Feline Immunodeficiency Virus

Overview and Taxonomy of Feline Immunodeficiency Virus

Feline immunodeficiency virus (FIV) stands as a globally significant lentivirus within the Retroviridae family, a pathogen of profound importance to both feline medicine and comparative virology. Since its initial isolation in 1986 from a cattery in Petaluma, California, FIV has been recognized as the etiologic agent of a progressive, ultimately fatal acquired immunodeficiency syndrome (AIDS) in domestic cats (Felis catus) [28, 32]. The virus is a member of the genus Lentivirus, a group characterized by slow, persistent infections, prolonged clinical latency, and eventual immune system collapse. This taxonomic placement places FIV in the same genus as the human immunodeficiency virus (HIV), simian immunodeficiency virus (SIV), and equine infectious anemia virus (EIAV), among others. The shared biological properties between FIV and HIV, including genetic organization, morphogenesis, cellular tropism for CD4+ T lymphocytes and macrophages, and the capacity to induce a state of profound immunosuppression, have established the FIV-cat model as an indispensable, naturally occurring small-animal system for studying HIV pathogenesis, neuropathogenesis, and therapeutic interventions [11, 17, 18, 23, 25]. The World Organisation for Animal Health (WOAH) recognizes FIV as a significant pathogen of domestic cats, though it is not a notifiable disease; its global distribution and impact on feline health, however, warrant continued surveillance and research.

Taxonomic Classification and Subtype Diversity

FIV is classified within the family Retroviridae, subfamily Orthoretrovirinae, genus Lentivirus. The viral genome, composed of two identical single-stranded positive-sense RNA molecules, is approximately 9.4 kb in length and encodes the canonical retroviral structural and enzymatic genes, gag, pol, and env, flanked by long terminal repeats (LTRs) [25]. A defining feature of lentiviruses is the presence of additional regulatory and accessory genes; FIV encodes vif, rev, and orfA (a functional analog of HIV’s tat), though it lacks a vpr or vpu ortholog [25, 26]. The vif gene is particularly critical, as it encodes the viral infectivity factor (Vif), which counteracts the host’s intrinsic restriction factor, feline APOBEC3 (feA3) proteins [10, 16, 26].

Phylogenetic analysis of FIV isolates, primarily based on the env and pol gene sequences, has delineated five major subtypes or clades, designated A through E, along with numerous circulating intersubtype recombinant forms (CRFs) [29]. This genetic diversity is a hallmark of lentiviral evolution, driven by the error-prone nature of reverse transcriptase and the high recombination rate during replication. Subtype distribution exhibits a marked geographic structure, reflecting historical viral spread and host population dynamics. Subtype A is the most prevalent in North America, Europe (including Ireland, where it accounts for the vast majority of strains), Australia, and Japan [3, 12, 29]. Subtype B is dominant in Central and Southern Europe, including Hungary, and is also found in parts of Asia and South America [4, 10]. Subtype C is primarily found in Canada and parts of the United States, while subtype D is prevalent in Japan and has been identified in Southeast Asia and Serbia [8, 29]. Subtype E is less common, with reports from Argentina and other South American nations [13, 22]. The existence of these distinct clades has profound implications for vaccine development, diagnostic test accuracy, and the interpretation of epidemiological studies, as cross-clade protection may be incomplete [27, 29].

Molecular Biology and Evolutionary Dynamics

The molecular architecture of FIV is remarkably similar to that of HIV, yet with key differences that illuminate lentiviral evolution. The matrix (MA) domain of the Gag protein, for instance, is N-terminally myristylated, a modification essential for targeting the Gag polyprotein to the plasma membrane during virus assembly. Nuclear magnetic resonance (NMR) studies have revealed that the myristyl group is sequestered within a hydrophobic pocket of the MA protein, a structural configuration analogous to that observed in HIV-1 [15]. Furthermore, FIV hijacks the cellular phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] signaling system to direct Gag trafficking, underscoring a conserved mechanism of lentiviral assembly [15]. The viral egress process is dependent on a “PSAP”-type late domain (L-domain) located in the C-terminal region of Gag, which recruits the cellular endosomal sorting complexes required for transport (ESCRT) machinery via interactions with Nedd4-2s ubiquitin ligase and ubiquitin [33].

A particularly compelling area of FIV molecular biology is the evolutionary arms race between the virus and its host’s intrinsic restriction factors. The feline genome encodes multiple APOBEC3 cytidine deaminases, most notably feA3Z3 (the ortholog of human APOBEC3H), which can be packaged into progeny virions and hypermutate the viral DNA during reverse transcription, thereby blocking infectivity [10, 16, 24]. To counter this, FIV Vif targets feA3 proteins for proteasomal degradation. Remarkably, FIV has evolved a dual strategy: in addition to Vif, the viral protease itself can cleave feA3 proteins within released virions, providing a secondary layer of defense [10]. This is a unique adaptation not seen in HIV. Furthermore, a naturally occurring variant of feA3Z3 (haplotype V) has been identified that is resistant to Vif-mediated degradation, suggesting that positive selection has driven the emergence of this variant in the domestic cat population approximately 60,000 years ago, likely in response to an ancestral FIV-like lentivirus [16]. This finding provides direct evidence of a co-evolutionary “arms race” between the cat and its lentivirus.

Epidemiological Overview and Host Range

FIV infection is a truly global phenomenon, with seroprevalence rates varying dramatically based on geographic region, cat population (healthy versus sick), and risk factors. In the United States, approximately 1.5 to 3 percent of healthy cats are infected, but this rate can exceed 15 percent in sick or high-risk populations [5, 19]. A large-scale study of 2,765 cats from the US and Canada in the late 1980s found a 14% infection rate among high-risk cats (those with clinical signs or outdoor access) versus only 1.2% in low-risk cats [30]. More recent data from the US (2000-2011) analyzing over 17,000 FIV-positive serology results revealed distinct geographic clustering, with a higher proportional morbidity ratio of FIV to FeLV in the southern and eastern states compared to the west [19]. In Europe, prevalence varies: Ireland reports 10.4% by ELISA [3], Hungary 9.9% [4], and Northern Serbia a striking 23.6% [8]. In Asia, studies from Thailand (8.3%) [2], Malaysia (10-31%) [9, 21], and Japan show substantial endemicity. In South America, particularly Brazil, prevalence ranges from 7.6% to 13.5% in various hospital and shelter populations [1, 6, 7]. Australia exhibits a notable geographic disparity, with Western Australia showing a significantly higher FIV seroprevalence (15-20%) compared to the eastern states, a phenomenon attributed to lower neutering rates in male cats [12].

The host range of FIV extends well beyond the domestic cat. Species-specific strains of FIV have been identified in at least 20 species of wild felids, including African lions (Panthera leo), cheetahs (Acinonyx jubatus), leopards (Panthera pardus), pumas (Puma concolor), and Pallas’ cats (Otocolobus manul) [22, 31, 34]. These non-domestic FIV strains, designated with species-specific suffixes (e.g., FIVple for lions, FIVoma for Pallas’ cats), form distinct monophyletic lineages that have co-evolved with their respective hosts over millennia. Critically, while FIV in domestic cats is clearly pathogenic, the impact of species-specific FIV in wild felids has been a subject of debate. However, longitudinal studies in free-ranging African lions have demonstrated that FIVple infection is associated with significant clinical, immunological, and pathological outcomes, including lymphadenopathy, gingivitis, tongue papillomas, lymphoid depletion, and elevated liver enzymes, mirroring the AIDS-defining conditions seen in domestic cats and humans [31]. Similarly, histopathological examination of Pallas’ cats suggests that FIVOma can cause immune depletion in its native host [34]. These findings challenge the long-held assumption that FIV is benign in its natural wild hosts and underscore the complex, context-dependent nature of lentiviral pathogenesis.

The primary mode of FIV transmission is through bite wounds, as the virus is present in high titers in the saliva of infected cats [28, 30]. This mechanism explains the strong epidemiological association between FIV infection and risk factors such as male sex (particularly intact males), outdoor access, adulthood, and aggressive behavior [2, 6, 7, 14, 21]. Vertical transmission (in utero or via milk) is inefficient under natural conditions, and sexual transmission is not considered a major route [28]. The virus is also transmitted efficiently via parenteral inoculation of blood or plasma, which is relevant to iatrogenic transmission [28]. The long latent period, often lasting years, allows infected cats to remain clinically healthy while serving as a source of infection for other cats, complicating control efforts [20, 32].

Viral Structure and Genomic Organization of Feline Immunodeficiency Virus

Feline immunodeficiency virus (FIV) is a complex retrovirus belonging to the genus Lentivirus within the family Retroviridae. Its structural and genomic architecture is fundamentally similar to that of its primate counterparts, including human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV), making it an invaluable comparative model for understanding lentiviral biology [25, 26]. However, FIV possesses unique organizational features and accessory genes that distinguish it from other lentiviruses and reflect its long co-evolution with its feline host [10, 16, 25]. A comprehensive understanding of FIV’s virion structure and genomic content is critical for elucidating mechanisms of viral replication, pathogenesis, immune evasion, and for the rational design of antiviral therapies and vaccines.

Virion Morphology and Structural Components

The mature FIV virion is an enveloped, spherical particle with a diameter of approximately 100-120 nanometers [25]. Its architecture conforms to the canonical lentiviral structure, comprising an outer lipid bilayer envelope derived from the host cell plasma membrane, an inner proteinaceous matrix shell, and a distinctive cone-shaped or cylindrical nucleocapsid core that houses the viral genome [17, 25].

The Envelope and Surface Glycoproteins: The virion envelope is embedded with trimeric spikes of the viral envelope glycoprotein (Env). These spikes are composed of two subunits: the surface unit (SU), known as gp100, and the transmembrane unit (TM), known as gp40 [25, 27]. The SU subunit is responsible for binding to the primary cellular receptor, CD134 (also known as OX40), and subsequently to a co-receptor, most commonly CXCR4 [25, 27]. The TM subunit, anchored in the viral membrane, facilitates the fusion of the viral and cellular membranes, a critical step for viral entry [25]. The Env glycoprotein is a primary target for the host humoral immune response, but its extensive glycosylation and high degree of conformational and sequence variability, particularly within the V3 loop, allow FIV to effectively evade neutralizing antibodies [27].

The Matrix Protein (MA): Beneath the viral envelope lies the matrix protein (MA), derived from the N-terminal region of the Gag polyprotein. The FIV MA protein is myristylated at its N-terminus, a post-translational modification that is essential for its stable association with the plasma membrane and for directing the Gag polyprotein to the site of virus assembly [15]. Nuclear magnetic resonance (NMR) studies of the myristylated FIV MA protein reveal a structure composed of a bundle of alpha-helices, which forms a hydrophobic pocket that sequesters the myristyl group in a manner structurally analogous to that observed in HIV-1 [15]. This sequestered conformation is likely influenced by binding to the cellular lipid, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2], a mechanism that FIV, like HIV, hijacks to direct intracellular Gag trafficking to the plasma membrane for efficient viral particle production [15].

The Capsid (CA) and Nucleocapsid (NC) Core: The conical core of the FIV virion is formed by the capsid protein (CA), which encapsulates the viral genomic RNA and several viral enzymes [25, 28]. The core’s integrity is crucial for the early stages of infection, including uncoating and reverse transcription. Within the core, the nucleocapsid (NC) protein, which contains conserved zinc finger motifs, binds and protects the dimerized viral RNA genome [25, 35]. This zinc finger domain is a validated target for antiviral compounds, as disruption of its structure can abrogate viral replication by preventing proper RNA encapsidation and early replication events [35].

Genomic Organization and Coding Strategy

The FIV genome is a single-stranded, positive-sense RNA molecule, approximately 9,400 nucleotides in length [25]. It is flanked by long terminal repeats (LTRs) that contain essential cis-acting regulatory elements for viral transcription, integration, and polyadenylation [25]. The genomic organization of FIV, while sharing the hallmark gag-pol-env gene order of all retroviruses, is distinguished by a unique complement of accessory genes that are not found in all primate lentiviruses.

The gag Gene: The gag gene encodes the precursor polyprotein Pr55Gag, which is cleaved by the viral protease (PR) during virion maturation to yield the structural proteins: matrix (MA, p15), capsid (CA, p24), and nucleocapsid (NC, p9) [25, 28]. While the overall function of Gag is conserved, the FIV Gag polyprotein possesses a critical late domain (L-domain) with the sequence "PSAP" (Pro-Ser-Ala-Pro), located in the C-terminal region of Gag [33]. This L-domain is essential for the final stages of virus budding, as it recruits components of the cellular endosomal sorting complexes required for transport (ESCRT) machinery. Unlike HIV-1, which utilizes a PTAP motif, FIV is strictly dependent on this PSAP motif for efficient particle release, even in the presence of a functional viral protease [33]. The process is also linked to Gag ubiquitination, and the Nedd4-2s ubiquitin ligase has been shown to enhance FIV Gag ubiquitination and can even rescue the release of L-domain mutants, highlighting a unique connection to the cellular budding machinery [33].

The pol Gene: The pol gene encodes the enzymatic machinery of the virus. It is translated as a Gag-Pol fusion polyprotein (Pr160Gag-Pol) via a ribosomal frameshifting mechanism and subsequently cleaved by the viral protease into several key enzymes:

• Protease (PR): Responsible for cleaving the Gag and Gag-Pol polyproteins during virion maturation, a process essential for producing infectious particles [25].
• Reverse Transcriptase (RT): The FIV RT is an RNA-dependent DNA polymerase that converts the viral RNA genome into double-stranded DNA. It is functionally and structurally similar to the HIV RT, exhibiting high in vitro susceptibility to many nucleoside and non-nucleoside RT inhibitors used in human antiretroviral therapy [17]. This high degree of similarity makes FIV an exceptional small-animal model for evaluating new RT-targeting compounds [17, 23].
• Integrase (IN): Catalyzes the integration of the newly synthesized viral DNA into the host cell chromosome, establishing a permanent proviral state [23, 25].

The env Gene: The env gene encodes the Env polyprotein (gp150), which is heavily glycosylated and cleaved by a cellular furin-like protease into the SU (gp100) and TM (gp40) subunits [25]. The genomic variability of env is the highest in the FIV genome, driving the classification of FIV into five distinct phylogenetic subtypes (A through E), which often predominate in different geographic regions [3, 29]. For example, subtype A is prevalent in North America and Europe, subtype B in Europe and Asia, subtype C in Taiwan and New Zealand, subtype D in Japan and Argentina, and subtype E in Argentina [3, 4, 29]. Crucially, circulating intersubtype recombinant forms have also been identified, further complicating the genetic landscape of FIV [29]. The envelope protein's variability is a primary mechanism for immune evasion and poses a major challenge for vaccine development [27, 29].

Accessory Genes: A Unique Repertoire: Unlike HIV-1, which possesses a suite of accessory genes including vif, vpr, vpu, and nef, FIV has a smaller and distinct set consisting of vif, orf-A, and a rev gene [25, 26]. The presence and function of these genes are central to FIV’s ability to replicate in its feline host and counteract intrinsic cellular defenses.

• Viral Infectivity Factor (Vif): The FIV Vif protein is essential for viral replication in vivo and functions to counteract the antiviral activity of the feline APOBEC3 (feA3) family of cytidine deaminases [10, 26]. Host APOBEC3 proteins are potent restriction factors that, if incorporated into budding virions, can hypermutate the viral DNA during reverse transcription, rendering the virus non-infectious. FIV Vif binds to feA3 proteins and targets them for ubiquitin/proteasome-dependent degradation [10, 24]. Remarkably, FIV has evolved a second, Vif-independent mechanism to antagonize feA3: the viral protease can directly cleave feA3 proteins within the virion, providing a redundant layer of defense against this host restriction [10]. This dual strategy is unique among lentiviruses. The evolutionary arms race between FIV Vif and feA3 is evident, as a naturally occurring variant of feA3 (haplotype V) has been selected for in the domestic cat population that is resistant to FIV Vif-mediated degradation, suggesting an ancient and ongoing conflict [16].
• Orf-A: The orf-A gene (also known as vpx-like or vpr-like) is a small open reading frame located between pol and env and is unique to FIV [25]. While its precise function is still being elucidated, it has been suggested to play a role in viral replication in certain cell types and may be involved in cell cycle arrest, similar to the function of HIV-1 Vpr [18, 25].
• Rev: The Rev protein is a regulatory protein that is critical for the nuclear export of unspliced and singly-spliced viral mRNAs, which encode the structural proteins (Gag, Gag-Pol) and Env [25]. By binding to the Rev-response element (RRE) within the viral RNA, Rev facilitates the transition from the early, multiply-spliced phase of viral gene expression (encoding regulatory proteins like Rev itself) to the late phase, allowing for the production of structural proteins and the assembly of new virions [25].

In summary, the structural and genomic organization of FIV reveals a highly adapted lentivirus that, while sharing a common replicative strategy with HIV, has evolved distinct mechanisms to manipulate its host environment. From its PI(4,5)P2-dependent membrane targeting by the myristylated MA protein to its PSAP-dependent budding mechanism and its unique dual-approach to neutralizing the feA3 restriction factor, FIV presents both striking parallels to and fascinating divergences from its primate cousins. This molecular architecture underpins its global distribution, its ability to establish persistent infection, and its utility as a robust and relevant model for understanding and combating lentiviral disease, including HIV/AIDS [11, 17, 29, 36].

Molecular Pathogenesis of Feline Immunodeficiency Virus

The Molecular Architecture of FIV Entry and Cellular Tropism

The molecular pathogenesis of feline immunodeficiency virus (FIV) is fundamentally rooted in its ability to establish a persistent, lifelong infection through a sophisticated interplay between viral determinants and host cellular machinery. As a member of the Lentivirus genus within the Retroviridae family, FIV shares a complex genomic organization with its human counterpart, HIV-1, encoding the canonical gag, pol, and env genes flanked by long terminal repeats (LTRs), alongside a suite of accessory genes including vif and rev [18, 25, 26]. The initial step in FIV pathogenesis, viral entry, is dictated by the envelope glycoprotein (Env), a trimeric complex of surface (SU, gp95) and transmembrane (TM, gp40) subunits. Unlike HIV-1, which utilizes CD4 as its primary receptor, FIV employs CD134 (OX40) as its primary binding receptor, a molecule expressed predominantly on activated CD4+ T lymphocytes [18, 25, 27]. This interaction induces conformational changes in Env that subsequently facilitate engagement with the chemokine coreceptor CXCR4, a process that ultimately triggers membrane fusion [25, 27]. The dependency on CD134 and CXCR4 explains the pronounced tropism of FIV for activated CD4+ T cells, macrophages, and microglial cells, establishing the cellular reservoirs that drive the progressive immunodeficiency characteristic of the disease. The structural details of the matrix (MA) protein further illuminate the molecular choreography of viral assembly. The N-terminal myristylation of the FIV MA domain is essential for targeting Gag polyproteins to the plasma membrane, a process critically dependent on the cellular phospholipid phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2]. Nuclear magnetic resonance (NMR) studies reveal that the myristyl group is sequestered within a hydrophobic pocket of the MA protein, a conformation that resembles the sequestered state observed in HIV-1 MA [15], underscoring the conserved evolutionary strategies lentiviruses employ to hijack cellular signaling systems for virion assembly.

The Intrinsic Battle: APOBEC3 Restriction and FIV Countermeasures

A pivotal arena of molecular pathogenesis is the evolutionary arms race between feline APOBEC3 (A3) cytidine deaminases and the FIV-encoded viral infectivity factor (Vif). APOBEC3 proteins are intrinsic restriction factors that, if packaged into progeny virions, deaminate cytosine to uracil on nascent single-stranded viral DNA during reverse transcription, leading to catastrophic G-to-A hypermutation and viral inactivation [10, 16, 24, 26]. The domestic cat genome encodes multiple APOBEC3 genes, with APOBEC3Z3 (A3Z3, an ortholog of human APOBEC3H) being a particularly potent suppressor of vif-deficient FIV [16]. To counteract this, FIV Vif recruits a ubiquitin ligase complex to target feline APOBEC3 proteins for proteasomal degradation. Critically, however, FIV has evolved a unique and sophisticated dual-strategy: in addition to Vif-mediated degradation, the FIV viral protease (PR) itself cleaves feline APOBEC3 within released virions, providing a second, independent mechanism of antagonism [10]. This discovery represents the first evidence that a lentivirus encodes two distinct anti-APOBEC3 factors.

This molecular struggle has driven dramatic evolutionary dynamics. Among naturally occurring domestic cats, seven haplotypes of A3Z3 (hap I through hap VII) have been identified. Remarkably, A3Z3 haplotype V is resistant to FIV Vif-mediated degradation and retains its antiviral activity even against wild-type, Vif-proficient FIV [16]. The amino acid residue at position 65 of A3Z3 is under positive selection, and phylogenetic analyses indicate that this resistant haplotype emerged approximately 60,000 years ago, likely driven by an ancient FIV epidemic [10, 16]. This provides compelling molecular evidence of an ongoing evolutionary arms race between the domestic cat and its cognate lentivirus. Furthermore, the Vif proteins of FIV subtype B exhibit significantly reduced anti-APOBEC3 activity compared to other subtypes, correlating with reduced genetic diversity and attenuated pathogenicity in this subtype [10]. The interplay extends beyond Vif; the feline foamy virus accessory protein Bet, as well as FIV Gag, can bind to APOBEC3 proteins, illustrating a complex network of host-viral interactions at the restriction factor interface [24].

Molecular Determinants of Viral Assembly, Egress, and Latency

The molecular pathogenesis of FIV is critically dependent on the late stages of the viral life cycle, assembly and budding. The FIV Gag polyprotein contains a PSAP-type late domain (L-domain) within its C-terminal region, a proline-rich motif that is absolutely required for efficient viral egress. This L-domain recruits components of the cellular endosomal sorting complex required for transport (ESCRT) machinery, specifically interacting with Tsg101 to hijack the multivesicular body (MVB) pathway for membrane fission and particle release [25, 33]. Unlike HIV-1, which can utilize alternative L-domains (PTAP, PPxY), FIV is strictly dependent on a functional PSAP motif. Remarkably, the ubiquitin ligase Nedd4-2s can rescue viral budding in the absence of the PSAP L-domain by enhancing Gag ubiquitination, linking the ubiquitin-proteasome system to viral egress [33]. This ubiquitination of Gag, which is dependent on an active L-domain, is a critical molecular step that couples FIV assembly to the cellular budding machinery.

Following entry and uncoating, the FIV RNA genome is reverse transcribed by the viral reverse transcriptase (RT). The RT of FIV is structurally and functionally homologous to HIV-1 RT, and this close similarity underlies the susceptibility of FIV to many nucleoside and non-nucleoside RT inhibitors used in human antiretroviral therapy [17, 23]. The resultant double-stranded viral DNA is integrated into the host genome as a provirus by the viral integrase. A defining feature of lentiviral pathogenesis, including FIV, is the establishment of latency. Following integration, the provirus can exist in a transcriptionally silent state, persisting as a long-lived reservoir within resting CD4+ T cells and cells of the monocyte/macrophage lineage [36]. This latency, maintained by epigenetic silencing and a lack of sufficient host transcription factors, allows the virus to evade immune surveillance and antiretroviral drugs, rendering FIV infection incurable. Reactivation from latency drives the recrudescent viremia and progressive immune dysfunction that characterize the chronic phase of infection [20, 36].

Neurotropism and CNS Pathogenesis at the Molecular Level

A particularly devastating aspect of FIV molecular pathogenesis is its capacity to invade the central nervous system (CNS) and establish a protected viral reservoir, leading to neuroinflammation and neurodegeneration. FIV enters the brain early after infection, likely within the first weeks, by exploiting the trafficking of infected immune cells across the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) at the choroid plexus [11, 41, 42]. In vitro studies using feline choroid plexus epithelial explants demonstrate that FIV significantly enhances the transmigration of both macrophages and peripheral blood mononuclear cells (PBMCs) across the epithelial barrier, indicating that the choroid plexus is an active portal of viral entry into the CSF [41]. Once within the CNS, FIV infects perivascular macrophages, microglial cells, and, to a lesser extent, astrocytes, establishing a compartmentalized viral reservoir that is partially shielded from systemic immune responses [11, 42]. The ensuing neuropathogenesis is driven not by direct neuronal infection (neurons are rarely infected) but by the release of soluble viral proteins and the induction of a sustained inflammatory response. Activated macrophages and microglia secrete neurotoxic factors, including pro-inflammatory cytokines (TNF-α, IL-1β), chemokines, and reactive oxygen species, leading to synaptic damage, dendritic simplification, and neuronal apoptosis [11]. This neuroinflammatory cascade recapitulates the pathology seen in HIV-associated neurocognitive disorders (HAND) in humans, making FIV infection of cats a uniquely valuable and validated model for investigating lentiviral CNS disease [11, 42]. The clinical relevance is underscored by molecular detection of FIV antigen within infiltrating T lymphocytes and macrophages in the myocardium of cats with hypertrophic cardiomyopathy and myocarditis, demonstrating that FIV-driven inflammation can extend beyond the immune and nervous systems to directly target cardiac tissue [38].

Systemic Immunopathogenesis and Altered Host Transcriptional Landscapes

On a systemic level, the molecular pathogenesis of FIV is characterized by a progressive, multifaceted assault on the feline immune system. The primary target is the CD4+ T lymphocyte population. Through direct viral cytopathicity, the induction of apoptosis in uninfected bystander cells, and the chronic activation and subsequent exhaustion of the immune system, FIV drives a relentless decline in CD4+ T cell numbers and a reversal of the CD4/CD8 ratio, culminating in an acquired immunodeficiency syndrome (AIDS) [18, 22, 32]. This immunosuppression, while often slower to develop than in HIV-infected patients, renders cats susceptible to a spectrum of opportunistic infections, including toxoplasmosis, haemoplasmosis, and chronic oral and respiratory tract infections [2, 8, 30, 37]. Transcriptomic analyses of FIV-infected feline T-cells using RNA-sequencing reveal a profound reprogramming of the host transcriptome. Within 24 hours of infection, 69 genes are significantly differentially expressed, many of which are also known to interact with HIV-1, including those involved in apoptosis, cell cycle regulation, and interferon signaling [40]. These early transcriptional changes set the stage for the persistent immune dysregulation that characterizes the chronic phase. The disruption of immune homeostasis is further reflected in alterations to the feline microbiota; FIV-infected cats exhibit significant shifts in the composition of their oral and conjunctival bacterial communities, including reduced abundances of protective phyla and increases in potential pathobionts, a dysbiosis that likely contributes to the high prevalence of oral disease in infected cats [39]. The molecular pathogenesis of FIV is therefore not merely a story of direct viral killing, but a complex systems-level disruption of host gene expression, immune cell trafficking, and commensal microbial ecology, all orchestrated by a lentivirus that has spent millennia co-evolving with its feline host.

Epidemiology and Risk Factors for FIV Infection

Global Prevalence and Geographic Distribution

Feline immunodeficiency virus (FIV) is a globally distributed lentivirus with a highly variable prevalence that reflects complex interactions between host population dynamics, ecological niches, and anthropogenic factors. The seroprevalence of FIV in domestic cat populations exhibits remarkable geographic heterogeneity, ranging from approximately 1.5–3% in healthy, low-risk populations in the United States to over 30% in certain high-risk cohorts [5, 30]. A landmark epidemiological investigation of 2,765 cats across the continental United States and Canada established that infection rates among cats considered by veterinarians to be at high risk reached 14%, whereas only 1.2% of low-risk or unknown-risk cats were seropositive [30]. This dichotomy between healthy and sick populations is a consistent theme across global studies: in southern Brazil, a retrospective analysis of 1,470 necropsied cats revealed an FIV prevalence of 13.5%, while a cross-sectional hospital-based study in the same region found an apparent prevalence of 7.65% among sick cats and only 2.20% among healthy cats [1, 6]. These data underscore a fundamental epidemiological principle: point-of-care testing in clinically ill populations will yield substantially higher positivity rates than screening of ostensibly healthy individuals, a fact that must inform diagnostic algorithms and client communication.

In Asia, prevalence estimates vary widely. A large retrospective Malaysian study of 2,230 cats tested between 2010 and 2016 reported an FIV seroprevalence of 10.0% [9], while a contemporaneous cross-sectional investigation in peninsular Malaysia documented an astonishing 31.3% seroprevalence [21]. The latter figure is among the highest reported globally and may reflect sampling bias toward multi-cat households, free-roaming animals, and clinically ill cats presenting to veterinary hospitals. In Bangkok, Thailand, the prevalence was lower at 8.3% among 480 cats, yet multi-cat ownership emerged as a significant risk factor in multivariate analysis [2]. European data from Ireland and Hungary indicate FIV seroprevalences of 7.8–10.4% and 9.9%, respectively, with the Hungarian study employing Bayesian analysis to estimate true prevalence after accounting for imperfect test sensitivity and specificity [3, 4]. A large Australian cross-sectional study of over 4,200 cats demonstrated a striking geographic gradient: seroprevalence was 6% among shelter cats in Western Australia, 15% among client-owned cats with outdoor access, and 14% among hospital patients, with Western Australia exhibiting significantly higher FIV seroprevalence compared to other Australian states [12]. The consistency of these findings across continents emphasizes that FIV is not a rare infection but rather an endemic pathogen whose prevalence is modulated by local ecological and behavioral determinants.

Host-Level Risk Factors: Age, Sex, and Reproductive Status

The strongest and most consistently reported host-level risk factor for FIV infection is male sex. Across virtually every epidemiological study that has examined sex as a variable, male cats are two to three times more likely to be FIV-seropositive than females. In the seminal North American investigation, male cats in the high-risk group were three times more likely to be infected than females [30]. A case-case study comparing risk factors for FIV versus FeLV seropositivity in 696 FIV-positive and 593 FeLV-positive cats from the United States and Canada found that the odds of FIV compared to FeLV positivity were significantly higher for intact males (OR = 3.14) and neutered males (OR = 2.68) relative to females [14]. Multivariate analysis of data from southern Brazil demonstrated that male cats were significantly more likely to be FIV-positive (p = 0.002) [7]. The biological plausibility of this association is rooted in the primary mode of FIV transmission: territorial aggression leading to bite wounds. Male cats, particularly intact males, engage in more frequent and severe fights than females, facilitating parenteral inoculation of virus-laden saliva into subcutaneous tissues [5, 14, 21]. Neutering reduces but does not eliminate this risk, as neutered males retain some territorial and defensive behaviors, albeit at reduced intensity [14].

Age is another critical determinant, albeit one with a more nuanced relationship. Most studies report that FIV seroprevalence increases with age, peaking in middle-aged to older cats (typically 3–10 years). In the Australian cohort, risk factors for FIV infection included age greater than 3 years [12]. Cats from Bangkok showed a similar pattern, with older cats at increased risk [2]. A Brazilian investigation calculated the mean age of FIV-positive cats at 64.25 months (approximately 5.3 years), significantly older than the mean age of 38.32 months for FeLV-positive cats [6]. The case-case study further demonstrated that adult cats had twice the odds of being FIV-positive compared to FeLV-positive (OR = 2.09) [14]. This age association is thought to reflect the cumulative probability of exposure to aggressive encounters over a cat's lifetime; older cats have simply had more opportunities to engage in fights and acquire infection. However, a paradoxical finding from the retrospective necropsy study in Brazil was that infected cats were significantly younger than uninfected cats [1]. This apparent contradiction likely reflects the specific population studied, necropsied cats represent a subset with severe, often terminal disease, and FIV-infected cats that die young may do so from secondary infections or comorbidities, skewing the age distribution toward younger individuals. The authors themselves noted that FIV infection was not associated with any specific disease condition in their sample, suggesting that the age difference was driven by other factors, possibly including co-infection with FeLV [1].

Breed has been inconsistently associated with FIV risk. Some studies have reported that domestic crossbred (non-pedigree) cats are at higher risk than purebred cats, likely because purebred cats are more frequently housed indoors and managed with greater veterinary oversight [7, 14]. In the Serbian study, non-pedigree breed was significantly associated with FIV seropositivity (OR = 5.5) [8]. However, this association is confounded by lifestyle and ownership patterns rather than representing a true genetic susceptibility.

Behavioral and Environmental Risk Factors: The Primacy of Outdoor Access

The behavioral epidemiology of FIV is dominated by a single, overwhelming risk factor: outdoor access and the associated propensity for inter-cat aggression. Cats with outdoor access are consistently at markedly elevated risk across diverse geographic settings. In the case-case study, outdoor access increased the odds of FIV seropositivity relative to FeLV by 2.58-fold [14]. The Serbian investigation reported that outdoor access conferred a 5.2-fold increased odds of FIV infection [8]. A study from Argentina concluded that male adult cats with outdoor access were at highest risk [13]. The mechanism is unambiguous: FIV is present in high titers in the saliva of infected cats, and because the virus is relatively fragile and poorly transmissible across mucosal surfaces, deep inoculation via bite wounds is the only epidemiologically significant route of transmission under natural conditions [5, 30]. Experimental transmission studies have confirmed that contact transmission between infected and susceptible specific-pathogen-free cats housed together for periods as long as 14 months did not occur, whereas parenteral inoculation with blood, plasma, or infective cell culture fluids readily transmitted infection [28]. This finding has profound implications for management: strict indoor confinement is the single most effective intervention for preventing FIV acquisition.

Multi-cat households represent another important risk factor, particularly when they include cats with outdoor access. In Bangkok, multivariate analysis identified multi-cat ownership as a significant predictor of retroviral infection [2]. Similarly, the Malaysian study found that living in a multi-cat household increased the risk of seropositivity [21]. The biological explanation is self-evident: increasing the number of cats in a household increases the probability that at least one cat will be infected and will engage in aggressive interactions with others. However, the risk is not uniform across all multi-cat environments. Households where cats are confined indoors exclusively and are stable in membership have much lower transmission rates than households where new cats are introduced frequently or where cats are allowed to roam and fight with neighborhood animals. Shelters and rescue facilities, which house large numbers of cats of unknown background, are particularly high-risk environments. The high prevalence of FIV among cats adopted from shelters or rescued from streets in Brazil, where most adopted cats had access to outdoors and lived with multiple cats, underscores this point [6].

Roaming behavior itself is a proxy for multiple risk factors: male sex, intact reproductive status, and territorial aggression. Cats that roam are more likely to encounter unfamiliar cats and to engage in fights. In the Serbian study, the association between FIV and outdoor access (OR = 5.2) was only slightly lower than the association with non-pedigree breed (OR = 5.5) and male gender (OR = 4.5), suggesting that these factors are partially independent but highly correlated [8]. The interplay between behavior and demography is perhaps best illustrated by the phenomenon of entire male cats in Western Australia, which had a lower neutering rate compared to the rest of Australia and correspondingly higher FIV seroprevalence [12]. This observation highlights a modifiable risk factor: promoting early neutering reduces roaming, fighting, and ultimately virus transmission.

Co-infection and Syndemic Interactions

FIV does not exist in an epidemiological vacuum; its prevalence and impact are profoundly influenced by co-infections with other pathogens, most notably feline leukemia virus (FeLV). Co-infection rates vary widely but are generally higher than would be expected by chance alone, indicating shared risk factors or biological facilitation. The Brazilian necropsy study found that 9.1% of 1,470 cats were co-infected with FeLV and FIV [1]. In Malaysia, 4.3% of cats harbored both viruses [21], while in Thailand, co-infection prevalence was 2.7% [2]. The case-case study noted that 16% of FeLV-infected cats in high- and low-risk groups were co-infected with FIV [30]. The epidemiological importance of co-infection lies in its synergistic pathogenicity: FeLV is more immunosuppressive and oncogenic than FIV, and co-infected cats have significantly higher odds of developing neoplastic diseases (OR = 1.9), bacterial diseases (OR = 2.8), and viral diseases (OR = 2.8) compared to singly infected cats [1]. Notably, the odds of lymphoma and leukemia were markedly elevated in co-infected cats (OR = 1.9 and 19.3, respectively) [1]. This syndemic interaction suggests that FeLV infection may accelerate FIV disease progression, or vice versa, through cumulative immunosuppression.

FIV also exhibits significant ecological associations with other infectious agents. The relationship with feline syncytium-forming virus (FeSFV) is particularly striking: 74% of FeSFV-infected cats in a high-risk study group were co-infected with FIV, compared to a 38% FIV infection rate among FeSFV-negative cats [30]. More recently, studies have documented associations between FIV and hemoplasma infections. In Northern Serbia, FIV seropositivity was significantly associated with hemoplasma infection (OR = 2.4), and multivariable analysis identified male gender, outdoor access, non-pedigree breed, and anemia as additional risk factors for hemoplasma carriage [8]. The direction of causality is uncertain, FIV-induced immunosuppression may predispose cats to hemoplasma infection, or shared risk factors (such as fighting and vector exposure) may account for the association. FIV infection has also been documented in cats from areas endemic for visceral leishmaniasis in Brazil, where 5.5% of 90 cats were FIV-positive, with co-infection with Leishmania infantum occurring in 8% of infected cats [44]. The role of FIV in modulating susceptibility to vector-borne and protozoal diseases remains an active area of investigation.

Genetic and Viral Subtype Variability

The epidemiology of FIV is further complicated by the existence of multiple viral subtypes (clades A–E) with distinct geographic distributions and potential differences in transmissibility and pathogenicity. Phylogenetic analyses have revealed substantial global diversity. In Ireland, all but one of eight FIV strains belonged to subtype A, with a single subtype B isolate [3]. In Hungary, all 22 sequenced strains belonged to subtype B, grouped into several monophyletic subgroups reflecting geographic origin, with overall mean genetic similarity of 98.2% [4]. In Northern Serbia, clade D was the most prevalent subtype [8]. This geographic structure suggests that FIV subtypes are not uniformly distributed but are shaped by founder effects, cat movement patterns, and possibly host genetic factors.

Of particular interest is the relationship between FIV subtype and pathogenicity. A study examining the interaction between FIV Vif and feline APOBEC3 proteins revealed that subtype B Vif is less active in degrading feline APOBEC3Z3 compared to other subtypes, and that subtype B exhibits significantly reduced genetic diversity [10]. The authors proposed that this attenuated anti-APOBEC3 activity may be associated with the lower pathogenicity of subtype B, suggesting that viral genetic factors contribute to the variable disease outcomes observed in naturally infected cats. Furthermore, a naturally occurring variant of feline APOBEC3Z3 (haplotype V) confers resistance to FIV Vif-mediated degradation and inhibits viral replication. Phylogenetic analyses indicate that this resistance haplotype emerged approximately 60,000 years ago, likely driven by positive selection from an ancestral FIV infection [16]. This evolutionary arms race between host restriction factors and viral countermeasures adds a layer of genetic complexity to the epidemiology of FIV, as populations with higher frequencies of protective APOBEC3 haplotypes may exhibit lower viral prevalence or slower disease progression.

Geographic Variation in the United States

A particularly informative analysis of the geographic distribution of FIV and FeLV in the United States examined counts of 17,108 FIV-positive and 30,017 FeLV-positive serology results from 48 contiguous states and the District of Columbia over an 11-year period (2000–2011) [19]. The study employed a proportional morbidity ratio (PMR) approach, comparing the relative frequency of FIV to FeLV diagnoses across regions. The results revealed a striking and statistically significant spatial pattern: the PMR of FIV to FeLV was higher in the southern and eastern United States, while FeLV diagnoses were proportionally more frequent in the western states [19]. The spatial scan test confirmed significant clustering of high FIV-to-FeLV ratios in the South and East (p < 0.05). The authors hypothesized that this geographic heterogeneity may reflect differences in vaccination rates against FeLV (which is more widely practiced in some regions than FIV vaccination), variations in virus strains, or differences in cat population density and management practices. Notably, the PMR approach controls for some confounding factors by comparing within the same testing dataset, but it cannot distinguish whether the observed pattern is driven by higher FIV prevalence in the South and East, lower FeLV prevalence in those regions, or both. Nevertheless, the finding underscores that even within a single country, FIV epidemiology is not uniform, and regional variations must be considered when designing testing and prevention strategies.

Transmission Dynamics in Non-Domestic Felids

While this section focuses on domestic cats, it is important to recognize that FIV infects a wide range of wild felid species, and these sylvatic cycles may influence the epidemiology in domestic populations through spillover or shared ecological niches. FIV strains specific to free-ranging African lions (Panthera leo), designated FIVple, have been associated with clinical and pathological outcomes similar to AIDS in domestic cats, including lymphadenopathy, gingivitis, tongue papillomas, lymphoid depletion, and abnormal hematological parameters [31]. A 25% seroprevalence of FIV has been documented in wild Mongolian Pallas’ cats (Otocolobus manul), with histopathological evidence of immune depletion in lymph node and spleen tissues [34]. These findings indicate that FIV is not a benign commensal in wild felids but rather a pathogen with significant fitness costs. The existence of species-specific FIV lineages in wild felids raises the possibility of bidirectional transmission between domestic and wild populations, particularly at the human–wildlife interface. However, current evidence suggests that domestic cat FIV strains are generally distinct from those circulating in wild felids, and cross-species transmission appears to be rare [22, 43]. Nonetheless, the conservation implications of FIV in endangered species such as the Florida panther and the Iberian lynx remain an area of active concern.

Clinical Manifestations and Disease Associations in FIV-Infected Cats

The clinical landscape of feline immunodeficiency virus (FIV) infection is characterized by a remarkable spectrum of outcomes, ranging from lifelong asymptomatic carriage to a progressive, ultimately fatal acquired immunodeficiency syndrome (AIDS). This variability is a hallmark of lentiviral infections, reflecting a complex interplay between viral pathogenicity, host genetic factors, environmental exposures, and the presence of concurrent infections. Unlike its more acutely pathogenic retroviral counterpart, feline leukemia virus (FeLV), FIV infection is not invariably associated with a rapid, predictable disease course. Instead, the clinical manifestations of FIV are largely secondary to the progressive immunosuppression it induces, rendering the infected cat vulnerable to a diverse array of opportunistic infections, inflammatory conditions, and neoplastic processes. A deep understanding of these clinical associations is critical for the veterinary practitioner, not only for diagnosis and prognosis but also for the development of targeted management strategies that can prolong and improve the quality of life for FIV-infected cats.

The Spectrum of Disease: From Asymptomatic Carriage to AIDS

The natural history of FIV infection is classically divided into three phases: an acute primary phase, a prolonged asymptomatic phase, and a terminal phase of immunodeficiency. The acute phase, occurring weeks to months post-infection, is often subclinical or manifests as a transient, non-specific illness. Experimentally, this phase is characterized by fever, neutropenia, and generalized lymphadenopathy, which can persist for several weeks [28]. Following this, the cat enters a protracted asymptomatic carrier state that can last for years. During this period, the virus continues to replicate at low levels, and the cat may appear clinically healthy, with no overt signs of disease [18, 20]. This latent period is a critical feature of FIV infection, and a positive test result in an otherwise healthy cat does not imply immediate suffering or a poor short-term prognosis [20]. However, the virus is relentlessly compromising the immune system, particularly through the progressive depletion of CD4+ T lymphocytes, which is the central immunopathological event [22, 31]. The terminal phase, which may not be reached in all cats, is marked by a profound state of immunosuppression, leading to the emergence of the clinical syndromes most commonly associated with FIV.

A critical insight from large-scale retrospective studies is that FIV infection, when present as a monoinfection, is not strongly associated with specific disease outcomes in the same way that FeLV is. A landmark necropsy study of 1,470 cats found that while FeLV infection and FeLV/FIV co-infection were significantly associated with neoplastic and infectious diseases, FIV infection alone did not differ significantly from uninfected cats in terms of the odds of developing these conditions [1]. This paradoxical finding underscores that the clinical impact of FIV is highly context-dependent. The virus creates a state of vulnerability, but the actual diseases that manifest are largely determined by the cat's environment, its exposure to other pathogens, and its individual genetic makeup. This contrasts sharply with FeLV, which is directly oncogenic and myelosuppressive, leading to a more predictable and severe disease profile [1, 46].

Oral Cavity Disease: The Most Prevalent Clinical Manifestation

Chronic oral cavity infections, particularly gingivitis and stomatitis, are arguably the most frequently reported clinical manifestation in FIV-infected cats. Early epidemiological studies identified chronic oral infections in over half (56%) of FIV-positive cats surveyed [30]. This condition, often referred to as feline chronic gingivostomatitis (FCGS), is characterized by severe, proliferative, and often ulcerative inflammation of the gingiva, fauces, and oral mucosa. The pathogenesis is complex and likely involves a dysregulated immune response to dental plaque and other oral antigens, exacerbated by the underlying FIV-induced immunodeficiency. The virus itself may also directly contribute to local inflammation. The oral cavity is a site of high viral load, and FIV infection has been shown to significantly alter the oral microbiota, with FIV-infected cats demonstrating a higher relative abundance of potentially pathogenic bacterial phyla such as Fusobacteria and Actinobacteria [39]. This dysbiosis likely contributes to the severity and chronicity of oral inflammation. The clinical consequence is often severe pain, dysphagia, ptyalism, and halitosis, making this one of the most debilitating conditions for affected cats.

Ocular, Respiratory, and Enteric Manifestations

Beyond the oral cavity, FIV infection predisposes cats to a range of other chronic or recurrent inflammatory conditions. Chronic upper respiratory tract disease (URTD) is another common finding, reported in approximately one-third of infected cats in early studies [30]. This is often a complex of rhinitis, sinusitis, and conjunctivitis, which can be caused by a variety of opportunistic viral (e.g., feline herpesvirus, calicivirus) and bacterial pathogens. Chronic conjunctivitis, in particular, has been noted as a distinct association [30]. The ocular and conjunctival microbiota is also altered in FIV-infected cats, which may contribute to the pathogenesis of these conditions [39]. Chronic enteritis, presenting as intermittent or persistent diarrhea, is also a recognized clinical problem, reported in nearly 20% of cases [30]. The etiology is likely multifactorial, involving opportunistic enteric pathogens, alterations in the gut-associated lymphoid tissue (GALT), and possibly direct viral effects on the intestinal mucosa.

Renal and Cardiovascular Disease: Emerging Associations

The parallels between FIV and HIV extend to the realm of organ-specific pathology. HIV is known to cause a spectrum of renal diseases, and FIV has emerged as a valuable natural model for studying lentivirus-associated nephropathy. Studies have demonstrated that both experimentally and naturally FIV-infected cats develop a range of renal morphological changes, including mesangial widening, glomerulonephritis, and the formation of proteinaceous tubular casts and microcysts [45]. Notably, naturally infected cats exhibited more severe pathology, including diffuse interstitial infiltrates and amyloidosis, which were not observed in experimentally infected animals, suggesting that the duration of infection and the presence of co-factors are important in disease progression [45]. This finding is clinically significant, as it suggests that FIV infection should be considered a potential contributing factor in cats presenting with chronic kidney disease (CKD), a common geriatric feline condition.

Perhaps one of the most striking and direct organ-specific associations is the link between FIV and hypertrophic cardiomyopathy (HCM) with myocarditis. A seminal histopathological and immunohistochemical study of five young cats (1–4 years old) presenting with HCM and myocarditis demonstrated the presence of FIV antigen within infiltrating inflammatory cells (T lymphocytes and macrophages) in the myocardium [38]. The absence of other common cardiotropic pathogens (feline calicivirus, coronavirus, FeLV, parvovirus, Chlamydia spp., and Toxoplasma gondii) in these lesions strongly implicated FIV as the direct causative agent of the myocarditis [38]. This finding is of profound clinical importance, as it suggests that FIV infection can be a primary cause of inflammatory heart disease and subsequent cardiomyopathy in cats, challenging the notion that FIV is merely a predisposing factor for secondary infections.

Neurological Involvement: A Model for HIV-Associated Neurocognitive Disorders

FIV is a neurotropic lentivirus, and its ability to invade the central nervous system (CNS) early after infection is a well-established feature [11, 42]. This neuroinvasion occurs via the blood-brain and blood-cerebrospinal fluid barriers, with the choroid plexus serving as an active site of immune cell trafficking in response to the virus [41]. The ensuing neuroinflammation and neuronal damage give rise to a spectrum of neurological deficits, collectively analogous to HIV-associated neurocognitive disorders (HAND) in humans [11]. Clinically, these can manifest as behavioral changes, sleep disturbances, motor deficits, and cognitive decline, although these signs may be subtle and easily overlooked in a standard veterinary examination. The neuropathogenesis involves both direct viral effects on neural cells and indirect damage mediated by activated macrophages and microglia, which release neurotoxic substances [11, 42]. This makes FIV an invaluable animal model for studying the mechanisms of lentiviral CNS disease and for testing potential neuroprotective therapies.

Hematological Abnormalities and Coinfections

Hematological abnormalities are a common laboratory finding in FIV-infected cats, reflecting the virus's impact on the bone marrow and immune system. Anemia is a frequently reported finding, with FIV-positive cats being significantly more likely to be anemic than their uninfected counterparts [2, 7]. This anemia can be multifactorial, resulting from chronic disease, immune-mediated destruction, or direct bone marrow suppression. Leukocytopenia, particularly neutropenia and lymphopenia, is also a characteristic finding, reflecting the depletion of CD4+ T cells and the overall immunosuppressed state [2, 7, 28]. These hematological changes are less severe than those seen in FeLV infection, which is more directly myelosuppressive [46].

The immunosuppression induced by FIV creates a permissive environment for a wide range of opportunistic infections. The risk of developing bacterial diseases is significantly increased in cats co-infected with FeLV and FIV [1]. Among the most clinically significant coinfections is toxoplasmosis. FIV-infected cats are at high risk for recrudescent toxoplasmosis, which can manifest as a severe, often fatal, disseminated disease. This risk is dramatically amplified by the use of immunosuppressive therapies, such as oclacitinib, which is used for allergic skin disease. A case report documented the rapid development of fatal disseminated toxoplasmosis in an FIV-positive cat after just five months of oclacitinib therapy [37]. This serves as a critical clinical warning: FIV-positive status should be considered a major contraindication for the use of potent immunosuppressive drugs.

Furthermore, FIV infection is a significant risk factor for infection with feline haemoplasmas, the causative agents of feline infectious anemia. A large study from Serbia found that FIV-seropositive cats were 2.4 times more likely to be infected with haemoplasmas than seronegative cats [8]. This co-infection exacerbates the risk of anemia, creating a synergistic pathological effect. The relationship between FIV and feline infectious peritonitis (FIP) is more complex. While some studies have found an increased odds of FIP in FeLV-infected cats, the association with FIV alone is less clear [1]. However, the immunosuppressive nature of FIV would logically increase susceptibility to any opportunistic viral infection.

Neoplastic Associations and the Role of Viral Subtype

The association between FIV and neoplasia is less direct and robust than that for FeLV. While FeLV is a direct oncogenic virus, FIV is thought to increase the risk of certain tumors indirectly through chronic immune dysregulation and inflammation. The odds of developing neoplasms are significantly higher in FeLV-infected and co-infected cats, but not in cats with FIV alone [1]. However, FIV infection has been linked to an increased incidence of B-cell lymphomas and other hematopoietic tumors in some studies, likely as a consequence of chronic antigenic stimulation and impaired immune surveillance. The specific FIV subtype may also influence the clinical course. For instance, FIV subtype B, which is prevalent in Europe and Asia, has been associated with a less active viral infectivity factor (Vif) and reduced genetic diversity, which may correlate with lower pathogenicity compared to other subtypes [10]. This suggests that the clinical outcome of FIV infection may be partially determined by the infecting viral strain.

In conclusion, the clinical manifestations of FIV infection are a tapestry woven from the threads of progressive immunosuppression, opportunistic infections, and direct viral pathology in select organ systems. The disease is not a monolith but a spectrum, heavily influenced by the cat's environment, genetics, and the presence of other pathogens. The most common presentations involve the oral cavity, but the potential for severe, life-threatening conditions such as myocarditis, nephropathy, and disseminated toxoplasmosis underscores the need for vigilant monitoring and proactive management. The recognition that FIV can directly cause disease, as in the case of myocarditis, and that it dramatically alters the risk-benefit profile of certain therapies, is paramount for the modern clinician.

Diagnostic Approaches for Feline Immunodeficiency Virus Infection

The accurate and timely diagnosis of feline immunodeficiency virus (FIV) infection is a cornerstone of clinical management, epidemiological surveillance, and research into lentiviral pathogenesis. As a lifelong, persistent infection that can remain subclinical for years, the diagnostic approach must be nuanced, accounting for the stage of infection, the cat’s vaccination history, and the inherent limitations of each testing modality. The landscape of FIV diagnostics has evolved significantly, driven by the need to differentiate naturally infected cats from those that have been vaccinated, and to improve sensitivity in early or latent infections. This section provides an exhaustive analysis of the current diagnostic armamentarium, from traditional serological methods to molecular techniques, with a critical evaluation of their performance characteristics, clinical applications, and interpretative challenges.

Serological Detection: The Mainstay of Point-of-Care Testing

For decades, the detection of circulating antibodies against FIV has been the primary method for diagnosing infection in clinical and shelter settings. This approach is predicated on the fact that, unlike some retroviruses, FIV induces a robust and persistent humoral immune response that is detectable within two to four weeks post-infection and remains present for the life of the animal [28]. The vast majority of commercially available point-of-care (PoC) tests are enzyme-linked immunosorbent assays (ELISA) or lateral flow immunochromatographic (IC) assays designed to detect antibodies to FIV structural proteins, most commonly p24 (capsid) and gp40 (transmembrane envelope protein) [12, 47].

The performance of these PoC tests has been rigorously evaluated. In a landmark comparative study of four commercially available combination test kits (SNAP FIV/FeLV Combo, Witness FeLV/FIV, Anigen Rapid FIV/FeLV, and VetScan FIV/FeLV), Levy et al. (2017) reported sensitivities for FIV detection ranging from 91.5% to 97.9% and specificities from 99.0% to 100% when compared against virus isolation as the gold standard [48]. The SNAP test demonstrated the highest sensitivity (97.9%) and specificity (99.0%) in that study, although no statistically significant differences were observed among the four kits for FIV detection [48]. These figures are generally consistent with other large-scale evaluations, which have reported sensitivities and specificities exceeding 95% for most modern PoC kits [47, 49].

However, the clinical utility of these tests is profoundly affected by the population being tested. In low-prevalence populations (e.g., 1-5% seroprevalence in healthy indoor cats), the positive predictive value (PPV) of even a highly specific test can be alarmingly low. Levy et al. (2017) explicitly cautioned that in such populations, a majority of positive results reported by most PoC devices could be false positives, potentially leading to unnecessary segregation, emotional distress for owners, or even unwarranted euthanasia [48]. This underscores the critical importance of confirmatory testing for all positive PoC results, especially in low-risk cats. Conversely, in high-risk populations (e.g., free-roaming, intact male cats with clinical signs), where seroprevalence can exceed 15%, the PPV is much higher, and a positive result is more likely to represent true infection [5, 12].

The Vaccination Conundrum and the Evolution of Diagnostic Strategy

The commercial release of an inactivated FIV vaccine (Fel-O-Vax FIV) in the United States in 2002, and its continued availability in Australia, New Zealand, and Japan, fundamentally altered the diagnostic landscape [47]. Vaccinated cats produce antibodies against FIV antigens that are indistinguishable from those generated by natural infection using conventional ELISA-based PoC tests. This created a significant clinical dilemma: how to determine the true infection status of a cat with a known or unknown vaccination history.

For years, the prevailing recommendation was to abandon serology in vaccinated cats and rely solely on nucleic acid detection via polymerase chain reaction (PCR). However, a series of pivotal studies by Westman et al. challenged this paradigm. Their research demonstrated that not all PoC antibody tests are created equal. Specifically, they found that two lateral flow immunochromatography test kits, Witness FeLV/FIV and Anigen Rapid FIV/FeLV, could accurately distinguish between FIV-vaccinated and FIV-infected cats [47, 49]. In a mixed population of 119 FIV-vaccinated and 239 unvaccinated cats, these two kits exhibited 100% sensitivity and 98-100% specificity for detecting true FIV infection, irrespective of vaccination history [49]. In contrast, the lateral flow ELISA-based SNAP FIV/FeLV Combo test could not differentiate vaccine-induced antibodies from infection-induced antibodies [49].

The mechanistic basis for this differential performance lies in the test format and antigen composition. The Witness and Anigen kits are lateral flow IC assays that detect antibodies to p24 and gp40, while the SNAP test is a lateral flow ELISA that detects antibodies to p15 and p24 [12]. It is hypothesized that the vaccine formulation (inactivated whole virus) may induce a different antibody profile or affinity maturation pattern compared to natural infection, which is differentially recognized by these test formats. This discovery has profound practical implications: in regions where the FIV vaccine is used, veterinarians can now use specific PoC tests (Witness or Anigen) as a reliable first-line screening tool, reserving PCR for cases where results are discordant or where the cat’s vaccination status is unknown and a different test kit was used [47].

Molecular Diagnostics: PCR and the Detection of Viral Nucleic Acid

Polymerase chain reaction (PCR) offers a direct method for detecting FIV proviral DNA or viral RNA, bypassing the host immune response. This is particularly advantageous in several specific scenarios: (1) in kittens born to FIV-infected queens, where maternal antibodies can cause false-positive serology results for up to 4-6 months; (2) in the early, pre-seroconversion window period (typically the first 2-4 weeks post-infection); (3) in cats with an unknown or positive vaccination history where serology is ambiguous; and (4) for confirmatory testing of positive PoC results [13, 49].

The sensitivity and specificity of PCR assays are highly variable and depend on the target gene, primer design, and laboratory protocols. Most commercial and in-house PCR assays target conserved regions of the pol (polymerase) or gag genes. Westman et al. (2015) evaluated a commercial FIV RealPCR assay and found a sensitivity of 92% and specificity of 99% against a composite reference standard [49]. This indicates that while PCR is highly specific, it is not perfectly sensitive. False-negative PCR results can occur due to low proviral load (particularly during the long asymptomatic latent stage), genetic variation in the target region (especially with diverse subtypes), or the presence of PCR inhibitors in the sample [13, 49].

The choice of sample type is also critical. Whole blood (EDTA-anticoagulated) is the standard specimen for proviral DNA detection, as it contains peripheral blood mononuclear cells (PBMCs) that harbor the integrated provirus. Plasma or serum can be used for viral RNA detection, but RNA is less stable and requires careful handling. Importantly, PCR does not differentiate between integrated provirus and unintegrated viral DNA, nor does it distinguish between replication-competent virus and defective genomes. Therefore, a positive PCR result confirms the presence of viral nucleic acid but does not necessarily equate to active viral replication or infectiousness.

Comparative studies have highlighted the discordance between serology and PCR. In a study of cats in Ireland, Szilasi et al. (2021) found that the apparent prevalence of FIV was 10.4% by ELISA but 9.3% by PCR, indicating that a small proportion of seropositive cats were PCR-negative [3]. Conversely, in a study from Hungary, the apparent prevalence was 9.9% by ELISA and 13.1% by PCR, suggesting that some cats may have been in the early stage of infection or had low antibody titers [4]. These discrepancies underscore the importance of using both serology and PCR in a complementary fashion, particularly in research or high-stakes clinical scenarios.

Virus Isolation and Advanced Techniques

Virus isolation (VI) remains the historical gold standard for FIV diagnosis, but it is rarely used in clinical practice due to its complexity, cost, and the requirement for specialized biosafety facilities. The technique involves co-culturing PBMCs from the suspect cat with mitogen-stimulated feline T-lymphoblastoid cells (e.g., MYA-1 cells) and monitoring for cytopathic effect and reverse transcriptase activity [28]. While highly specific, VI is insensitive during periods of low viral load and can take several weeks to yield a result. It is primarily reserved for research applications, such as characterizing new viral isolates or confirming infection in experimental settings.

Other advanced techniques, such as immunohistochemistry (IHC) and in situ hybridization (ISH), have been used primarily in research and post-mortem diagnostics. IHC has been employed to detect FIV antigen in tissue sections, demonstrating viral presence in lymphocytes, macrophages, and even myocardial cells in cases of FIV-associated myocarditis [38]. These techniques are invaluable for understanding viral tropism and pathogenesis but are not practical for routine antemortem diagnosis.

Diagnostic Algorithm and Interpretative Pitfalls

Given the strengths and limitations of each diagnostic modality, a rational, stepwise approach is essential. For a healthy, low-risk cat with a negative PoC test, no further testing is typically warranted. For any cat with a positive PoC test, confirmatory testing is strongly recommended. The choice of confirmatory test depends on the cat’s vaccination status and the specific PoC test used. If the initial test was a Witness or Anigen kit, and the cat is from a region where FIV vaccination is practiced, a positive result is highly predictive of true infection, and PCR confirmation may be reserved for cases where the result is unexpected or has serious consequences (e.g., shelter euthanasia decisions). If the initial test was a SNAP or other ELISA-based kit, or if the vaccination history is unknown, PCR is the recommended confirmatory test [47, 49].

Several pitfalls must be considered. First, maternally derived antibodies can persist in kittens for up to 6 months, leading to false-positive serology results. Testing kittens under 6 months of age should be interpreted with extreme caution, and PCR is the preferred method for determining infection status in this age group. Second, cats in the terminal stage of FIV infection may become seronegative due to profound immune exhaustion and B-cell dysfunction, leading to false-negative serology results. In such cases, PCR or virus isolation may be necessary. Third, the presence of concurrent FeLV infection or other immunosuppressive conditions can affect antibody production and test performance [1, 2]. Finally, the genetic diversity of FIV, with at least five subtypes (A-E) and circulating recombinants, can impact the sensitivity of both serological and molecular tests [4, 10, 29]. Most PoC tests are designed to detect antibodies to conserved epitopes and are generally robust across subtypes, but PCR assays targeting variable regions may fail to amplify divergent strains.

In conclusion, the diagnosis of FIV infection requires a sophisticated understanding of test methodology, host immune dynamics, and epidemiological context. The integration of high-quality PoC serology with confirmatory PCR, guided by the cat’s signalment and vaccination history, provides the most reliable framework for clinical decision-making. As the FIV vaccine continues to be used in certain regions, the ability of specific PoC tests to discriminate between vaccinated and infected cats represents a major advance, allowing veterinarians to maintain the convenience of serological screening without sacrificing diagnostic accuracy.

Management, Therapeutic Strategies, and Prevention of FIV

The clinical management of feline immunodeficiency virus (FIV) infection requires a paradigm shift from a diagnosis of imminent fatality to one of chronic disease management. Contemporary veterinary science recognizes that FIV-infected cats, when provided with optimal husbandry and proactive medical oversight, can experience lifespans comparable to their uninfected counterparts. The cornerstone of effective management rests upon a tripartite foundation: rigorous prevention of secondary infections, strategic suppression of viral replication where feasible, and implementation of biosecurity measures to curtail further transmission within feline populations. This section delineates a comprehensive, evidence-based framework for the clinical stewardship of FIV-infected cats, integrating epidemiological risk assessment, therapeutic interventions, and prophylactic strategies.

Environmental and Supportive Management of the FIV-Positive Cat

The fundamental principle governing the care of the FIV-positive cat is the minimization of exposure to opportunistic pathogens. Given that FIV induces a progressive immunocompromised state, particularly characterized by CD4+ T-lymphocyte depletion and qualitative defects in humoral and cell-mediated immunity, the infected host is rendered susceptible to a spectrum of secondary infections [18, 32]. The primary management directive, therefore, is strict indoor confinement. This singular intervention addresses two critical objectives: it protects the immunocompromised cat from encounters with infectious agents prevalent in the outdoor environment (e.g., Toxoplasma gondii, feline herpesvirus, calicivirus, and bacterial pathogens), and it simultaneously prevents the index cat from perpetuating the transmission cycle through the principal route of FIV spread, territorial fighting and bite wounds [5, 20, 30]. Epidemiological data consistently demonstrate that free-roaming, intact male cats constitute the demographic at highest risk for FIV acquisition and transmission, reinforcing the logic of enforced indoor habitation for all diagnosed individuals [6, 14, 30].

Neutering of all FIV-positive cats is an indispensable component of management. Castration of males reduces testosterone-driven aggression and roaming behavior, thereby decreasing the likelihood of bite-related transmission. Ovariohysterectomy in females eliminates the risk of reproductive cycling and associated stressors, and, critically, prevents vertical transmission to kittens, although lactogenic transmission is inefficient and intrauterine transmission is considered rare under natural conditions [28]. Nutritional optimization is equally paramount. A high-quality, balanced commercial diet supports the integrity of the gastrointestinal mucosal barrier and the overall nutritional status of the cat, which can be compromised by the chronic inflammatory state associated with lentiviral infection. Dietary supplements, such as omega-3 fatty acids for their anti-inflammatory properties and probiotics to support gut microbiome stability, may be considered, although robust clinical trial data in FIV-infected populations remain limited. Regular veterinary health monitoring, comprising semi-annual physical examinations, complete blood counts, serum biochemistry panels, and urinalysis, is essential for the early detection of emerging comorbidities. Hematological abnormalities, including anemia, leukopenia, and lymphopenia, are significantly more prevalent in FIV-positive cats compared to uninfected controls and serve as critical sentinel markers of disease progression and bone marrow compromise [2, 7]. Furthermore, the oral cavity warrants meticulous attention; chronic gingivostomatitis is a hallmark clinical manifestation of FIV infection, often requiring professional dental scaling, extraction of affected teeth, and long-term anti-inflammatory or immunomodulatory therapy [30, 32]. Regular monitoring for the development of neoplasia, particularly lymphoma, and for renal disease, given the documented association between FIV infection and glomerulonephritis and interstitial nephritis, is also clinically prudent [1, 45, 46].

Therapeutic Strategies: Antiviral and Immunomodulatory Interventions

The therapeutic armamentarium for FIV infection is less expansive and less standardized than that for human immunodeficiency virus (HIV), yet several avenues warrant consideration. The structural and functional homology between the reverse transcriptase (RT) enzymes of FIV and HIV renders FIV susceptible to several nucleoside analogue RT inhibitors (NRTIs) commonly used in human antiretroviral therapy [17]. Zidovudine (AZT) is the most extensively studied compound in this context. In vitro studies have demonstrated potent inhibition of FIV replication, and in vivo clinical trials have documented improvements in oral health, reduction in viral load, and amelioration of neurological signs in some treated cats [17, 23]. However, the clinical use of AZT in cats is tempered by its potential for hematological toxicity, particularly bone marrow suppression leading to anemia and neutropenia, necessitating diligent monitoring of complete blood counts throughout therapy. The clinical utility of other NRTIs, such as didanosine (ddI) and lamivudine (3TC), has been explored, but data supporting their routine use in FIV-infected cats are insufficient to recommend them as standard of care. A critical limitation of antiretroviral monotherapy in any lentiviral infection is the rapid emergence of drug-resistant viral quasispecies, a phenomenon well-documented in HIV and likely operative in FIV. Combination antiretroviral therapy (cART), analogous to highly active antiretroviral therapy (HAART) in humans, has been attempted in experimental settings but is not a practical reality in clinical feline practice due to cost, drug availability, and the lack of feline-optimized formulations of protease inhibitors and integrase strand transfer inhibitors [23].

Beyond direct antivirals, immunomodulatory agents have been investigated. Recombinant feline interferon-omega (rFeIFN-ω), licensed in several countries for the treatment of feline viral infections, has shown some benefit in improving clinical signs and reducing mortality in FIV-infected cats, particularly when administered early in the course of disease. The proposed mechanism involves enhancement of innate antiviral immune responses and modulation of the cytokine milieu. However, the evidence for profound or durable virologic suppression with interferon therapy is lacking, and its use is generally considered adjunctive rather than curative [17]. The management of secondary infections remains the most impactful therapeutic intervention. Bacterial infections of the skin, respiratory tract, and urinary tract require prompt identification and targeted antimicrobial therapy based on culture and sensitivity testing whenever possible. Protozoal infections, particularly toxoplasmosis, pose a significant threat to the immunocompromised FIV-positive cat. A compelling case report details fatal disseminated toxoplasmosis in an FIV-positive cat receiving oclacitinib, a Janus kinase (JAK) inhibitor, for allergic skin disease, underscoring the critical need to avoid potent immunosuppressive agents in this population and to maintain a high index of suspicion for recrudescent opportunistic infections [37]. Similarly, mycoplasmal infections, including those caused by Mycoplasma haemofelis and Candidatus Mycoplasma haemominutum, are significantly more prevalent in FIV-seropositive cats and can induce or exacerbate hemolytic anemia, necessitating appropriate antimycoplasmal therapy such as doxycycline or a fluoroquinolone [8, 44].

Prevention of FIV Transmission and Infection

Prevention of FIV infection in the general cat population relies on a dual strategy: reduction of exposure risk and active immunization. The epidemiological profile of FIV, transmitted predominantly through bite wounds during aggressive encounters between territorial male cats, dictates that the most effective preventive measure is the elimination of high-risk behaviors. This is achieved through early neutering, which reduces the hormonal drive for fighting and roaming, and through strict indoor housing, which prevents contact with potentially viremic free-roaming cats [14, 30]. Multi-cat households, particularly those with a history of conflict or with outdoor access, represent environments of elevated risk, and careful management, including gradual introductions, provision of adequate resources (feeding stations, litter boxes, hiding places), and separation of incompatible individuals, is essential to minimize aggressive interactions and the attendant risk of fomite or bite transmission [2, 21].

Diagnostic screening prior to introduction of a new cat into a household is a cornerstone of prevention. Point-of-care (PoC) antibody tests, such as the SNAP FIV/FeLV Combo Test and the Witness FeLV/FIV test, are widely used, but their performance characteristics must be understood. The SNAP test demonstrates high sensitivity (97.9%) and specificity (99.0%) for FIV antibody detection in unvaccinated populations [48]. However, a critical diagnostic challenge arises in cats that have received the FIV vaccine (Fel-O-Vax FIV), which is commercially available in some countries, including the United States, Australia, New Zealand, and Japan. Vaccinated cats develop antibodies to FIV antigens that are indistinguishable from those produced during natural infection by most PoC antibody tests, leading to false-positive results. Fortunately, not all test kits are equally affected; the Witness and Anigen Rapid tests have been shown to accurately distinguish between vaccinated and naturally infected cats, as they employ different antigen targets (gp40 versus p24) that yield differential reactivity [47, 49]. In populations where FIV vaccination is practiced, the use of a vaccine-compatible test kit is imperative, or alternatively, nucleic acid amplification testing (PCR) for proviral DNA should be employed for confirmatory diagnosis. PCR testing, however, also has limitations, including variable sensitivity related to viral load and proviral integration site, and its cost may be prohibitive for routine screening in some settings [13, 47]. For the clinician managing a multi-cat environment, the decision to isolate or integrate a newly diagnosed FIV-positive cat must balance the risk of transmission, which is low in stable, non-aggressive households where all cats are neutered, against the welfare considerations of the infected individual. General consensus supports that FIV-positive cats can be housed with FIV-negative cats provided that all cats are neutered, aggressive interactions are minimal, and the negative cats are tested and deemed negative prior to introduction [20].

Vaccination: Efficacy, Limitations, and Strategic Considerations

A commercial inactivated whole-virus vaccine (Fel-O-Vax FIV) has been available in several countries since the early 2000s. This vaccine, containing inactivated subtypes A and D viruses formulated with an adjuvant, was demonstrated in laboratory efficacy trials to provide protection against homologous and certain heterologous FIV strains, including circulating intersubtype recombinants [29]. The mechanism of vaccine-induced protection is thought to be mediated primarily by cellular immune responses, including FIV-specific T-cell immunity, rather than by broadly neutralizing antibodies, which are notoriously difficult to elicit against lentiviral envelope glycoproteins [27]. Despite its availability, several factors complicate the widespread recommendation of FIV vaccination. The vaccine does not provide sterilizing immunity; vaccinated cats can still become infected if challenged with a sufficiently high dose of virus or with a heterologous strain not covered by the vaccine. Furthermore, the vaccine’s ability to protect against all five global subtypes (A–E) and circulating recombinants has been questioned, and breakthrough infections have been documented [29].

The most significant practical impediment to FIV vaccination is the diagnostic interference it creates. As detailed above, vaccinated cats seroconvert and produce antibodies that are detected by most PoC test kits, rendering serological diagnosis of natural infection unreliable in vaccinated individuals. This poses a major problem for shelters, rescue organizations, and multi-cat households where determining the true infection status of each cat is critical for management and adoption decisions. The vaccine is also a non-core vaccine, meaning it is not universally recommended for all cats; its use is generally reserved for cats at high risk of exposure (e.g., outdoor access, intact males, those living in multi-cat households with a known FIV-positive cat). The vaccine’s safety profile is acceptable, although injection-site sarcomas remain a theoretical concern with any adjuvanted feline vaccine, and the vaccine is not licensed for use in pregnant queens or kittens under eight weeks of age. Given these complexities, the decision to vaccinate must be made on a case-by-case basis, after thorough client education about the vaccine’s limitations, the need for adjunctive testing strategies, and the paramount importance of combining vaccination with behavioral risk reduction (confinement, neutering). In regions where the vaccine is unavailable, the entire focus of prevention shifts to environmental management and diagnostic screening. The global variability in FIV prevalence, which can range from less than 3% in healthy pet cats in the United States to over 15% in high-risk or sick populations in certain geographic regions, further underscores the need for regionally tailored prevention strategies that account for local epidemiological patterns, prevalence of specific subtypes, and available diagnostic and prophylactic resources [5, 12, 21].

References

[1] Mello LDd, Ribeiro PR, Almeida BAd, Bandinelli M, Sonne L, Driemeier D, et al.. Diseases associated with feline leukemia virus and feline immunodeficiency virus infection: A retrospective study of 1470 necropsied cats (2010-2020).. Comparative Immunology, Microbiology & Infectious Diseases. 2023. DOI: https://doi.org/10.1016/j.cimid.2023.101963

[2] Rungsuriyawiboon O, Jarudecha T, Hannongbua S, Choowongkomon K, Boonkaewwan C, Rattanasrisomporn J. Risk factors and clinical and laboratory findings associated with feline immunodeficiency virus and feline leukemia virus infections in Bangkok, Thailand. Veterinary World. 2022. DOI: https://doi.org/10.14202/vetworld.2022.1601-1609

[3] Szilasi A, Dénes L, Krikó E, Murray C, Mándoki M, Balka G. Prevalence of feline leukaemia virus and feline immunodeficiency virus in domestic cats in Ireland.. Acta Veterinaria Hungarica. 2021. DOI: https://doi.org/10.1556/004.2020.00056

[4] Szilasi A, Dénes L, Krikó E, Heenemann K, Ertl R, Mándoki M, et al.. Prevalence of feline immunodeficiency virus and feline leukaemia virus in domestic cats in Hungary. JFMS open reports. 2019. DOI: https://doi.org/10.1177/2055116919892094

[5] Phillips T. Feline Immunodeficiency Virus. Clinical Small Animal Internal Medicine. 2020. DOI: https://doi.org/10.32388/htmi25

[6] Biezus G, Machado G, Ferian PE, Costa UMd, Pereira L, Withoeft J, et al.. Prevalence of and factors associated with feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) in cats of the state of Santa Catarina, Brazil.. Comparative Immunology, Microbiology & Infectious Diseases. 2019. DOI: https://doi.org/10.1016/J.CIMID.2018.12.004

[7] Costa F, Valle SF, Machado G, Corbellini L, Coelho E, Rosa RB, et al.. Hematological findings and factors associated with feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) positivity in cats from southern Brazil. Pesquisa Veterinaria Brasileira. 2017. DOI: https://doi.org/10.1590/S0100-736X2017001200028

[8] Sarvani E, Tasker S, Filipović MK, Andrić JF, Andrić N, Aquino L, et al.. Prevalence and risk factor analysis for feline haemoplasmas in cats from Northern Serbia, with molecular subtyping of feline immunodeficiency virus. JFMS open reports. 2018. DOI: https://doi.org/10.1177/2055116918770037

[9] Sivagurunathan A, Atwa AM, Lobetti R. Prevalence of feline immunodeficiency virus and feline leukaemia virus infection in Malaysia: a retrospective study. JFMS open reports. 2018. DOI: https://doi.org/10.1177/2055116917752587

[10] Yoshikawa R, Takeuchi J, Yamada E, Nakano Y, Misawa N, Kimura Y, et al.. Feline Immunodeficiency Virus Evolutionarily Acquires Two Proteins, Vif and Protease, Capable of Antagonizing Feline APOBEC3. Journal of Virology. 2017. DOI: https://doi.org/10.1128/JVI.00250-17

[11] Meeker R, Hudson L. Feline Immunodeficiency Virus Neuropathogenesis: A Model for HIV-Induced CNS Inflammation and Neurodegeneration. Veterinary Sciences. 2017. DOI: https://doi.org/10.3390/vetsci4010014

[12] Westman M, Paul A, Malik R, McDonagh P, Ward M, Hall E, et al.. Seroprevalence of feline immunodeficiency virus and feline leukaemia virus in Australia: risk factors for infection and geographical influences (2011–2013). JFMS open reports. 2016. DOI: https://doi.org/10.1177/2055116916646388

[13] Novo SG, Bucafusco D, Diaz LM, Bratanich A. Viral diagnostic criteria for Feline immunodeficiency virus and Feline leukemia virus infections in domestic cats from Buenos Aires, Argentina.. Revista Argentina de Microbiología. 2016. DOI: https://doi.org/10.1016/j.ram.2016.07.003

[14] Chhetri BK, Berke O, Pearl D, Bienzle D. Comparison of risk factors for seropositivity to feline immunodeficiency virus and feline leukemia virus among cats: a case-case study. BMC Veterinary Research. 2015. DOI: https://doi.org/10.1186/s12917-015-0339-3

[15] Brown LA, Cox C, Baptiste JL, Summers H, Button R, Bahlow K, et al.. NMR Structure of the Myristylated Feline Immunodeficiency Virus Matrix Protein. Viruses. 2015. DOI: https://doi.org/10.3390/v7052210

[16] Yoshikawa R, Izumi T, Yamada E, Nakano Y, Misawa N, Ren F, et al.. A Naturally Occurring Domestic Cat APOBEC3 Variant Confers Resistance to Feline Immunodeficiency Virus Infection. Journal of Virology. 2015. DOI: https://doi.org/10.1128/JVI.02612-15

[17] Hartmann K, Wooding A, Bergmann M. Efficacy of Antiviral Drugs against Feline Immunodeficiency Virus. Veterinary Sciences. 2015. DOI: https://doi.org/10.3390/vetsci2040456

[18] Taniwaki S, Figueiredo AS, Jr JPA. Virus–host interaction in feline immunodeficiency virus (FIV) infection. Comparative Immunology, Microbiology & Infectious Diseases. 2013. DOI: https://doi.org/10.1016/j.cimid.2013.07.001

[19] Chhetri BK, Berke O, Pearl D, Bienzle D. Comparison of the geographical distribution of feline immunodeficiency virus and feline leukemia virus infections in the United States of America (2000–2011). BMC Veterinary Research. 2013. DOI: https://doi.org/10.1186/1746-6148-9-2

[20] Pedersen N, Schellekens H, Horzinek MC. Feline Immunodeficiency Virus Infection. Canine and Feline Infectious Diseases. 2013. DOI: https://doi.org/10.1016/B978-1-4377-0795-3.00021-1

[21] Bande F, Arshad S, Hassan L, Zakaria Z, Sapian NA, Rahman NA, et al.. Prevalence and risk factors of feline leukaemia virus and feline immunodeficiency virus in peninsular Malaysia. BMC Veterinary Research. 2012. DOI: https://doi.org/10.1186/1746-6148-8-33

[22] Teixeira BM, Hagiwara M, Cruz JCM, Hosie M. Feline Immunodeficiency Virus in South America. Viruses. 2012. DOI: https://doi.org/10.3390/v4030383

[23] Mohammadi H, Bienzle D. Pharmacological Inhibition of Feline Immunodeficiency Virus (FIV). Viruses. 2012. DOI: https://doi.org/10.3390/v4050708

[24] Chareza S, Lukic DS, Liu Y, Räthe A, Münk C, Zabogli E, et al.. Molecular and functional interactions of cat APOBEC3 and feline foamy and immunodeficiency virus proteins: different ways to counteract host-encoded restriction.. Virology. 2012. DOI: https://doi.org/10.1016/j.virol.2011.12.017

[25] Kenyon J, Lever A. The Molecular Biology of Feline Immunodeficiency Virus (FIV). Viruses. 2011. DOI: https://doi.org/10.3390/v3112192

[26] Zielonka J, Münk C. Cellular Restriction Factors of Feline Immunodeficiency Virus. Viruses. 2011. DOI: https://doi.org/10.3390/v3101986

[27] Hosie M, Pajek D, Samman A, Willett B. Feline Immunodeficiency Virus (FIV) Neutralization: A Review. Viruses. 2011. DOI: https://doi.org/10.3390/v3101870

[28] Yamamoto J, Sparger E, Ho EW, Andersen PR, O'Connor TP, Mandell CP, et al.. Pathogenesis of experimentally induced feline immunodeficiency virus infection in cats.. American Journal of Veterinary Research. 1988. DOI: https://doi.org/10.2460/ajvr.1988.49.08.1246

[29] Yamamoto J, Sanou MP, Abbott J, Coleman JK. Feline Immunodeficiency Virus Model for Designing HIV/AIDS Vaccines. Current HIV Research. 2010. DOI: https://doi.org/10.2174/157016210790416361

[30] Yamamoto J, Hansen H, Ho EW, Morishita T, Okuda T, Sawa TR, et al.. Epidemiologic and clinical aspects of feline immunodeficiency virus infection in cats from the continental United States and Canada and possible mode of transmission.. Journal of the American Veterinary Medical Association. 1989. DOI: https://doi.org/10.2460/javma.1989.194.02.213

[31] Roelke M, Brown M, Troyer J, Winterbach H, Winterbach C, Hemson G, et al.. PATHOLOGICAL MANIFESTATIONS OF FELINE IMMUNODEFICIENCY VIRUS (FIV) INFECTION IN WILD AFRICAN LIONS. Virology. 2009. DOI: https://doi.org/10.1016/j.virol.2009.04.011

[32] Hartmann K. Clinical aspects of feline immunodeficiency and feline leukemia virus infection. Veterinary Immunology and Immunopathology. 2011. DOI: https://doi.org/10.1016/j.vetimm.2011.06.003

[33] Calistri A, Vecchio C, Salata C, Celestino M, Celegato M, Göttlinger H, et al.. Role of the Feline Immunodeficiency Virus L-domain in the presence or absence of Gag processing: involvement of ubiquitin and Nedd4-2s ligase in viral egress. Journal of Cellular Physiology. 2009. DOI: https://doi.org/10.1002/jcp.21587

[34] Brown M, Munkhtsog B, Troyer J, Ross S, Sellers R, Fine AE, et al.. FELINE IMMUNODEFICIENCY VIRUS (FIV) IN WILD PALLAS’ CATS. Veterinary Immunology and Immunopathology. 2009. DOI: https://doi.org/10.1016/j.vetimm.2009.10.014

[35] Asquith C, Meli M, Konstantinova L, Laitinen T, Poso A, Rakitin O, et al.. Novel fused tetrathiocines as antivirals that target the nucleocapsid zinc finger containing protein of the feline immunodeficiency virus (FIV) as a model of HIV infection.. Bioorganic & Medicinal Chemistry Letters. 2015. DOI: https://doi.org/10.1016/j.bmcl.2014.12.047

[36] McDonnel S, Sparger E, Murphy B. Feline immunodeficiency virus latency. Retrovirology. 2013. DOI: https://doi.org/10.1186/1742-4690-10-69

[37] Moore A, Burrows A, Malik R, Ghubash R, Last R, Remaj B. Fatal disseminated toxoplasmosis in a feline immunodeficiency virus‐positive cat receiving oclacitinib for feline atopic skin syndrome. Veterinary dermatology (Print). 2022. DOI: https://doi.org/10.1111/vde.13097

[38] Rolim V, Casagrande R, Wouters AT, Driemeier D, Pavarini S. Myocarditis caused by Feline Immunodeficiency Virus in Five Cats with Hypertrophic Cardiomyopathy. Journal of Comparative Pathology. 2016. DOI: https://doi.org/10.1016/j.jcpa.2015.10.180

[39] Weese S, Nichols J, Jalali M, Litster A. The oral and conjunctival microbiotas in cats with and without feline immunodeficiency virus infection. Veterinary Research. 2015. DOI: https://doi.org/10.1186/s13567-014-0140-5

[40] Ertl R, Klein D. Transcriptional profiling of the host cell response to feline immunodeficiency virus infection. Virology Journal. 2014. DOI: https://doi.org/10.1186/1743-422X-11-52

[41] Meeker R, Bragg D, Poulton W, Hudson L. Transmigration of macrophages across the choroid plexus epithelium in response to the feline immunodeficiency virus. Cell and Tissue Research. 2012. DOI: https://doi.org/10.1007/s00441-011-1301-8

[42] Fletcher N, Meeker R, Hudson L, Callanan J. The neuropathogenesis of feline immunodeficiency virus infection: Barriers to overcome. The Veterinary Journal. 2010. DOI: https://doi.org/10.1016/j.tvjl.2010.03.022

[43] . feline immunodeficiency virus. CABI Compendium. 2022. DOI: https://doi.org/10.1079/cabicompendium.96141

[44] Marcondes M, Hirata KY, Vides J, Sobrinho LSV, Azevedo J, Vieira TS, et al.. Infection by Mycoplasma spp., feline immunodeficiency virus and feline leukemia virus in cats from an area endemic for visceral leishmaniasis. Parasites & Vectors. 2018. DOI: https://doi.org/10.1186/s13071-018-2716-9

[45] Poli A, Tozon N, Guidi G, Pistello M. Renal Alterations in Feline Immunodeficiency Virus (FIV)-Infected Cats: A Natural Model of Lentivirus-Induced Renal Disease Changes. Viruses. 2012. DOI: https://doi.org/10.3390/v4091372

[46] Collado VM, Doménech A, Miró G, Martín S, Escolar E, Gómez-Lucia E. Epidemiological Aspects and Clinicopathological Findings in Cats Naturally Infected with Feline Leukemia Virus (FeLV) and/or Feline Immunodeficiency Virus (FIV). Open Journal of Veterinary Medicine. 2012. DOI: https://doi.org/10.4236/OJVM.2012.21003

[47] Westman ME, Malik R, Norris JM. Diagnosing feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV) infection: an update for clinicians.. Australian Veterinary Journal. 2019. DOI: https://doi.org/10.1111/avj.12781

[48] Levy JK, Crawford PC, Tucker S. Performance of 4 Point‐of‐Care Screening Tests for Feline Leukemia Virus and Feline Immunodeficiency Virus. Journal of Veterinary Internal Medicine. 2017. DOI: https://doi.org/10.1111/jvim.14648

[49] Westman M, Malik R, Hall E, Sheehy P, Norris J. Determining the feline immunodeficiency virus (FIV) status of FIV-vaccinated cats using point-of-care antibody kits.. Comparative Immunology, Microbiology & Infectious Diseases. 2015. DOI: https://doi.org/10.1016/j.cimid.2015.07.004