCR) systems.

Conventional Diagnostic Approaches: Clinical, Hematological, and Serological Foundations

Initial diagnostic suspicion for FPV has historically been, and continues to be, rooted in the recognition of characteristic clinical and clinicopathological abnormalities. The disease presents acutely, particularly in unvaccinated juveniles, with a classic triad of fever, profound depression, and vomiting followed by diarrhea, often hemorrhagic [45, 55, 56]. Hematological evaluation remains a cornerstone of initial assessment, as FPV exhibits a marked tropism for rapidly dividing cells in the bone marrow, lymphoid tissues, and intestinal crypt epithelium. The resulting panleukopenia, a severe, often precipitous reduction in total white blood cell count, is a pathognomonic finding, though its absence does not rule out infection, particularly in peracute cases or early in the disease course [50, 52, 56]. Advanced hematological parameters, such as the quick Sepsis-related Organ Failure Assessment (qSOFA) score and the systemic inflammatory response index (SIRI), have been investigated as prognostic tools, with correlations established between these composite scores and survival outcomes in panleukopenic cats [62, 64].

Serological assays, while valuable for population-level seroprevalence studies and vaccine response monitoring, are of limited utility for acute diagnosis. The detection of anti-FPV antibodies via hemagglutination inhibition (HI) or enzyme-linked immunosorbent assay (ELISA) cannot distinguish between active infection and prior vaccination or exposure, given that maternally derived antibodies can persist for weeks in kittens and that vaccine-induced titers may be indistinguishable from those generated by natural infection [52, 74, 78]. Notably, the HI assay using recombinant VP2 protein expressed in insect cells has been refined as a standardized, non-infectious alternative to traditional live-virus HI tests, enhancing laboratory safety while maintaining diagnostic accuracy for serosurveillance [29].

Rapid Antigen Detection Tests: Point-of-Care Convenience with Inherent Limitations

Immunochromatographic (IC) antigen test kits, commonly referred to as rapid tests, have become ubiquitous in veterinary practice due to their operational simplicity, speed (results in 10–15 minutes), and minimal equipment requirements. These lateral-flow devices detect FPV VP2 antigen in fecal or rectal swab samples, providing a binary yes/no result that is immediately actionable in a clinical setting [45, 54, 59, 67]. However, a critical and systematic evaluation of their diagnostic performance, benchmarked against PCR as the reference standard, reveals significant accuracy concerns.

A rigorous comparative study conducted in Bangladesh demonstrated that while the IC test detected FPV in 84% of clinically suspected cases, PCR confirmed infection in only 60%, yielding a substantial 24% false-positive rate [71]. This discordance likely arises from the lower analytical sensitivity of the IC test, its susceptibility to interference from fecal matrix components, and the subjective interpretation of faint test lines. Furthermore, the detection limit of these kits is typically in the range of 10⁵–10⁶ virus particles per gram of feces, rendering them incapable of identifying low-level shedders. This is particularly relevant in breeding catteries, where subclinical shedding of FPV DNA has been documented in 2.5% of healthy, vaccinated cats, often in association with diarrheic feces [34]. The practical consequence is that reliance solely on a negative IC test result cannot reliably rule out FPV infection, especially in cases with a high clinical index of suspicion. For confirmation, particularly in outbreak settings, multi-cat environments, or when deciding on isolation protocols, PCR-based methods are unequivocally superior.

Conventional PCR and Gel-Based Detection

The introduction of conventional endpoint polymerase chain reaction (PCR) targeting the VP2 gene represented a paradigm shift in FPV diagnostics, offering orders of magnitude greater sensitivity than antigen detection [46, 51, 71, 73]. Standard PCR assays amplify a specific DNA fragment (commonly 237–695 bp of VP2), which is then visualized by gel electrophoresis. This method is highly specific, capable of confirming infection even when viral loads are too low for IC test detection, and provides a definitive genetic template for downstream sequencing and phylogenetic analysis [21, 46, 47, 51].

However, conventional PCR is not without significant drawbacks. It is a time- and labor-intensive process requiring thermocyclers, gel electrophoresis equipment, and skilled personnel. The post-amplification handling of PCR products creates a substantial risk of amplicon carryover contamination. Moreover, conventional PCR is fundamentally qualitative, it reports the presence or absence of target DNA but cannot quantify the viral load, a parameter of increasing clinical relevance. To address these limitations, several refinements have been developed. NanoPCR, which incorporates gold nanoparticles into the reaction mixture to enhance thermal conductivity and improve amplification efficiency, has demonstrated a 100-fold increase in sensitivity (detection limit: 7.97 × 10² copies/μL) compared to conventional PCR, while maintaining the operational simplicity and lower cost relative to qPCR [63]. Similarly, duplex PCR assays have been established for the simultaneous detection of FPV and canine parvovirus type 2 (CPV-2), allowing differentiation between these closely related but epidemiologically distinct pathogens in a single reaction [22].

Quantitative Real-Time PCR: The Gold Standard Quantification

Quantitative real-time PCR (qPCR) has superseded conventional PCR as the gold standard for FPV diagnosis due to its superior sensitivity, specificity, and ability to provide absolute quantification of viral genome copies. By monitoring fluorescence emission in real-time, qPCR eliminates the need for post-amplification processing, dramatically reducing turnaround time and contamination risk. The cycle threshold (Ct) value is inversely proportional to the initial viral load, enabling precise quantification that is invaluable for assessing disease severity, monitoring treatment response, and understanding shedding dynamics.

The diagnostic supremacy of qPCR is illustrated by studies examining vaccine virus shedding. Following administration of modified-live FPV vaccines, qPCR detected FPV DNA in feces of 21.6% of shelter cats, predominantly at day 7 post-vaccination, albeit with significantly lower copy numbers (median 1.13 × 10⁷ copies/g) compared to clinically diseased cats (median 5.01 × 10⁸ copies/g) [49]. This quantitative capacity is clinically critical: a low copy number in a recently vaccinated cat may represent benign vaccine virus shedding, whereas a high copy number in an unvaccinated cat with compatible clinical signs is diagnostic of pathogenic infection. qPCR has also been instrumental in characterizing shedding in breeding catteries, identifying low-level shedding (Ct values ranging from 24–37) in cats that would be missed by antigen testing, and demonstrating a significant association between FPV DNA shedding and diarrheic feces (odds ratio: 9.9) [34].

Advanced Multiplex qPCR Systems: Simultaneous Pathogen Detection

The recognition that feline respiratory and enteric disease complexes frequently involve polymicrobial infections has driven the development of multiplex qPCR platforms capable of detecting multiple pathogens in a single reaction. This approach is not merely a technical convenience; it reflects the biological reality of co-infections, which can influence clinical presentation, disease severity, and treatment strategies. For instance, FPV co-infection with canine circovirus (CanineCV) has been shown to significantly enhance FPV replication through suppression of the host type I interferon response, as demonstrated by qPCR analysis of IFN-α, IFN-β, MxA, and ISG15 gene expression [33]. Similarly, FPV is frequently detected alongside feline coronavirus (FCoV), feline calicivirus (FCV), feline herpesvirus 1 (FHV-1), and feline leukemia virus (FeLV) in clinical samples [44, 57, 76].

Several meticulously optimized and validated multiplex qPCR assays have been published, each targeting distinct genomic regions for optimal specificity. Wang et al. (2025) established a quadruplex TaqMan MGB qPCR assay targeting the VP2 gene of FPV, the TK gene of FHV-1, the ORF2 gene of FCV, and the N gene of FIPV [44]. Primer and probe concentrations were rigorously optimized (FPV primers at 0.08 μM, probes at 0.08 μM; annealing temperature of 59°C), achieving limits of detection as low as 41.25–53.21 copies/μL for recombinant plasmid standards. The assay demonstrated no cross-reactivity with common feline pathogens, high amplification efficiencies (96.28%–105.05%), and excellent reproducibility (inter- and intra-batch CVs of 0.14%–1.37%). In a field evaluation of 381 fecal samples, the quadruplex assay detected FPV, FHV-1, FCV, and FIPV at rates of 13.65%, 18.37%, 26.77%, and 9.71%, respectively, with 100% agreement to conventional single-plex PCR and commercial kits [44].

Another significant contribution is the one-step triplex TaqMan RT-qPCR developed by He et al. (2024) for the simultaneous detection of FCoV, FPV, and FeLV [57]. This assay targets the FCoV 5′ UTR, FPV VP2, and FeLV pol genes, achieving limits of detection of approximately 140–152 copies/reaction. Importantly, in a large clinical validation of 1,175 samples, the triplex method demonstrated a clinical sensitivity of 93.07% and specificity of 97.99% compared to single-plex reference assays, with FPV positivity rates of 19.91% [57]. A one-step duplex TaqMan qPCR for FCoV and FPV (FPLV) further underscores the robustness of this approach, achieving a limit of detection of 50 copies/μL (approximately 20-fold more sensitive than conventional PCR) with CVs below 2% [75]. This assay was notable for its ability to detect a broad range of FCoV genotypes and FPV variants, reflecting the careful selection of conserved primer and probe binding sites.

Isothermal Amplification and Emerging Biosensor Platforms

While qPCR remains the reference standard, its requirement for expensive thermal cycling equipment and specialized training limits deployment in resource-constrained settings. To bridge this gap, isothermal amplification methods have emerged as viable point-of-care alternatives. Recombinase polymerase amplification (RPA) combined with lateral flow dipstick detection (RPA-LFDA) represents a particularly promising technology. Hong et al. (2024) developed an RPA-LFDA assay for FPV that operates at a constant temperature of 39°C, with a detection limit of 10 target copies per reaction, comparable to qPCR [72]. In clinical validation, RPA-LFDA detected 39 of 44 qPCR-positive samples (sensitivity 100%, specificity 83.33%, overall accuracy 97.73%), significantly outperforming the colloidal gold method (CGM), which detected only 26 of the same positives [72]. The strand exchange amplification (SEA) method offers another isothermal option, capable of detecting FPV DNA after a 40-minute incubation at 61°C, with visual colorimetric readout eliminating the need for specialized detection instruments [77].

Innovative biosensor platforms are also being explored. A novel colorimetric DNA sensor utilizing ZnFe₂O₄ (ZFO) nanoparticles exploits the peroxidase-like activity of these nanoparticles to catalyze oxidation of TMB substrate, producing a blue color that is inhibited upon target DNA hybridization [70]. This assay achieved a detection limit of 300 ng/μL for amplified PCR product and 10³ virus particles per gram of feces, offering a simple, stable, and highly selective platform suitable for low-resource environments [70].

Diagnostic Considerations for Novel and Emerging FPV Variants

A critical dimension of FPV diagnostics is the capability to detect and characterize emerging genetic variants, particularly those associated with altered antigenicity or host range. The FPV VP2 gene is under continuous evolutionary pressure, with several amino acid substitutions now recognized as dominant in circulating strains. The A91S substitution in VP2 has increased in prevalence from 15.63% in 2017 to 100% in 2024 in China, forming a distinct phylogenetic cluster [7]. Similarly, the I101T substitution has reached an equilibrium frequency, while a novel T440A substitution co-occurring with N564S and A568G, a combination characteristic of CPV-2c, has been identified in recently isolated strains, potentially conferring enhanced immune evasion [16]. The G299E substitution, first identified in a giant panda isolate (Giant panda/CD/2018) and found in the top region of the VP2 surface loop 3, is notable for its potential impact on host range and antigenicity [20, 25].

These evolutionary dynamics have direct diagnostic implications. Primer and probe sets designed against older, prototype FPV sequences (e.g., the 1964 Johnson snow leopard strain) may exhibit reduced amplification efficiency or complete failure against contemporary variants. Indeed, the detection of a vaccine-origin FPV in wastewater that was 100% identical in VP2 to the 1964 vaccine strain underscores the genetic distance between vaccine lineages and currently circulating field strains [5]. Consequently, ongoing bioinformatic surveillance and periodic updating of diagnostic primer/probe sequences are essential to maintain diagnostic sensitivity. Multiplex qPCR assays that target highly conserved regions of VP2, NS1, or other genomic loci are theoretically more resilient to sequence drift, but continuous validation against contemporary field isolates remains a best practice [44, 57, 75]. The integration of sequencing, whether Sanger sequencing of VP2 amplicons or next-generation sequencing of complete genomes, into diagnostic workflows not only confirms pathogen identity but provides invaluable epidemiological data for tracking viral spread, identifying cross-species transmission events, and informing vaccine strain selection [13, 19, 24, 47, 71].

Prevention and Control: Vaccine Development and Cross-Protection

The prevention and control of feline panleukopenia virus (FPV) hinges on robust vaccination strategies, yet the evolving genetic landscape of the virus presents formidable challenges to cross-protection. The cornerstone of FPV prophylaxis has historically been the administration of modified-live virus (MLV) or inactivated vaccines, which have demonstrated considerable efficacy in reducing morbidity and mortality. However, the emergence of phylogenetically divergent strains, characterized by specific amino acid substitutions in the VP2 capsid protein, raises critical questions about the durability of vaccine-induced immunity against contemporary field isolates. This section provides an exhaustive analysis of vaccine development paradigms, the molecular underpinnings of antigenic variation, and the implications for cross-protection in domestic and wild felid populations.

The Molecular Basis of Antigenic Drift and Vaccine Escape

The VP2 capsid protein is the primary target of neutralizing antibodies and the principal determinant of host range and antigenicity. Recent molecular epidemiological studies have documented a pronounced shift in the genetic composition of circulating FPV strains, particularly in Asia. The A91S substitution in VP2 has emerged as a dominant mutation, with its prevalence increasing from 15.63% to 100% among Chinese FPV strains between 2017 and 2024 [7]. This substitution is located within a hypervariable region of VP2, and structural modeling predicts that it extends the random coil from residues 92-95 to 91-95, potentially altering surface charge distribution and receptor-binding dynamics [27]. Concurrently, the I101T mutation has achieved a state of adaptive equilibrium, suggesting that it confers a selective advantage without compromising viral fitness [8, 16]. Critically, these mutations are not merely neutral markers of genetic drift; they are associated with enhanced virulence and immune evasion. For instance, the FPV ZZ202303 strain, which harbors both I101T and a prevalent A91S-like substitution, demonstrated 1.8- to 2.3-fold greater pathogenicity compared to contemporary field strains and formed a distinct phylogenetic cluster diverging from vaccine lineages [1]. This divergence is not confined to Asia. In Portugal, FPV strains have exhibited slight but consistent divergence from older vaccine strains, and in the United States, vaccine-origin FPV has been detected in wastewater, indicating that even vaccine-derived viruses can circulate in the environment [5, 53]. The detection of a T440A substitution, co-occurring with N564S and A568G, a combination characteristic of CPV-2c, in newly isolated Chinese strains further underscores the potential for antigenic drift to erode vaccine efficacy [16]. These findings collectively indicate that the genetic distance between vaccine strains and circulating field strains is widening, necessitating a re-evaluation of vaccine strain selection.

Vaccine Platforms: From Traditional to Next-Generation Approaches

Traditional FPV vaccines, both MLV and inactivated, have been the mainstay of prevention. MLV vaccines, typically administered subcutaneously, induce robust humoral and cell-mediated immunity. A field study comparing an inactivated adjuvanted vaccine with an MLV vaccine demonstrated that while both elicited protective hemagglutination inhibition (HI) titers (≥1:32), the MLV vaccine produced significantly higher antibody titers at 4 weeks and 12 months post-vaccination [80]. However, MLV vaccines carry a risk of residual virulence and post-vaccination shedding. In a shelter setting, 21.6% of cats shed FPV DNA in feces following MLV vaccination, with peak shedding occurring on day 7 post-vaccination, though viral copy numbers were significantly lower than those observed in clinically ill cats [49]. This shedding complicates diagnostic interpretation and may pose a risk to immunocompromised animals. Inactivated vaccines, while safer, often require adjuvants to enhance immunogenicity, and concerns about feline injection-site sarcomas (FISS) have driven recommendations for alternative injection sites. Importantly, a study evaluating an inactivated vaccine administered in the scruff, distal hindlimb, or proximal tail found no significant differences in protective antibody responses, supporting the use of distal sites to mitigate FISS risk [80].

The limitations of traditional platforms have spurred the development of next-generation vaccines. Virus-like particles (VLPs) represent a particularly promising approach. Using a baculovirus expression vector system (BEVS), researchers have produced FPV VLPs self-assembled from VP2 protein. These VLPs, formulated with Seppic adjuvant, elicited a strong hemagglutination inhibition titer of 1:216 in cats and provided 100% protection against challenge with a virulent FPV variant (Ala91Ser/Ile101Thr) [43]. A separate study confirmed that a VLP vaccine, even at a low dose of 5 μg, induced significantly higher HI and virus-neutralizing (VN) antibody titers compared to controls, and vaccinated cats exhibited no clinical signs or leukopenia post-challenge [39]. The VLP platform offers distinct advantages: it is non-infectious, lacks viral nucleic acid, and can be produced in scalable insect cell cultures, addressing safety concerns associated with MLV vaccines and production complexities of inactivated vaccines.

Another innovative approach is the development of oral vaccines using recombinant Lactobacillus plantarum expressing VP2. This platform leverages the natural mucosal adjuvanticity of lactic acid bacteria. A recombinant L. plantarum NC8/VP2 strain, based on the prevalent Chinese FPV-251 isolate, effectively colonized the feline intestinal tract and induced high levels of neutralizing antibodies after 30 days of continuous oral dosing. Kittens were significantly protected against FPV-251 challenge, with no fatalities [79]. This oral delivery system circumvents the stress and adverse reactions associated with injectable vaccines and is particularly advantageous for unvaccinated kittens that may be exposed to FPV during veterinary visits. The use of a locally prevalent strain as the antigen source is critical, as it may provide superior protection against regional variants.

Cross-Protection and Host Range Expansion

The capacity of current vaccines to protect against heterologous strains is a pressing concern. The emergence of FPV variants with mutations in key antigenic sites, such as G299E in the VP2 protein, which is located on the top region of interconnecting surface loop 3, a region involved in host range and antigenicity, raises the specter of vaccine failure [20, 25]. This mutation was first identified in a giant panda isolate and has since been detected in other captive wildlife. Similarly, the A300P substitution, which enables FPV to infect canine cell lines and cause gastrointestinal disease in dogs, represents a significant host range expansion [6]. While FPV has traditionally been considered host-restricted to felids, these findings demonstrate that single amino acid changes can facilitate cross-species transmission. The detection of FPV in dogs from Italy and Egypt, as well as in raccoon dogs in China, further underscores the potential for inter-species spread [13, 26]. A canine-derived FPV strain, when inoculated into specific pathogen-free kittens, caused only mild, self-limiting diarrhea but resulted in prolonged viral shedding and systemic dissemination, suggesting that such strains may act as stealth vectors for viral maintenance [36].

The implications for vaccination are profound. Vaccines developed against historical FPV strains may not provide optimal protection against these emerging variants. For instance, the Chinese epidemic strain FPLV-CC19-02, which carries the Ala91Ser and Ile101Thr substitutions, is phylogenetically distant from all major commercial vaccine strains (Pfizer, MSD, Virbac, Merial, Nobivac) [3]. Despite this distance, vaccination with a traditional inactivated vaccine still demonstrated good immunogenicity in cats, suggesting that some degree of cross-protection is retained. However, the study also noted that the variant exhibited greater virulence for host cells and cats, implying that while vaccination may prevent mortality, it may not fully prevent infection or shedding. This is consistent with observations in captive Siberian tigers, where vaccinated animals succumbed to FPV infection due to insufficient protective immunity or maternal antibody interference [19]. The presence of three common VP2 mutations (I101T, I232V, L562V) in the infecting strain, which are also found in global FPV strains, indicates that even widely circulating variants can overcome vaccine-induced immunity under certain conditions.

Vaccination in Special Populations and the Role of Maternal Antibodies

The efficacy of vaccination is profoundly influenced by the presence of maternal-derived antibodies (MDA). In kittens, MDA can neutralize vaccine antigens, leading to vaccine failure. This is a well-documented phenomenon and was identified as the likely cause of FPV infection in vaccinated tiger cubs in a Korean zoo [19]. The timing of the primary vaccination series is therefore critical. Current guidelines recommend starting vaccination at 8-12 weeks of age, with boosters every 3-4 weeks until 16-20 weeks of age. However, even with this schedule, a subset of kittens may remain susceptible. A study evaluating antibody responses in kittens with and without intestinal parasites found that 23.5% of parasitized kittens and 42.1% of non-parasitized kittens had pre-vaccination HI titers ≥1:40, indicating that MDA was still present at the time of first vaccination [74]. Importantly, the presence of parasites did not significantly impair the immune response, suggesting that deworming, while generally recommended, is not a prerequisite for effective vaccination.

Asymptomatic retrovirus-infected cats (feline leukemia virus [FeLV] and feline immunodeficiency virus [FIV]) represent another special population. A pilot study found that 100% of such cats had pre-vaccination FPV antibodies, and their response to MLV vaccination was similar to that of non-infected cats, with no adverse effects observed [78]. This suggests that retrovirus-infected cats can be safely and effectively vaccinated against FPV, though the study was limited by small sample size. In shelter environments, where FPV outbreaks are common and devastating, rapid induction of immunity is paramount. The use of MLV vaccines is preferred in this setting due to their faster onset of protection, but the potential for post-vaccination shedding must be managed through appropriate isolation and diagnostic interpretation [49].

Strategic Considerations for Future Vaccine Development

The accumulating evidence of genetic divergence, host range expansion, and vaccine breakthrough necessitates a proactive approach to vaccine strain selection. The World Organisation for Animal Health (WOAH) and national veterinary authorities should consider establishing a global surveillance network to monitor FPV VP2 sequence diversity and antigenic drift. This would enable the periodic updating of vaccine strains, analogous to the system used for influenza virus. The use of local epidemic strains, such as the FPLV-CC19-02 variant, as vaccine candidates has been proposed, and preliminary data suggest that such vaccines can induce robust immune responses [3]. However, the development of broadly protective vaccines, perhaps incorporating multiple VP2 variants or conserved epitopes, may be a more sustainable long-term strategy. The VLP and recombinant Lactobacillus platforms offer the flexibility to rapidly incorporate new antigenic variants, making them well-suited for this purpose. Furthermore, the development of DIVA (Differentiating Infected from Vaccinated Animals) vaccines, which allow serological distinction between vaccinated and naturally infected animals, would greatly enhance epidemiological surveillance and outbreak management. The detection of vaccine-origin FPV in wastewater highlights the need for such tools to accurately track viral circulation [5]. Ultimately, the control of FPV will require a multifaceted approach that combines optimized vaccination protocols, rigorous biosecurity, and continuous molecular surveillance to anticipate and mitigate the impact of emerging variants.

Emerging Threats and Future Perspectives

The landscape of feline panleukopenia virus (FPV) epidemiology and pathogenesis is undergoing a profound transformation, driven by accelerating viral evolution, expanding host ranges, and the emergence of complex co-infection dynamics. As a pathogen that has circulated among felids for over a century, FPV was once considered a genetically stable, well-controlled disease. However, the convergence of molecular, ecological, and anthropogenic factors has unveiled a series of emergent threats that challenge existing paradigms of diagnosis, prophylaxis, and biosecurity. Concurrently, a wave of technological innovation in diagnostics, vaccinology, and therapeutics offers unprecedented opportunities to confront these challenges. This section provides an exhaustive analysis of these emerging threats and charts the future perspectives that will define the next era of FPV research and clinical management.

Accelerating Genetic Evolution and the Emergence of Vaccine-Evasive Variants

Perhaps the most pressing emerging threat is the documented acceleration of FPV genetic evolution, particularly within the VP2 capsid protein gene, which governs host range, antigenicity, and receptor binding. The evolutionary rate of FPV has been calculated at approximately 1.13 × 10⁻⁴ substitutions per site per year, a figure approaching that of many RNA viruses and remarkably high for a single-stranded DNA virus [8]. This rapid evolution has driven the emergence of specific amino acid substitutions that are now becoming fixed in circulating populations, with profound implications for vaccine efficacy and cross-protection.

The paradigm-shifting discovery is the global ascendancy of the A91S substitution in the VP2 protein. Multiple independent surveillance studies from China have documented a dramatic temporal increase in the prevalence of this mutation, from 15.63% of field strains in 2017 to nearly 100% by 2024 [7, 27]. Phylogenetic analyses reveal that A91S variants form a distinct evolutionary branch, diverging significantly from traditional vaccine lineages [7]. The structural consequences of this substitution are non-trivial: molecular modeling predicts an extension of the random coil at amino acid residues 91–95, altering the surface topology of the capsid in an antigenically critical region [27]. This mutation is now being detected across Asia, Europe, and the Americas, indicating a global selective sweep [27].

Alongside A91S, the I101T substitution has also reached high frequency and appears to have stabilized at an adaptive equilibrium [16]. While A91S and I101T were initially identified in the emergent FPV ZZ202303 strain, which exhibited 1.8- to 2.3-fold greater pathogenicity compared to contemporary field strains [1], these mutations are now ubiquitous. The combination of I101T with V232I and V562L, commonly observed in Korean and global strains, further underscores the adaptive evolution occurring in the VP2 protein [19]. Critically, these mutations are located within antigenic determinant regions, raising legitimate concerns about the ability of vaccines derived from mid-20th century prototypes (e.g., the Cu-4 strain or the Johnson snow leopard strain) to confer sterilizing immunity against these modern variants [3, 5].

Beyond these high-frequency mutations, the discovery of novel, rare substitutions with potentially profound biological consequences is accelerating. The unprecedented identification of an A300P substitution in a feline-derived FPV strain (FPV-251) represents a landmark event, as this isolate demonstrated the ability to replicate efficiently in canine cell lines (MDCK and A72 cells) and, for the first time, to cause clinical disease in dogs via oral infection [6]. This single amino acid change at residue 300, located on the surface loop of VP2, appears to have expanded the host range of a classical feline virus into a canine host, blurring the traditional boundary between FPV and canine parvovirus (CPV). Similarly, the G299E mutation, first identified in a giant panda isolate, occurs at the top of the interconnecting surface loop 3, a region known to control host range and antigenicity [20, 25]. The detection of T440A, co-occurring with N564S and A568G in Beijing isolates, is particularly alarming as this combination of residues is characteristic of CPV-2c, suggesting ongoing convergent evolution between FPV and CPV lineages [16]. The capsid surface location of residue 440 suggests a potential role in immune evasion [16].

Broadening Host Range and Cross-Species Spillover into Wildlife

The expanding host range of FPV represents a critical emerging threat, both for the conservation of endangered species and for the creation of novel viral reservoirs that could fuel future human or domestic animal outbreaks. Historically considered a pathogen of domestic cats, FPV is now documented with increasing frequency in a staggering diversity of wild carnivores and even non-carnivoran mammals. The implications for global biodiversity are severe, and the World Organisation for Animal Health (WOAH) has recognized the significance of such emergent pathogens in wildlife.

The most consequential cross-species spillover events involve endangered felids and other charismatic megafauna. Fatal outbreaks have been documented in captive Siberian tigers in the Republic of Korea, where genetic analysis confirmed transmission from stray domestic cats [19]. In China, FPV has caused fatal disease in giant pandas, with one isolate (pFPV-sc) exhibiting a remarkable ability to infect human cell lines in vitro, raising questions about zoonotic potential [14]. The identification of FPV in a captive giant panda with mild diarrhea, harboring the unique G299E mutation, further underscores the vulnerability of this endangered species [25]. The threat extends beyond captive settings, as evidenced by the detection of FPV in free-ranging jaguars (Panthera onca) in Brazil, a species already vulnerable to extinction due to habitat loss [23]. The first report of FPV-induced cerebellar hypoplasia in a wild Amur leopard cat confirms that the virus can cause the same debilitating neurological sequelae in wild felids as it does in domestic cats, hindering survival and reproduction [2].

Perhaps more alarming is the expansion of FPV into non-felid hosts. The detection of FPV in a Marsican brown bear and a crested porcupine in Italy extends the known host range to novel mammalian orders [11]. The isolation of FPV from wild raccoon dogs in residential areas of Shanghai marks the first report of this virus in canids in a natural setting, suggesting that urban wildlife can serve as a bridge host for transmission between domestic and wild animal populations [13]. Reports of FPV infection in a banded linsang in Thailand further illustrate the breadth of susceptible species [32]. The role of the illegal wildlife trade as a vector for FPV dissemination cannot be overstated, as demonstrated by the case of a juvenile puma rescued from illegal trade in Colombia, which succumbed to FPV infection [12]. This trade creates unnatural contact between stressed, immunocompromised animals of different species and geographic origins, providing ideal conditions for viral spillover and adaptation.

Synergistic Co-Infections and the Emergence of Novel Pathogen Complexes

A second major emerging threat is the increasing recognition that FPV rarely acts alone. The field is moving away from a single-pathogen paradigm toward an understanding of FPV as a keystone pathogen that facilitates, and is facilitated by, complex polymicrobial interactions. Co-infection rates are high, and the synergistic effects can dramatically alter disease severity, transmission dynamics, and diagnostic interpretation.

The most compelling evidence for synergistic pathogenesis comes from the discovery of a novel truncated replication protein (Rep’) mutant of Canine Circovirus (CanineCV) that demonstrates a direct, molecular-level interaction with FPV. This Rep’ protein significantly enhances the cytotoxicity of CanineCV against feline cells, and while the full-length Rep protein promotes FPV replication more potently, both proteins suppress the host type I interferon (IFN-I) response [33]. This suppression of interferon-α, interferon-β, and interferon-stimulated genes (MxA, ISG15) creates a permissive cellular environment for enhanced FPV replication. This finding establishes a mechanistic basis for the clinical observation that FPV-positive cats are frequently co-infected with other pathogens.

Clinical studies corroborate this molecular evidence. In a study of breeding catteries, all FPV DNA-shedding cats were co-infected with at least one other gastrointestinal pathogen, including Clostridium spp., Cryptosporidium spp., Giardia spp., and feline coronavirus (FCoV) [34]. The odds of FPV shedding were nearly 10 times higher in cats with diarrheic feces compared to those with normal feces, indicating that co-infection drives both viral shedding and clinical disease [34]. Multiplex PCR surveys from China reveal that co-infections with FPV, feline herpesvirus-1 (FHV-1), feline calicivirus (FCV), and feline infectious peritonitis virus (FIPV) are commonplace, with detection rates of 13.65%, 18.37%, 26.77%, and 9.71%, respectively, in fecal samples [44]. The simultaneous presence of these pathogens complicates diagnosis, treatment, and prognosis.

Beyond viral co-infections, FPV-induced immunosuppression creates a gateway for opportunistic bacterial invaders. Necropsy studies of FPV-positive cats have revealed unexpected lesions including necrotizing bronchopneumonia, hepatic and renal degeneration, and even hydrocephalus with central nervous system degeneration [61]. Bacteriological examination of these cases identified opportunistic pathogens such as Stenotrophomonas maltophilia, a multi-drug-resistant nosocomial pathogen, suggesting that FPV-mediated immunosuppression facilitates bacterial invasion and contributes to multisystemic tissue damage [61]. This underscores the need for comprehensive antimicrobial management in FPV cases, not merely supportive care.

Diagnostic Challenges and the Need for High-Throughput, Point-of-Care Solutions

The current diagnostic landscape for FPV presents a critical bottleneck in both clinical management and epidemiological surveillance. Conventional approaches, including immunochromatographic (IC) antigen tests and standard PCR, are increasingly inadequate in the face of viral evolution and the complexity of co-infections. The emerging threat is not a lack of diagnostic tools, but rather a failure of existing tools to provide accurate, timely, and comprehensive information.

The limitations of rapid IC tests are starkly illustrated by data from Bangladesh, where the IC test exhibited a 24% false-positive rate compared to PCR, detecting FPV in 84% of suspected cases compared to only 60% by PCR [71]. This over-diagnosis leads to unnecessary isolation, euthanasia, and economic losses. Conversely, qPCR, while more accurate, cannot differentiate between virulent field virus and vaccine virus shedding, a critical distinction in shelter and cattery settings. A study of modified-live FPV vaccination in a shelter found that 21.6% of vaccinated cats shed FPV DNA detectable by qPCR, with shedding peaking at day 7 post-vaccination [49]. Without the ability to distinguish vaccine from field virus, shelters risk misdiagnosis and inappropriate culling of healthy, vaccinated animals.

These diagnostic limitations are being addressed by a wave of technological innovation. High-throughput, multiplex platforms are essential for managing polymicrobial infections. The quadruplex TaqMan MGB fluorescent quantitative PCR, capable of simultaneously detecting FPV, FHV-1, FCV, and FIPV with detection limits as low as 41.25 copies/μL and no cross-reactivity, represents a significant advance for comprehensive clinical workup [44]. Similarly, triplex RT-qPCR assays for FCoV, FPV, and FeLV have demonstrated high sensitivity (93.07%) and specificity (97.99%) in screening 1,175 clinical samples [57]. For resource-limited settings, isothermal amplification technologies such as recombinase polymerase amplification coupled with lateral flow dipsticks (RPA-LFDA) offer a compelling alternative, with a detection limit of just 10 copies per reaction, sensitivity of 100%, and specificity of 83.33% compared to qPCR, all without requiring thermal cyclers [72]. NanoPCR, which incorporates gold nanoparticles into the reaction, enhances sensitivity 100-fold over conventional PCR while remaining more convenient and cost-effective than qPCR [63].

Perhaps the most innovative diagnostic approach to emerge is the use of colorimetric genosensors based on ZnFe₂O₄ nanoparticles with peroxidase-like activity. This platform, which produces a visually detectable color change upon FPV DNA hybridization, offers a limit of detection of 300 ng/μL for amplified targets or 10³ virus particles per gram of feces, with high stability and reproducibility [70]. This technology could be adapted for field-deployable, smartphone-based diagnostics in remote or wildlife settings. The Food and Agriculture Organization (FAO) has emphasized the need for such portable diagnostic tools for transboundary animal disease surveillance.

Novel Vaccine Strategies and the Imperative for Updated Prophylaxis

Conventional FPV vaccines, while generally effective, are facing obsolescence due to the antigenic divergence of contemporary field strains. The emergence of A91S, I101T, and other mutations in circulating viruses has created a genetic distance from vaccine strains that may compromise cross-protection, as evidenced by vaccine failures in captive Siberian tigers [19]. The development of next-generation vaccines that provide broader, more durable, and safer immunity is a critical future priority.

Virus-like particle (VLP) vaccines represent a transformative approach. By recombinantly expressing the VP2 capsid protein in a baculovirus expression vector system (BEVS), VLPs self-assemble into immunogenic structures that mimic the native virus but lack infectious genetic material. A Chinese epidemic strain-derived VLP vaccine, adjuvanted with Seppic, elicited a hemagglutination inhibition titer of 1:216 and conferred 100% protection against challenge with a virulent Ala91Ser/Ile101Thr variant [43]. Similarly, a second-generation VLP vaccine produced via baculovirus expression and purified by size-exclusion chromatography induced robust neutralizing antibodies and prevented clinical signs and leukopenia upon challenge [39]. These platforms eliminate the safety risks associated with modified-live vaccines, including reversion to virulence and shedding, while providing superior antigenic match to contemporary strains.

The most radical departure from traditional injectable vaccines is the development of oral vaccines based on recombinant Lactobacillus plantarum expressing VP2. This approach leverages the probiotic bacterium as a live delivery vector that colonizes the feline intestinal tract and induces mucosal and systemic immunity. Oral administration of L. plantarum NC8/VP2 to kittens elicited high levels of neutralizing antibodies and provided significant protection against FPV-251 infection after 30 days of continuous dosing [79]. This strategy eliminates the stress of injection, reduces the risk of injection-site sarcomas (FISSs), and could be deployed in mass vaccination campaigns for free-roaming or wild felid populations, where capture and injection are logistically challenging.

The need for adjuvanted inactivated vaccines that can be safely administered in the hindlimb or tail to mitigate FISS risk has also been addressed. Field studies confirm that an inactivated adjuvanted vaccine administered in the distal hindlimb or proximal tail elicits comparable protective antibody titers to scruff administration, supporting evidence-based vaccination site recommendations [80]. Furthermore, the immunogenicity of vaccines can be enhanced through serial passage in cell culture. A study of a new inactivated trivalent vaccine (FPV, FCV, FHV-1) demonstrated that 70 passages of FPV in CRFK cells significantly increased viral titers, and the resulting vaccine produced mean HI titers of 259.9 against FPV in cats [35]. The selection of appropriate cell substrates and adjuvant systems (e.g., Carbopol vs. Rehydragel) can further optimize immune responses to meet the challenge of emerging variant strains [35].

Advanced Therapeutic Approaches and the Promise of Immunomodulation

The treatment of FPV has historically been limited to supportive care, fluid therapy, antiemetics, broad-spectrum antibiotics, as no specific antiviral drug is approved for this indication [17, 55, 66]. However, a new frontier of targeted immunomodulatory and antiviral therapies is emerging, driven by a deeper understanding of FPV pathogenesis and host-virus interactions.

Granulocyte colony-stimulating factor (G-CSF), specifically the human recombinant form filgrastim, has shown significant promise in counteracting the hallmark leukopenia of FPV. In naturally infected cats, filgrastim therapy was demonstrated to be effective and safe, with treated animals showing improved white blood cell counts and survival outcomes [58]. When combined with inactivated parapoxvirus ovis (iPPVO), an immunomodulator known to stimulate non-specific innate immunity, the therapeutic effect was enhanced. The combination of standard treatment (ST) plus iPPVO plus filgrastim produced significantly higher levels of white blood cells, neutrophils, lymphocytes, and monocytes compared to ST alone, and the neutrophil-to-lymphocyte ratio and systemic inflammatory response index (SIRI) emerged as valuable prognostic markers [62]. These findings suggest that combinatorial immunomodulatory therapy, targeting both myeloid recovery and innate immune stimulation, may become the future standard of care.

The discovery of specific host-encoded microRNAs (miRNAs) that regulate FPV replication opens the door to RNA-based therapeutics. FPV infection upregulates cellular miR-92a-1-5p, which in turn enhances interferon-α/β expression by targeting SOCS5, a negative regulator of NF-κB signaling, thereby inhibiting viral replication [37]. Conversely, FPV also upregulates miR-1343-5p, which suppresses the IFN-I response by targeting IRAK1, and overexpression of this miRNA strongly promotes FPV replication [31]. The identification of these competing regulatory axes suggests that therapeutic manipulation of miRNA levels, either by delivering miR-92a-1-5p mimics or by inhibiting miR-1343-5p with antagomirs, could tip the balance in favor of host immunity.

Transcriptomic analyses of FPV-infected feline kidney cells have identified additional potential therapeutic targets. RNA-seq profiling of F81 cells infected with a Chinese epidemic strain revealed 3,116 differentially expressed genes, including upregulation of key immune-associated genes such as IGSF6, IFI44L, IFI6, IFITM10, IL1R1, and JAK3, alongside activation of Toll-like receptor, JAK-STAT, IL-17, and TNF signaling pathways [4]. Long non-coding RNAs (lncRNAs) are also dysregulated during FPV infection, with 291 lncRNAs and 873 mRNAs showing differential expression, many of which are involved in MAPK signaling and immune regulation [42]. These lncRNAs may serve as novel biomarkers or therapeutic targets.

Even unconventional approaches, such as the use of Nigella sativa (black cumin) extract containing the bioactive compound thymoquinone, have shown anecdotal promise. In a case report, oral administration of black cumin extract to a FPV-positive cat with severe leukopenia (450 cells/μL) was associated with progressive improvement in total leukocyte count and differential leukocyte profile [60]. While controlled studies are urgently needed, this highlights the growing interest in natural immunomodulators as adjunctive therapies.

Wastewater Surveillance and Environmental Monitoring as a Sentinel for Emergence

A groundbreaking future perspective is the application of wastewater-based epidemiology (WBE) to FPV surveillance. Traditionally used for human pathogens like poliovirus and SARS-CoV-2, WBE has now been successfully applied to detect and characterize parvoviruses in community wastewater. A landmark study from Arizona, USA, coupled wastewater surveillance with long-range PCR and MinION long-read sequencing to monitor parvovirus diversity in a population of ~500,000 people [5]. This approach not only detected CPV-2a, 2b, and 2c variants but also identified a vaccine-origin FPV (100% identical in VP2 to the 1964 Johnson snow leopard strain used in the Felocell vaccine), marking the first detection of vaccine-origin FPV in wastewater [5].

The implications are profound. WBE offers a non-invasive, population-level complement to clinical case surveillance that can capture viral diversity in both symptomatic and asymptomatic shedders, including stray and feral animals not under veterinary care. It can detect early signals of variant emergence before clinical cases are recognized, and can monitor the environmental dissemination of vaccine strains. The Centers for Disease Control and Prevention (CDC) has recognized the utility of WBE for infectious disease surveillance, and its extension to veterinary pathogens is a natural and necessary evolution. Establishing sentinel wastewater monitoring sites in urban centers, wildlife parks, and intensive breeding facilities could provide early warning of emergent FPV variants and inform targeted control measures.

The principal threats and opportunities outlined above are summarized in the table below:

The convergence of these factors, genetic dynamism, host promiscuity, polymicrobial synergy, and diagnostic inadequacy, demands a paradigm shift in how we approach FPV. A multi-pronged strategy integrating continuous molecular surveillance, high-throughput diagnostics, antigenically updated and novel-route vaccines, targeted immunomodulatory therapies, and environmental monitoring is no longer aspirational but imperative. The future of FPV control depends on our ability to move beyond reactive, single-pathogen approaches toward a proactive, One Health framework that recognizes the interconnectedness of domestic animal, wildlife, and environmental health.

References

[1] Wang M, Zhao S, Wang S, Du M, Ming S, Zeng L. Feline Panleukopenia Virus ZZ202303 Strain: Molecular Characterization and Structural Implications of the VP2 Gene Phylogenetic Divergence. International Journal of Molecular Sciences. 2025. DOI: https://doi.org/10.3390/ijms26104573

[2] Han J, Oh J, Ahn SY, Kim J, Lee DN, Suh YD, et al.. Cerebellar hypoplasia in an Amur leopard cat (Prionailurus bengalensis euptilura) with feline panleukopenia virus infection. Veterinární Medicína. 2025. DOI: https://doi.org/10.17221/29/2025-VETMED

[3] Feng E, Ye Z, Yan M, Zhou Y, Wu D, Cheng S, et al.. Genetic and Biological Properties of an Epidemic Feline Panleukopenia Virus Strain (Ala91Ser) in China. Veterinary Sciences. 2025. DOI: https://doi.org/10.3390/vetsci12070668

[4] Feng E, Wu S, Cheng S, Cheng Y. Host–Virus Interactions in Feline Kidney Cells Infected with a Chinese Epidemic Strain of Feline Panleukopenia Virus Analysed Using RNA-Seq. Veterinary Sciences. 2025. DOI: https://doi.org/10.3390/vetsci12080748

[5] Vargas J, Bermudez-Rivera B, Block I, Shaffer G, Estrada L, Dadd T, et al.. Canine Parvovirus and Vaccine-Origin Feline Panleukopenia Virus in Wastewater, Arizona, USA: July 2022–June 2023. Microorganisms. 2025. DOI: https://doi.org/10.3390/microorganisms13092124

[6] Li J, Peng J, Zeng Y, Wang Y, Li L, Cao Y, et al.. Isolation of a feline-derived feline panleukopenia virus with an A300P substitution in the VP2 protein and confirmation of its pathogenicity in dogs. Animal Diseases. 2024. DOI: https://doi.org/10.1186/s44149-023-00108-5

[7] Wen Y, Tang Z, Wang K, Geng Z, Yang S, Guo J, et al.. Epidemiological and Molecular Investigation of Feline Panleukopenia Virus Infection in China. Viruses. 2024. DOI: https://doi.org/10.3390/v16121967

[8] Zhang H, Zhang W, Pan Y, Li H, He T, Dong Q, et al.. Evolutionary Dynamics and Pathogenicity Analysis of Feline Panleukopenia Virus in Xinjiang, China. Microorganisms. 2024. DOI: https://doi.org/10.3390/microorganisms12112205

[9] Wei J, Shi Y, Wang X, He S, Qi X, Lu R, et al.. The first outbreak of feline panleukopenia virus infection in captive Pallas’s cats in Xining Wildlife Park. Frontiers in Veterinary Science. 2024. DOI: https://doi.org/10.3389/fvets.2024.1418553

[10] Akter J, Nath C, Hossain MS, Ahaduzzaman M. The coding complete genomic sequence of a feline panleukopenia virus detected from a domestic cat (Felis catus) in Chattogram, Bangladesh. Microbiology Resource Announcements. 2024. DOI: https://doi.org/10.1128/mra.00209-24

[11] Diakoudi G, Lanave G, Berjaoui S, Desario C, Teodoro GD, Vasinioti V, et al.. Feline Panleukopenia Virus in a Marsican Brown Bear and Crested Porcupine, Italy, 2022–2023. Emerging Infectious Diseases. 2024. DOI: https://doi.org/10.3201/eid3012.240505

[12] Corzo KA, Duarte-Rodriguez L, Arango-Lozano J. Illegal Wildlife Trade gives insights into suspected Feline Panleukopenia Virus (FPV) Occurrence in Puma concolor. Mammalogy Notes. 2024. DOI: https://doi.org/10.47603/mano.v10n2.424

[13] Chen C, Tao J, Tang L, Sun T, Sun Z, Xu H, et al.. First Isolation and Characterization of Feline Panleukopenia Virus From Wild Raccoon Dogs in the Residential Area of Shanghai, China. Veterinary Medicine and Science. 2024. DOI: https://doi.org/10.1002/vms3.70071

[14] Zhao S, Hu H, Lan J, Yang Z, Peng Q, Yan L, et al.. Characterization of a fatal feline panleukopenia virus derived from giant panda with broad cell tropism and zoonotic potential. Frontiers in Immunology. 2023. DOI: https://doi.org/10.3389/fimmu.2023.1237630

[15] Xue H, Hu C, Ma H, Song Y, Zhu K, Fu J, et al.. Isolation of feline panleukopenia virus from Yanji of China and molecular epidemiology from 2021 to 2022. Journal of Veterinary Sciences. 2023. DOI: https://doi.org/10.4142/jvs.22197

[16] Su X, Zhou H, Jiang WG, Xu F, Xiao B, Zhang J, et al.. Isolation and evolutionary analysis of feline panleukopenia virus strains FPV-BJ-J2 and FPV-BJ-J3 (T440A, N564S, A568G) in Beijing, China. Frontiers in Veterinary Science. 2026. DOI: https://doi.org/10.3389/fvets.2026.1745551

[17] Apoorva HJ, Lathamani VS, Ramesh P, Murag S, Sudharani R, Veena M. FELINE PANLEUKOPENIA VIRUS INFECTION: A COMPREHENSIVE REVIEW (2010–2025). Indian Journal of Veterinary and Animal Sciences Research. 2026. DOI: https://doi.org/10.56093/ijvasr.v55i1.175110

[18] Ye Z, Wu D, Jiang X, Liu L, Luo G, Wang Z, et al.. Genetic Diversity and Evolutionary Dynamics of Feline Panleukopenia Virus in China: Phylogenetic Analysis and Substitution Patterns in NS1 and VP2 Proteins. Viruses. 2026. DOI: https://doi.org/10.3390/v18050562

[19] Yeo Y, Kim H, Park J, Kim J, Shin Y, Lee K, et al.. Epidemiological and Molecular Approaches for a Fatal Feline Panleukopenia Virus Infection of Captive Siberian Tigers (Panthera tigris altaica) in the Republic of Korea. Animals. 2023. DOI: https://doi.org/10.3390/ani13182991

[20] Yang Y, Geng Y, Ping O, Li Y, Guo H, Deng H, et al.. Identification of a Feline Panleukopenia Virus from Captive Giant Pandas (Ailuropoda melanoleuca) and Its Phylogenetic Analysis. Transboundary and Emerging Diseases. 2023. DOI: https://doi.org/10.1155/2023/7721487

[21] Dang T, Tran T, Van TM, Le QT, Ngoc BT. First molecular report of Feline panleukopenia virus infection in diarrheic cats at Can Tho City, Vietnam. Veterinary Integrative Sciences. 2023. DOI: https://doi.org/10.12982/vis.2024.043

[22] Magouz A, Elkon I, Khaled E, Alkhalefa N. First report of detection of Canine Parvovirus type 2 in naturally infected domestic cats in Egypt by duplex PCR for simultaneous detection of Canine Parvovirus type 2 and Feline Panleukopenia virus. Journal of the Hellenic Veterinary Medical Society. 2023. DOI: https://doi.org/10.12681/jhvms.28838

[23] Cruz TP, Morgado T, Ribeiro KR, Nakazato L, Dutra V. Detection of feline panleukopenia virus (Carnivore protoparvovirus 1) in free-ranging Panthera onca in Brazil. Pesquisa Veterinária Brasileira. 2023. DOI: https://doi.org/10.1590/1678-5150-pvb-7331

[24] Li S, Chen X, Hao Y, Zhang G, Lyu Y, Wang J, et al.. Characterization of the VP2 and NS1 genes from canine parvovirus type 2 (CPV-2) and feline panleukopenia virus (FPV) in Northern China. Frontiers in Veterinary Science. 2022. DOI: https://doi.org/10.3389/fvets.2022.934849

[25] Yi S, Liu S, Meng X, Huang P, Cao Z, Jin H, et al.. Feline Panleukopenia Virus With G299E Substitution in the VP2 Protein First Identified From a Captive Giant Panda in China. Frontiers in Cellular and Infection Microbiology. 2022. DOI: https://doi.org/10.3389/fcimb.2021.820144

[26] Diakoudi G, Desario C, Lanave G, Salucci S, Ndiana LA, Zarea A, et al.. Feline Panleukopenia Virus in Dogs from Italy and Egypt. Emerging Infectious Diseases. 2022. DOI: https://doi.org/10.3201/eid2809.220388

[27] Chen X, Wang J, Zhou Y, Yue H, Zhou N, Tang C. Circulation of heterogeneous Carnivore protoparvovirus 1 in diarrheal cats and prevalence of an A91S feline panleukopenia virus variant in China.. Transboundary and Emerging Diseases. 2022. DOI: https://doi.org/10.1111/tbed.14641

[28] Odend'hal S. feline panleukopenia virus. CABI Compendium. 2022. DOI: https://doi.org/10.1016/B978-0-12-524180-9.50057-1

[29] Yang D, Park Y, Park Y, Yoo J, An S, Park J, et al.. Expression of the VP2 protein of feline panleukopenia virus in insect cells and use thereof in a hemagglutination inhibition assay. . 2021. DOI: https://doi.org/10.14405/kjvr.2021.61.e19

[30] Hoang M, Wu C, Lin C, Nguyen H, Le V, Chiou M, et al.. Genetic characterization of feline panleukopenia virus from dogs in Vietnam reveals a unique Thr101 mutation in VP2. PeerJ. 2020. DOI: https://doi.org/10.7717/peerj.9752

[31] Zhang L, Zhao J, Zhai Z, Liang L, Liang R, Cui S. Cellular microRNA, miR-1343-5p, modulates IFN-I responses to facilitate feline panleukopenia virus replication by directly targeting IRAK1 gene.. Veterinary Microbiology. 2020. DOI: https://doi.org/10.1016/j.vetmic.2020.108691

[32] Inthong N, Sutacha K, Kaewmongkol S, Sinsiri R, Sribuarod K, Sirinarumitr K, et al.. Feline panleukopenia virus as the cause of diarrhea in a banded linsang (Prionodon linsang) in Thailand. Journal of Veterinary Medical Science. 2019. DOI: https://doi.org/10.1292/jvms.19-0238

[33] Li S, Xin J, Peng Y, Liu H, Zhou Z, Zhong Z, et al.. Investigation of truncated replication protein mutant of Canine Circovirus: synergistic interaction with Feline Panleukopenia Virus. Frontiers in Veterinary Science. 2025. DOI: https://doi.org/10.3389/fvets.2025.1536913

[34] Bergmann M, Klein-Richers U, Zablotski Y, Hartmann K. Shedding of feline panleukopenia virus DNA in cats in breeding catteries. Tierärztliche Praxis Ausgabe K: Kleintiere / Heimtiere. 2025. DOI: https://doi.org/10.1055/a-2694-0145

[35] Yang D, Park Y, Kim E, Lee H, Oh S, Hyun B. Immunogenicity of a new inactivated vaccine against feline panleukopenia virus, calicivirus, and herpesvirus-1 for cats. Korean Journal of Veterinary Research. 2023. DOI: https://doi.org/10.14405/kjvr.20220042

[36] Wu J, Chen Q, Zhang Y, Hu X, Li Y, Tang M, et al.. Pathogenicity of a Canine-Derived Feline Panleukopenia Virus in Cats and NS1 Evolution Associated with Adaptation to Dogs. Animals. 2026. DOI: https://doi.org/10.3390/ani16071061

[37] Liang R, Liang L, Zhao J, Liu W, Cui S, Zhang X, et al.. SP1/miR-92a-1-5p/SOCS5: A novel regulatory axis in feline panleukopenia virus replication.. Veterinary Microbiology. 2022. DOI: https://doi.org/10.1016/j.vetmic.2022.109549

[38] Carreño CH, Navarro CO, Jara MA. Design of primers in the molecular detection of Feline Panleukopenia Virus. World Journal of Biology Pharmacy and Health Sciences. 2021. DOI: https://doi.org/10.30574/wjbphs.2021.8.3.0129

[39] Wang T, Wu H, Wang Y, Guan Y, Cao Y, Wang L, et al.. Virus-like Particle Vaccine for Feline Panleukopenia: Immunogenicity and Protective Efficacy in Cats. Vaccines. 2025. DOI: https://doi.org/10.3390/vaccines13070684

[40] Leal E, Liang R, Liu Q, Villanova F, Shi L, Liang L, et al.. Regional adaptations and parallel mutations in Feline panleukopenia virus strains from China revealed by nearly-full length genome analysis. PLoS ONE. 2020. DOI: https://doi.org/10.1371/journal.pone.0227705

[41] Zhang L, Liang R, Zhang G, Zhai Z, Deng Y, Li J, et al.. Analysis of the microRNA expression profiles in feline kidney cell line infected with feline panleukopenia virus.. Infection, Genetics and Evolution. 2019. DOI: https://doi.org/10.1016/j.meegid.2019.103945

[42] Zhang L, Liang R, Raheem A, Liang L, Zhang X, Cui S. Transcriptomics analysis reveals key lncRNAs and genes related to the infection of feline kidney cell line by panleukopenia virus.. Research in Veterinary Science. 2023. DOI: https://doi.org/10.1016/j.rvsc.2023.03.027

[43] Feng E, Luo G, Wang C, Liu W, Yan R, Bai X, et al.. Generation and Immunogenicity of Virus-like Particles Based on the Capsid Protein of a Chinese Epidemic Strain of Feline Panleukopenia Virus. Veterinary Sciences. 2025. DOI: https://doi.org/10.3390/vetsci12050503

[44] Wang H, Xue L, Wang L, Liu Y, Chen J, Sun Y, et al.. The quadruplex TaqMan MGB fluorescent quantitative PCR method for simultaneous detection of feline panleukopenia virus, feline herpesvirus 1, feline calicivirus and feline infectious peritonitis virus. Frontiers in Cellular and Infection Microbiology. 2025. DOI: https://doi.org/10.3389/fcimb.2025.1581946

[45] Ullah M, Gofur M, Sarder M. Antigenic characterization and clinical manifestations of feline Panleukopenia virus in Cats. Veterinary Research Notes. 2025. DOI: https://doi.org/10.5455/vrn.2025.e63

[46] Zochampuii T, Rajkhowa TK, Jayappa K. Molecular detection and phylogenetic analysis of feline panleukopenia virus in domestic cat population of Mizoram state, India. Veterinary Research Forum. 2025. DOI: https://doi.org/10.30466/vrf.2025.2039338.4410

[47] Dishow M, Alsarhan QT, Al-Naqshabendy A. Molecular Diagnosis, Risk factors and Phylogenetic Analysis of Feline Panleukopenia Virus in Cats in Duhok Province, Iraq. Egyptian Journal of Veterinary Sciences. 2024. DOI: https://doi.org/10.21608/ejvs.2024.265950.1809

[48] Kabir A, Habib T, Chouhan CS, Hassan J, A.KM.AnisurRahman I, Nazmul K, et al.. Epidemiology and molecular characterization of Feline panleukopenia virus from suspected domestic cats in selected Bangladesh regions. PLoS ONE. 2023. DOI: https://doi.org/10.1371/journal.pone.0282559

[49] Jacobson L, Janke KJ, Ha K, Giacinti J, Weese J. Feline panleukopenia virus DNA shedding following modified live virus vaccination in a shelter setting.. The Veterinary Journal. 2021. DOI: https://doi.org/10.1016/j.tvjl.2021.105783

[50] Trang NTT, Thành NV, Nam NX, Giang LH, Quyết NV, Trường VĐ. Sự thay đổi chỉ số sinh lý máu trên mèo mắc bệnh giảm bạch cầu do virus Feline panleukopenia gây ra. Tạp chí Khoa học và Công nghệ Lâm nghiệp. 2024. DOI: https://doi.org/10.55250/jo.vnuf.13.5.2024.032-039

[51] Awad R, Khalil W, Attallah A. Epidemiology and diagnosis of feline panleukopenia virus in Egypt: Clinical and molecular diagnosis in cats. Veterinary World. 2018. DOI: https://doi.org/10.14202/vetworld.2018.578-584

[52] Zenad M, Radhy AM. Clinical, serological and antigenic study of feline panleukopenia virus in cats in Baghdad, Iraq. Iraqi journal of Veterinary Sciences. 2020. DOI: https://doi.org/10.33899/IJVS.2019.125960.1201

[53] Miranda C, Vieira M, Silva E, Carvalheira J, Parrish CR, Thompson G. Genetic Analysis of Feline Panleukopenia Virus Full‐length VP2 Gene in Domestic Cats Between 2006–2008 and 2012–2014, Portugal. Transboundary and Emerging Diseases. 2017. DOI: https://doi.org/10.1111/tbed.12483

[54] Maharani A, Yuliawati PS, Yenni L, Lestari F. Literatur Review: Analisis Risiko, Peneguhan Diagnosa dan Evaluasi pada Feline Panleukopenia Virus. Jurnal Sain Veteriner. 2025. DOI: https://doi.org/10.22146/jsv.92291

[55] Syarifuddin A, Safitri JA, Wibowo SE, Damhuri D, Triningsih AT. The Treatment of A Case of Feline Panleukopenia Virus in a Mixed Persian Cat. Journal of Applied Veterinary Science and Technology. 2025. DOI: https://doi.org/10.20473/javest.v6.i1.2025.81-84

[56] Đỉnh NN, Anh NTN, Hiếu NQ, Trọng N, Thái N. MỘT SỐ ĐẶC ĐIỂM BỆNH DO FELINE PANLEUKOPENIA VIRUS Ở MÈO TẠI THÀNH PHỐ BUÔN MA THUỘT, TỈNH ĐẮK LẮK. TNU Journal of Science and Technology. 2025. DOI: https://doi.org/10.34238/tnu-jst.12073

[57] He M, Feng S, Shi K, Shi Y, Long F, Yin Y, et al.. One-step triplex TaqMan quantitative reverse transcription polymerase chain reaction for the detection of feline coronavirus, feline panleukopenia virus, and feline leukemia virus. Veterinary World. 2024. DOI: https://doi.org/10.14202/vetworld.2024.946-955

[58] Dascalu M, Bocaneti FD, Soreanu O, Țuțu P, Cozma A, Morosan S, et al.. Filgrastim Efficiency in Cats Naturally Infected with Feline Panleukopenia Virus. Animals. 2024. DOI: https://doi.org/10.3390/ani14243582

[59] Wibisono C, Purnamaningsih H, Widiyono I. Laporan Kasus: Gambaran Klinikopatologi Kucing yang Terinfeksi Feline Panleukopenia Virus. Indonesia Medicus Veterinus. 2024. DOI: https://doi.org/10.19087/imv.2024.13.1.47

[60] Putri AA, Datrianto DS. Case Report: Uses of Nigella sativa in A Cat with Feline Panleukopenia Virus Infection. BIO Web of Conferences. 2026. DOI: https://doi.org/10.1051/bioconf/202622901005

[61] Purec D, Călugărița A, Cristescu M, Udrea A, Vlad F, Iorgoni V, et al.. Atypical gross pathological findings in feline panleukopenia virus positive cats: a case series. Romanian Journal of Veterinary Sciences. 2026. DOI: https://doi.org/10.59463/rjvs.2026.1.17

[62] Aydın Ö, Aktaş G. Investigation of the prognostic effects of inactivated parapoxvirus ovis and combined filgrastim therapy in naturally infected cats with feline panleukopenia virus.. Acta Veterinaria Hungarica. 2026. DOI: https://doi.org/10.1556/004.2025.01231

[63] Xue H, Liang Y, Gao X, Song Y, Zhu K, Yang M, et al.. Development and Application of nanoPCR Method for Detection of Feline Panleukopenia Virus. Veterinary Sciences. 2023. DOI: https://doi.org/10.3390/vetsci10070440

[64] Gülersoy E, Erol B, Ok M, Sevinç M. Evaluation of qSOFA and variation of hematochemical profile in cats naturally infected with feline panleukopenia virus. Open Veterinary Science. 2023. DOI: https://doi.org/10.1515/ovs-2022-0118

[65] Widyasanti NWH, Putra I. Abortus pada kucing akibat infeksi feline panleukopenia virus. ARSHI Veterinary Letters. 2023. DOI: https://doi.org/10.29244/avl.7.1.9-10

[66] Rahmawati I, Dwi L, Wardhani K, Mardijanto A, Ngabut H. Treatment of Feline Panleukopenia Virus Disease at Miracle Animal Clinic, Surabaya. Jurnal Riset Veteriner Indonesia (Journal of The Indonesian Veterinary Research). 2023. DOI: https://doi.org/10.20956/jrvi.v7i2.27623

[67] Albab U, Febrianth A, Hermawan IP, Kurnianto A. Studi kasus : Feline panleukopenia virus pada kucing abel. VITEK : Bidang Kedokteran Hewan. 2022. DOI: https://doi.org/10.30742/jv.v12i2.113

[68] Isaya R, Ciccarelli S, Enache D, Specchi S, Pesaresi M, Ferri F, et al.. Gastrointestinal Ultrasonographic Findings in Cats With Feline Panleukopenia Virus: a Case Series. . 2020. DOI: https://doi.org/10.21203/rs.3.rs-18066/v1

[69] Porporato F, Horzinek MC, Hofmann-Lehmann R, Ferri F, Gerardi G, Contiero B, et al.. Survival estimates and outcome predictors for shelter cats with feline panleukopenia virus infection.. Journal of the American Veterinary Medical Association. 2018. DOI: https://doi.org/10.2460/javma.253.2.188

[70] Abdoli Z, Staji H, Bahrami Z. A novel colorimetric DNA sensor for rapid detection of feline panleukopenia virus using ZnFe2O4 nanoparticles and TMB modulation. Scientific Reports. 2025. DOI: https://doi.org/10.1038/s41598-025-20071-0

[71] Jeba N, Mia R, Masum M, Tarafder B, Mozumder A, Farazi R, et al.. Comparative Detection and Genetic Characterization of Feline Panleukopenia Virus in Bangladesh. Veterinary Medicine and Science. 2025. DOI: https://doi.org/10.1002/vms3.70594

[72] Hong L, Huang Q, Zhou Y, Zheng Q, Wang S, Chen F, et al.. Development and Application of an RPA-Based Rapid Point-of-Care Testing (POCT) Method for the Detection of Feline Panleukopenia Virus. Transboundary and Emerging Diseases. 2024. DOI: https://doi.org/10.1155/2024/3680778

[73] Saranraj, P, Manigandan, M, Nisha, R, Yuvalakshmi. L. Molecular Detection of Feline Panleukopenia Virus From Clinical Cases in India. Al-Anbar Journal of Veterinary Sciences. 2024. DOI: https://doi.org/10.37940/ajvs.2024.17.4

[74] Weidinger A, Hartmann K, Barutzki D, Truyen U, Wahed AAE, Zablotski Y, et al.. Antibody response after feline panleukopenia virus vaccination in kittens with and without intestinal parasites. Journal of feline medicine and surgery. 2024. DOI: https://doi.org/10.1177/1098612X241264731

[75] Liu Z, Jiang Q, Yang Y, Qi R, Gu H, Chen M, et al.. Establishment of one-step duplex TaqMan real-time PCR for detection of feline coronavirus and panleukopenia virus. Applied Microbiology and Biotechnology. 2025. DOI: https://doi.org/10.1007/s00253-024-13394-x

[76] Zhang Q, Niu J, Yi S, Dong G, Yu D, Guo Y, et al.. Development and application of a multiplex PCR method for the simultaneous detection and differentiation of feline panleukopenia virus, feline bocavirus, and feline astrovirus. Archives of Virology. 2019. DOI: https://doi.org/10.1007/s00705-019-04394-8

[77] Liu M, Li M, Ma C, Shi C. Detection of canine parvovirus and feline panleukopenia virus in fecal samples by strand exchange amplification. Journal of Veterinary Diagnostic Investigation. 2020. DOI: https://doi.org/10.1177/1040638720962067

[78] Bergmann M, Schwertler S, Speck S, Truyen U, Hartmann K. Antibody response to feline panleukopenia virus vaccination in cats with asymptomatic retrovirus infections: a pilot study. Journal of feline medicine and surgery. 2019. DOI: https://doi.org/10.1177/1098612X18816463

[79] Li J, Zeng Y, Li L, Peng J, Yan Q, Ye Z, et al.. Development of a recombinant Lactobacillus plantarum oral vaccine expressing VP2 protein for preventing feline panleukopenia virus.. Veterinary Microbiology. 2024. DOI: https://doi.org/10.1016/j.vetmic.2024.110257

[80] Westman M, Hall E, Norris JM, Weir W, Dunbar D, McDonald M, et al.. EXPRESS: Antibody responses to inactivated (adjuvanted) and attenuated (non-adjuvanted) feline panleukopenia virus vaccination, including in different injection sites. Journal of feline medicine and surgery. 2026. DOI: https://doi.org/10.1177/1098612x261450889