Feline Papillomavirus

Overview and Taxonomy of Feline Papillomavirus

The family Papillomaviridae encompasses a remarkably diverse and ancient assemblage of small, non-enveloped, double-stranded DNA viruses that exhibit a profound degree of host specificity and tropism for cutaneous and mucosal epithelial tissues. Within the domestic cat (Felis catus), the study of papillomaviruses (PVs) has undergone a paradigm shift over the past three decades, transitioning from a state of relative obscurity prior to 1990 to a position of central importance in feline oncogenic virology [1]. Feline papillomaviruses (FcaPVs) are now recognized as significant etiological agents in a spectrum of proliferative and neoplastic skin and oral diseases, ranging from benign viral plaques and papillomas to malignant lesions such as Bowenoid in situ carcinoma (BISC), squamous cell carcinoma (SCC), and Merkel cell carcinoma (MCC) [2, 1]. The taxonomy of these viruses, their genomic architecture, and their phylogenetic relationships are critical for understanding their biological behavior, host interactions, and the molecular mechanisms that underpin their oncogenic potential.

Taxonomic Classification and Phylogenetic Context

The taxonomy of Felis catus papillomaviruses is grounded in the International Committee on Taxonomy of Viruses (ICTV) criteria, which define a new PV type based on less than 90% nucleotide identity in the L1 major capsid protein gene compared to any other known type [2]. To date, seven distinct FcaPV types have been fully sequenced and formally recognized: FcaPV-1 through FcaPV-7 [2, 3]. However, the total diversity is almost certainly underestimated, as metagenomic surveys and PCR-based studies of atypical lesions continue to yield sequences representing putative novel types [4, 5, 6]. These viruses are distributed across several genera within the Papillomaviridae family, reflecting a complex evolutionary history that includes both long-term virus-host co-speciation and more recent host-switching events.

Phylogenetic analyses have placed FcaPV-1 and FcaPV-2 within the Lambdapapillomavirus genus, a lineage that also includes PVs from a wide range of other mammalian species, including other felids, canids, and bovids [2, 7]. The Lambdapapillomavirus genus is of particular interest because it appears to have co-diverged with its mammalian hosts over millions of years. Seminal work by Rector et al. demonstrated that the evolutionary relationships among feline PVs perfectly mirror those of their feline hosts across species such as the domestic cat, bobcat (Lynx rufus), puma (Puma concolor), lion (Panthera leo), and snow leopard (Uncia uncia) [7]. This co-speciation pattern allowed for the first precise calibration of PV evolutionary rates, estimated at 1.95 × 10⁻⁸ nucleotide substitutions per site per year for the viral coding genome, a rate that underscores the remarkably slow evolutionary pace of these viruses [7]. However, the evolutionary trajectory for the domestic cat PVs may be more nuanced. The discovery of Leopardus wiedii papillomavirus type 1 (LwiePV1) has provided evidence suggesting a polyphyletic origin for feline Lambdapapillomaviruses, indicating that strict co-divergence may be punctuated by periods of host-independent evolution or even ancient cross-species transmission events, complicating the simple co-speciation model [8].

In contrast, the more recently described types, FcaPV-3, FcaPV-4, FcaPV-5, and FcaPV-6, are classified within the Taupapillomavirus genus [2, 6]. This genus-level distinction is significant because it correlates with differences in genome organization and, potentially, in pathogenic mechanisms and host tissue tropism. The seventh recognized type, FcaPV-7, has been proposed to also belong to the Taupapillomavirus genus based on its L1 gene sequence similarity, although its full taxonomic placement requires further confirmation [3]. Furthermore, a papillomavirus sequence amplified from a cutaneous papilloma on a cat’s nasal planum exhibited such a low level of similarity to all known feline PVs that it may represent a member of an entirely novel, unnamed genus [5]. This highlights that the current taxonomic framework for FcaPVs is likely incomplete, and continued surveillance of feline lesions for novel PV sequences is warranted. The co-detection of bovine papillomavirus (BPV) DNA, specifically feline sarcoid-associated PV (FeSarPV), in bovine cutaneous warts further suggests the potential for cross-species transmission events that could blur taxonomic boundaries and epidemiological patterns [9].

Genetic Architecture and Genomic Diversity

The FcaPV genome is a circular double-stranded DNA molecule, typically ranging from approximately 7,450 to 8,070 base pairs in length [10, 11], with a GC content typically around 54-55% [3, 6]. All characterized FcaPV genomes share a canonical structure comprising an early (E) region, a late (L) region, and a non-coding long control region (LCR) that contains the origin of replication and transcriptional regulatory elements. The early region encodes up to six proteins essential for viral replication, transcription, and cellular transformation: E1 (helicase), E2 (transcription/replication factor), E4 (involved in genome amplification and viral egress), E6, E7 (major oncoproteins), and, in some genera, E5 [2, 12]. The late region encodes the major and minor capsid proteins, L1 and L2, respectively, which are responsible for virion assembly and infectivity [2].

The genomic organization of FcaPVs exhibits genus-level variation. For Lambdapapillomaviruses like FcaPV-1 and FcaPV-2, a distinctive feature is the presence of a second noncoding region located between the early and late protein coding sequences, a structural hallmark not observed in all PV genera [7]. The E6 and E7 oncogenes of FcaPV-2 have been the subject of intense investigation due to their proven capacity to drive neoplastic transformation. The FcaPV-2 E6 protein, for example, has been shown to bind the cellular ubiquitin ligase E6AP, promoting the formation of a ternary E6-E6AP-p53 complex that results in the poly-ubiquitination and proteasomal degradation of the p53 tumor suppressor protein [13]. This mechanism is strikingly similar to that employed by high-risk human papillomaviruses (HR-HPVs). The E7 protein, in turn, binds and inactivates the retinoblastoma protein (pRb), leading to the release of E2F transcription factors, cell cycle progression, and the subsequent overexpression of p16CDKN2A, a surrogate biomarker commonly used in both human and feline PV-associated neoplasia [14, 15, 16]. This functional conservation across species underscores the translational relevance of the feline model for understanding PV-induced carcinogenesis, a point recognized by the World Health Organization’s (WHO) ongoing efforts to classify PV-associated cancers across species.

The genomic variability among FcaPV types is also reflected in their L1 sequences. The novel FcaPV-6 genome (7,453 bp) shares its highest nucleotide identity with FcaPV-3, yet it is sufficiently distinct to warrant designation as a separate type [6]. Similarly, the sequence of a putative novel PV from a feline basal cell carcinoma (BCC) was 75.1% similar to FcaPV-6, which meets the criteria for a new type [4]. This diversity suggests that the Taupapillomavirus genus may be particularly prone to genetic diversification, potentially driven by different ecological niches or host immune pressures.

Diversity of Recognized Feline Papillomavirus Types and Their Clinical Associations

The biological relevance of FcaPV taxonomy is directly linked to the disease phenotypes with which each type is associated. FcaPV-1 is predominantly linked to benign lesions, including oral papillomas and less commonly cutaneous viral plaques, and is not generally considered a high-risk oncogenic type [2, 17]. In contrast, FcaPV-2 is the most frequently detected and most extensively studied oncogenic PV in cats. A robust body of evidence, derived from PCR, in situ hybridization (ISH), and immunohistochemical studies, has established a causative role for FcaPV-2 in the development of feline cutaneous SCCs, particularly those arising in ultraviolet (UV)-protected haired skin, as well as BISC and viral plaques [10-12, 22]. FcaPV-2 DNA and/or transcripts have been identified in 56% of cutaneous SCCs from UV-protected sites, with expression patterns consistent with a non-productive, transforming infection [18]. Intriguingly, FcaPV-2 is also the primary viral agent implicated in feline Merkel cell carcinoma (MCC), a rare but highly aggressive neuroendocrine tumor. Over 90% of feline MCC cases are FcaPV-2-positive, and whole genome sequencing has confirmed the integration of the FcaPV-2 genome into the host cellular genome, a key hallmark of viral oncogenesis [14, 15, 16]. This association is so consistent that FcaPV-2 detection has become a diagnostic and mechanistic criterion for a distinct subset of feline MCC.

FcaPV-3, alongside FcaPV-4 and FcaPV-5, are members of the Taupapillomavirus genus and have been identified in a range of neoplastic lesions, including BISC and SCC, although with lower and geographically variable prevalence compared to FcaPV-2 [19, 20]. FcaPV-3 and FcaPV-4 have been detected in oral lesions, including oral in situ carcinomas and a subset of oral SCCs (FOSCC), suggesting a potential role in oral carcinogenesis [21, 22, 20]. A comprehensive multicenter study found FcaPV-2 DNA in 7.5% of FOSCC samples, followed by FcaPV-1 (6.2%), FcaPV-3 (5.3%), and FcaPV-5 (1.8%), while FcaPV-6 was not detected in oral SCCs [22]. These data indicate that multiple FcaPV types can be present in oral neoplasia, but their etiological contribution remains less definitive than for cutaneous lesions. FcaPV-6 was initially identified from a nasal planum SCC in a cat with concurrent lymphoma and has been confirmed as a distinct type within the Taupapillomavirus genus [6]. FcaPV-7, the most recently characterized type, was amplified from a BCC that exhibited unusual histological features and intense p16 immunostaining, but appears to be a rare infection in cats, as screening of a large panel of skin and oral samples failed to detect it [3]. This pattern of a few highly prevalent types and many rare types mirrors the diversity seen in human PV infections.

Epidemiologically, the prevalence of FcaPV-2 in cutaneous SCC is not uniform across geographic regions. A comparative study between Taiwan and Japan reported FcaPV-2 DNA in 46.9% of feline SCCs from Taiwan but only 8.6% from Japan, suggesting that environmental or host genetic factors may influence viral transmission and pathogenesis [23]. Conversely, FcaPV-3 and FcaPV-4 have been detected in SCCs from Japanese cats where FcaPV-2 was notably absent, further underscoring the geographic heterogeneity of FcaPV type distribution [23, 20]. The genoprevalence of FcaPV-2 in dermatologically healthy cats is astoundingly high, with one study reporting a rate of 98% via quantitative PCR, while the seroprevalence for anti-FcaPV-2 L1 antibodies was only 22% [24]. This striking discrepancy indicates that FcaPV-2 is a near-ubiquitous commensal on feline skin, with most infections being subclinical and non-productive, and that a robust immune response is only mounted in a subset of animals or during periods of active viral replication [24]. This high background prevalence poses a significant challenge for studies using PCR alone to establish causality, emphasizing the necessity for transcript-based or in situ detection methods to confirm active, transforming infection. The detection of transcriptionally active FcaPV-2 in the blood of healthy cats (25% of samples) further suggests that hematogenous spread may be a route for viral dissemination and intra-individual transmission, potentially contributing to the high prevalence observed on skin [17]. Understanding the taxonomy and natural history of these viruses is therefore not merely a taxonomic exercise but a prerequisite for the rational design of prophylactic or therapeutic interventions, a goal that aligns with the WOAH (World Organisation for Animal Health) priorities for managing emerging infectious diseases in companion animals.

Molecular Pathogenesis of Feline Papillomavirus Infection

The molecular pathogenesis of feline papillomavirus (FcaPV) infection represents a paradigm of virus-driven oncogenesis that shares profound mechanistic similarities with high-risk human papillomavirus (HPV) infections, yet exhibits distinct features rooted in the unique biology of the feline host. The constellation of FcaPV types, currently encompassing FcaPV-1 through FcaPV-7, with additional putative novel types awaiting formal classification, displays a spectrum of pathogenic potential ranging from asymptomatic colonization to the induction of highly aggressive malignancies, including Merkel cell carcinoma (MCC), Bowenoid in situ carcinoma (BISC), cutaneous squamous cell carcinoma (SCC), and oral squamous cell carcinoma (FOSCC) [14, 2, 3, 6]. Central to the oncogenic capacity of these viruses are the early genes E6 and E7, whose encoded oncoproteins subvert critical tumor suppressor pathways, thereby dismantling the cellular safeguards that normally prevent uncontrolled proliferation and genomic instability.

The E6 Oncoprotein and p53 Degradation: A Conserved Mechanism of Tumor Suppressor Inactivation

Among the most well-characterized molecular events in FcaPV-driven carcinogenesis is the E6-mediated degradation of the p53 tumor suppressor protein. High-risk HPV E6 promotes p53 ubiquitination and proteasomal degradation by recruiting the cellular ubiquitin ligase E6AP (UBE3A), forming a ternary complex that targets p53 for destruction. Compelling evidence demonstrates that FcaPV-2 E6 employs an analogous mechanism. In transfected CRFK cells expressing FcaPV-2 E6, the viral oncoprotein binds directly to E6AP, and this interaction facilitates the recruitment of p53 into a multimolecular complex [13]. The functional consequence is a marked increase in p53 poly-ubiquitination and accelerated proteasomal turnover, as evidenced by p53 stabilization upon treatment with proteasome inhibitors and the accumulation of p53 following E6AP gene knockdown [13]. This mechanistic conservation extends to naturally occurring feline SCC cell lines, where E6 mRNA levels correlate directly with the degree of p53 poly-ubiquitination and inversely with p53 protein abundance [13]. The biological significance of this pathway in vivo is underscored by immunohistochemical analyses of FcaPV-2-positive tumors: both MCC and cutaneous SCC consistently exhibit reduced or absent p53 immunoreactivity relative to virus-negative counterparts, consistent with E6-driven p53 depletion [14, 15, 16]. Intriguingly, however, p53 missense mutations have been identified in a substantial proportion of FcaPV-2-positive MCC cases (8 of 10 cases in one study), suggesting that while E6-mediated degradation may be the primary mechanism of p53 inactivation, secondary mutational events can occur, possibly reflecting the genomic instability that ensues from loss of p53-dependent checkpoint control [15]. This duality, viral oncoprotein-driven degradation coupled with acquired mutations, mirrors the complexity observed in HPV-associated human cancers and underscores the multifactorial nature of p53 pathway disruption in FcaPV-associated tumorigenesis.

E7-Mediated Rb Inactivation and the p16^{INK4A} Surrogate Marker

The retinoblastoma protein (pRb) serves as a critical gatekeeper of the G1/S cell cycle checkpoint, and its functional inactivation is a hallmark of papillomavirus-induced transformation. FcaPV-2 E7, like its high-risk HPV counterpart, binds to pRb and promotes its proteasomal degradation, thereby releasing E2F transcription factors that drive S-phase entry and unscheduled cellular proliferation [14, 16, 12]. The downstream consequence of pRb inactivation is a compensatory upregulation of p16^{INK4A} (hereafter p16), a cyclin-dependent kinase inhibitor that is normally subject to negative feedback regulation by functional pRb. In the context of E7-mediated pRb loss, this feedback loop is disrupted, leading to p16 overexpression that serves as a reliable surrogate marker of oncogenic PV activity [14, 15, 16]. Across multiple studies, FcaPV-2-positive MCC, BISC, and cutaneous SCC consistently exhibit intense, diffuse nuclear and cytoplasmic p16 immunolabeling, whereas virus-negative tumors display low or absent p16 expression [1, 8-10]. The diagnostic utility of p16 immunohistochemistry as a proxy for PV etiology in feline lesions is now well established, analogous to its role in human cervical and oropharyngeal cancer screening. However, it is critical to note that p16 overexpression is not entirely specific for PV infection; a subset of FOSCC cases, for instance, exhibit high p16 immunoreactivity in the absence of detectable PV DNA, suggesting alternative mechanisms of pRb pathway dysregulation, such as CDKN2A mutation or epigenetic silencing [25]. Conversely, some FcaPV-2-positive BISC lesions may harbor viral DNA without exhibiting the canonical p16 overexpression pattern, indicating that the relationship between viral infection and p16 induction is not absolute and may be influenced by additional factors, including viral integration status and host genetic background [26].

Viral Genome Integration and the Shift from Productive to Nonproductive Infection

A critical juncture in PV-associated carcinogenesis is the transition from a productive, episomal infection to a nonproductive, integration-driven state. In productive infections, viral replication is tightly linked to keratinocyte differentiation, with E6 and E7 expression restricted to the basal and parabasal layers and late gene expression (L1, L2) confined to the terminally differentiated superficial epithelium. This pattern is recapitulated in feline PV infections: in hyperplastic skin adjacent to FcaPV-2-positive SCC, RNAscope in situ hybridization reveals intense nuclear E6/E7 signals in the superficial epidermis, indicative of active viral replication, with punctate signals in the basal layers reflecting lower-level oncogene transcription [18]. However, within invasive SCC lesions, this differentiation-dependent pattern is lost; E6/E7 transcripts are detected uniformly across all epidermal layers, with progressive attenuation of the intense nuclear signals characteristic of productive replication [18]. This transcriptional dysregulation is a hallmark of malignant progression and reflects the loss of viral late gene expression and the uncoupling of oncogene transcription from epithelial differentiation.

Whole genome sequencing of FcaPV-2-positive MCC has provided definitive evidence for viral integration into the host genome, a finding that profoundly alters the molecular trajectory of infection [14, 15]. Integration disrupts the viral genome, typically within the E1 or E2 open reading frames, leading to loss of the E2 repressor protein and consequent deregulation of E6 and E7 transcription from the integrated viral sequences [14, 15]. The integrated viral DNA is maintained as a stable genetic element within the tumor cell population, ensuring constitutive oncogene expression that drives sustained proliferation and resistance to apoptosis. Importantly, not all FcaPV-2-positive tumors exhibit integration; episomal viral genomes can persist in some lesions, and the precise frequency of integration across different FcaPV-associated neoplasms remains an area of active investigation. Nevertheless, the demonstration of clonal integration in MCC provides a compelling mechanistic framework for understanding how FcaPV-2 contributes to malignant transformation, mirroring the integration-dependent pathogenesis of high-risk HPV in human cervical cancer.

Type-Specific Pathogenesis: Divergent Roles for Different FcaPV Types

While FcaPV-2 has emerged as the predominant oncogenic type in feline cutaneous neoplasia, the pathogenic landscape is more nuanced, with different FcaPV types exhibiting distinct tissue tropisms and disease associations. FcaPV-3 and FcaPV-4, for instance, have been identified in a subset of cutaneous SCC in Japan, where FcaPV-2 prevalence is notably lower than in other geographic regions, suggesting that the relative contribution of each viral type to SCC burden may vary geographically [23, 20]. FcaPV-4 has also been detected in BISC lesions, with chromogenic in situ hybridization localizing viral DNA specifically within neoplastic keratinocytes, providing strong evidence for a causative role [19]. In the oral cavity, FcaPV-1 and FcaPV-3 DNA have been amplified from in situ carcinomas of the tongue and oral mucosa, with intense p16 immunolabeling confirming PV-driven oncogenic activity [21]. The detection of FcaPV-3 DNA in a lingual metastasis from a pulmonary adenocarcinoma, however, serves as a cautionary note: viral presence does not necessarily imply viral etiology, and careful distinction between passenger viruses and true oncogenic drivers is essential [27].

The discovery of novel FcaPV types continues to expand the pathogenic repertoire. A putative novel PV type, most closely related to FcaPV-6, was identified in a feline basal cell carcinoma that contained widespread PV-induced cytopathic changes and intense p16 immunostaining, suggesting that this virus may contribute to tumorigenesis [4]. Similarly, FcaPV-7, a rare Tau-papillomavirus, was amplified from a basal cell carcinoma with unusual histological features, further broadening the spectrum of PV-associated lesions [3]. Cutaneous papillomas, long considered benign lesions with limited pathogenic significance, have also been associated with novel PV sequences, indicating that even ostensibly benign infections may harbor oncogenic potential under appropriate conditions [5]. The existence of at least six characterized FcaPV types, plus multiple putative novel types, underscores the genetic diversity of feline PVs and the need for comprehensive typing in both research and diagnostic settings.

Transcriptional Regulation and Viral Gene Expression Patterns

The expression profile of FcaPV genes within neoplastic lesions provides critical insights into the stage of infection and the nature of virus-host interaction. In FcaPV-2-positive FOSCC, reverse transcription PCR has detected mRNA transcripts for L1, E2, and E6/E7 in a subset of cases, with E6/E7 expression observed in 50% of DNA-positive tumors [28]. The detection of L1 mRNA, which encodes the major capsid protein, is particularly noteworthy, as it suggests that a proportion of FcaPV-2-associated oral SCC may support productive viral replication, a feature that is atypical for HPV-associated cancers in humans, where capsid gene expression is usually silenced in invasive lesions [28]. This finding raises the possibility that FcaPV-2 infection in the oral cavity may follow a different trajectory than in cutaneous sites, perhaps reflecting differences in the local immune environment or keratinocyte differentiation program. The expression of E2, which encodes a multifunctional regulatory protein involved in viral replication and transcription, further supports the notion of active, potentially productive infection in some FOSCC cases [28].

In the blood of healthy cats, FcaPV-2 DNA is detectable in approximately 25% of individuals, and importantly, mRNA transcripts for L1, E2, E6, and E7 are expressed, indicating active and possibly productive infection in non-epithelial tissues [17]. The detection of viral transcripts in peripheral blood raises intriguing questions about the potential for hematogenous dissemination of FcaPV-2 and its role in the establishment of infection at distant epithelial sites. FcaPV-1, in contrast, is not detectable in the blood, suggesting fundamental differences in the biology of these two viral types, with FcaPV-2 exhibiting a broader tissue tropism and greater capacity for systemic spread [17]. The high genoprevalence of FcaPV-2 on the skin of healthy cats, approaching 98% in some studies, combined with a relatively low seroprevalence (22%), suggests that most infections are subclinical and do not elicit a robust humoral immune response [24]. This immunological tolerance may facilitate persistent infection and create a permissive environment for oncogenic progression in susceptible individuals.

Downstream Cellular Pathways: Telomerase, Matrix Metalloproteinases, and Epithelial-Mesenchymal Transition

Beyond the core E6/E7-p53/pRb axis, FcaPV oncoproteins influence a broader network of cellular pathways that contribute to the malignant phenotype. In HPV-associated human cancers, E6 activates telomerase reverse transcriptase (TERT) expression, thereby promoting telomere maintenance and cellular immortalization. In feline oral SCC cell lines (SCCF2 and SCCF3) that harbor FcaPV-2, telomerase activity and TERT expression are readily detectable, along with expression of the TERT transcriptional activator c-Myc [29]. However, siRNA-mediated knockdown of FcaPV-2 E6 in these cells does not alter TERT, c-Myc, or matrix metalloproteinase (MMP) levels, suggesting that, unlike high-risk HPV E6, FcaPV-2 E6 does not directly regulate these pathways in the feline system [29]. This divergence highlights an important species-specific difference in viral pathogenesis: while the core mechanism of p53 degradation is conserved, the downstream consequences may differ, with FcaPV-2 relying on alternative, as-yet-uncharacterized mechanisms to drive telomerase activation and invasive behavior.

The expression of MMPs, which facilitate extracellular matrix degradation and tumor invasion, has been characterized in feline SCC with respect to PV status. In a pilot study of oral and cutaneous SCC, MMP-2, -9, -13, and -14 were consistently expressed, with MMP-2 and MMP-9 exhibiting strong cytoplasmic and nuclear immunostaining [30]. Notably, TIMP-2, a tissue inhibitor of MMPs, was significantly higher in PV-negative samples, suggesting that PV-positive tumors may exhibit a more invasive phenotype due to reduced protease inhibition [30]. The relationship between PV infection and the epithelial-mesenchymal transition (EMT) has also been explored. Feline head and neck SCCs, regardless of PV status, display features of partial EMT, including co-expression of epithelial markers (keratins, E-cadherin) and mesenchymal markers (vimentin, N-cadherin), along with the presence of CD44/CD271 double-positive cells at the invasive front that likely represent cancer stem cells [31]. Whether PV oncoproteins actively drive EMT or merely create a cellular context permissive for its induction remains to be determined, but the parallels with human HNSCC are striking and support the use of feline SCC as a comparative model.

The Role of p63, p73, and Stem Cell Pathways in Merkel Cell Carcinoma Pathogenesis

Feline MCC, which is >90% FcaPV-2-positive, provides a unique window into the molecular interplay between viral infection and cell lineage determination. Merkel cells are specialized neuroendocrine cells that share features with both epithelial and neuronal lineages, and their transformation in MCC involves the dysregulation of transcription factors that govern cell fate. Immunohistochemical profiling of FcaPV-2-positive MCC reveals a distinctive phenotype: tumor cells are uniformly negative for the basal cell markers p40 and p63, positive for p73, and express the neuroendocrine markers CK18, CK20, synaptophysin, and CD56, along with the stem cell-associated transcription factor SOX2 [32]. This pattern is notably different from that of normal Merkel cells, which are p73-negative, and from basal cells, which are p40-, p63-, and SOX2-positive [32]. The loss of p63 expression in MCC is particularly intriguing, given that p63 is essential for the proliferation and differentiation of keratinocyte stem cells and is required for the PV life cycle. The absence of p63 in FcaPV-2-positive MCC may reflect a dedifferentiation process in which infected cells have lost their basal cell identity and acquired a neuroendocrine phenotype, possibly as a consequence of viral oncogene expression disrupting normal differentiation programs [32]. The expression of SOX2, a key regulator of stem cell pluripotency and Merkel cell development, further supports the notion that FcaPV-2 drives tumor cells toward a primitive, stem-like state. This molecular reprogramming may be a critical determinant of the aggressive behavior and unique histology of feline MCC.

Comparative and Evolutionary Considerations

The molecular pathogenesis of FcaPV infection must be understood within the broader context of papillomavirus evolution and host adaptation. Phylogenetic analyses of feline PVs have revealed a remarkable pattern of virus-host co-speciation within the Felidae family, with the evolutionary relationships among feline PVs perfectly mirroring those of their feline hosts [7]. This long-term co-divergence, estimated at millions of years, has shaped the molecular interactions between virus and host, resulting in a finely tuned balance between viral persistence and pathogenicity. The overall evolutionary rate of feline PV coding genes is approximately 1.95 × 10⁻⁸ nucleotide substitutions per site per year, which is consistent with slowly evolving DNA viruses and confirms that feline PVs have co-evolved with their hosts over extended evolutionary timescales [7]. The finding that FcaPV-2 is highly prevalent on the skin of healthy cats, yet causes disease in only a minority of infected individuals, reflects this evolutionary equilibrium: the virus has evolved to replicate efficiently without causing excessive harm to its host, but occasional disruptions, whether due to immune suppression, genetic predisposition, environmental cofactors, or viral integration, can tip the balance toward malignant transformation. Understanding the molecular basis of this equilibrium is essential for identifying the factors that determine why some infected cats develop cancer while most remain disease-free.

Epidemiology and Risk Factors for Feline Papillomavirus

The epidemiological landscape of feline papillomavirus (FcaPV) infection is characterized by a complex interplay of viral diversity, host susceptibility, environmental co-factors, and geographical variation, reflecting a pathogen that is both ubiquitous in the domestic cat population and yet selectively pathogenic in a minority of infected individuals. Understanding the distribution, prevalence, and determinants of FcaPV-associated disease is critical for developing preventive strategies and for elucidating the mechanisms by which these viruses transition from commensal organisms to oncogenic drivers.

Global Prevalence and Geographical Variation in FcaPV Detection

The prevalence of FcaPV DNA in feline tissues varies dramatically depending on the anatomical site, the presence of lesions, the viral type under investigation, and the geographic origin of the sampled population. A seminal finding that underscores the ubiquity of these viruses is the observation that FcaPV-2 DNA can be detected on the skin of up to 98% of dermatologically healthy cats when highly sensitive quantitative PCR (qPCR) assays are employed [24]. This extraordinarily high genoprevalence in clinically normal animals indicates that FcaPV-2 is a near-commensal member of the feline cutaneous microbiome, a finding that fundamentally complicates the interpretation of PCR-based studies that merely detect viral DNA without localizing it within lesions. The seroprevalence of antibodies against the FcaPV-2 L1 major capsid protein in these same healthy cats is markedly lower, at approximately 22%, suggesting that while viral DNA is almost universally present on the skin, productive viral replication that elicits a robust humoral immune response is relatively uncommon [24]. This disparity between genoprevalence and seroprevalence implies that most cats harbor the virus in a latent or low-level replicative state, with only occasional episodes of active viral production that expose the immune system to capsid antigens.

Geographic variation in FcaPV prevalence is particularly striking and has been documented through comparative molecular epidemiological studies. A direct comparison of FcaPV-2 detection rates in feline squamous cell carcinoma (SCC) samples from Taiwan and Japan revealed a dramatic disparity: 46.9% of SCCs from Taiwan were PCR-positive for FcaPV-2, compared to only 8.6% of those from Japan [23]. This five-fold difference cannot be attributed solely to methodological differences, as the same PCR assays targeting the E1 and E7 genes were employed in both cohorts. Similarly, a multicentric study encompassing samples from Italy and Austria detected FcaPV-2 DNA in 7.5% of feline oral squamous cell carcinoma (FOSCC) specimens, with variable prevalence rates between the two countries [22]. In Japan, earlier studies had even failed to detect FcaPV-2 in feline SCCs altogether, instead identifying FcaPV-3 and FcaPV-4 as the predominant types in that geographic region [20]. These data collectively suggest that the distribution of FcaPV types is not uniform across the globe, and that regional differences in viral circulation, host genetics, environmental exposures, or diagnostic methodologies may influence the apparent association between specific FcaPV types and neoplastic disease. The detection of FcaPV-2 DNA in the blood of 25% of healthy cats in an Italian study further indicates that hematogenous dissemination may contribute to viral spread within and between individuals, potentially facilitating infection of anatomically distant epithelial sites [17].

Viral Type Distribution Across Lesion Types

The association between specific FcaPV types and distinct clinical and histopathological entities is a cornerstone of feline papillomavirus epidemiology. FcaPV-2 is by far the most extensively studied and most frequently implicated type in neoplastic disease. In feline Merkel cell carcinoma (MCC), the association is remarkably strong, with over 90% of cases testing positive for FcaPV-2 DNA and demonstrating expression of viral oncogenes E6 and E7 [14, 16]. This near-universal association, coupled with the demonstration of viral genome integration into the host genome and the consequent dysregulation of p53 and retinoblastoma (Rb) tumor suppressor pathways, provides compelling evidence that FcaPV-2 is a causative agent in feline MCC [15, 16]. The expression of FcaPV-2 E6 and E7 RNA has been localized within neoplastic cells of cutaneous SCCs using RNAscope in situ hybridization, with a hybridization pattern that mirrors that seen in human papillomavirus-induced cancers: punctate nuclear signals within all layers of the epidermis, indicative of unregulated oncogene transcription and loss of productive viral replication [18]. This pattern was observed in 56% of SCCs from ultraviolet (UV)-protected sites but in none of the SCCs from UV-exposed sites, suggesting that FcaPV-2 may play a more significant role in SCCs arising in anatomical locations not subjected to chronic solar damage [18].

FcaPV-2 is also the predominant type found in Bowenoid in situ carcinoma (BISC), a pre-invasive neoplastic lesion of the feline epidermis. Quantitative PCR and chromogenic in situ hybridization (CISH) studies have demonstrated FcaPV-2 DNA in 15 of 18 BISC cases, with viral DNA localized as nuclear dots within grouped neoplastic keratinocytes [19]. Fluorescence in situ hybridization (FISH) has confirmed the presence of intralesional FcaPV-2 DNA in 35.7% of BISCs, with probe annealing predominantly within the nuclei of koilocytes in the upper epidermal strata [26]. The lower detection rate by FISH compared to PCR likely reflects the ability of FISH to distinguish true intralesional virus from superficial contamination or commensal carriage, a critical distinction that PCR alone cannot provide.

Other FcaPV types exhibit more restricted or less consistent associations with disease. FcaPV-3 and FcaPV-4 have been detected in a subset of feline SCCs, particularly in Japan where FcaPV-2 prevalence is low [20]. FcaPV-3 DNA has been identified in oral in situ carcinomas, with 5 of 7 such lesions in one case series containing FcaPV-3 DNA [21]. FcaPV-4 has been detected in BISC lesions by both qPCR and CISH, with viral DNA localized specifically within neoplastic cells [19]. FcaPV-1, historically associated with oral papillomas, has been detected in 2 of 7 oral in situ carcinomas [21] and in a small proportion of FOSCCs [22]. The recently identified FcaPV-7, discovered within a basal cell carcinoma exhibiting unusual histological features and intense p16 immunostaining, appears to be a rare infection, with specific primers failing to amplify its DNA from 60 routine skin and oral samples [3]. This suggests that some FcaPV types may be highly adapted to specific niches or may cause disease only under exceptional circumstances.

Host-Related Risk Factors: Age, Breed, and Immunosuppression

Age is one of the most consistently identified risk factors for FcaPV-associated neoplastic disease. Feline MCC, which is almost universally FcaPV-2-positive, occurs predominantly in aged individuals, mirroring the epidemiology of human MCC [14]. Similarly, FOSCC, a subset of which is associated with FcaPV infection, primarily affects older, non-pedigree cats, with a median age at diagnosis typically exceeding 10 years [33, 34]. The average age of cats with FOSCC exhibiting high p16 immunoreactivity, a surrogate marker for papillomavirus involvement, is significantly lower than that of cats with low p16 expression, suggesting that FcaPV-associated oral cancers may arise in a slightly younger demographic than those driven by other etiologies [25]. This age-related susceptibility likely reflects the cumulative effects of immune senescence, prolonged viral persistence, and the accrual of genetic mutations over time.

Breed predisposition for FcaPV-associated disease is not well established, but available evidence suggests that non-pedigree domestic shorthair cats are overrepresented in most case series [33, 34, 35]. This may simply reflect the demographic composition of the general feline population rather than a true genetic susceptibility. No clear sex predisposition has been identified for most FcaPV-associated lesions, although one study of feline cystadenomatosis, a condition not strongly linked to papillomavirus, found a 2.24-fold higher relative risk in male cats [35]. For FOSCC, the literature remains equivocal, with some studies reporting no sex predilection and others suggesting a slight male predominance [33, 34].

Immunosuppression, whether iatrogenic or disease-induced, is a critical risk factor for FcaPV-associated disease. The development of multiple Bowenoid in situ carcinomas in a feline immunodeficiency virus (FIV)-positive cat, with lesions containing FcaPV-2 DNA sequences, illustrates the permissive effect of viral-induced immunosuppression on papillomavirus pathogenesis [36]. FIV infection compromises cell-mediated immunity, which is the primary defense against papillomavirus infection and the clearance of virus-infected cells. The presence of concurrent FIV infection may facilitate the establishment of persistent FcaPV infection, allow for higher viral loads, and promote the progression from subclinical infection to neoplastic transformation. Similarly, the observation that papillomas in older dogs occur due to systemic immunosuppression, warranting diagnostic investigation, has parallels in feline medicine, although systematic studies of immunosuppression as a risk factor for FcaPV disease in cats are lacking [37].

Environmental and Iatrogenic Risk Factors

Environmental exposures play a significant and potentially modifiable role in the etiology of FcaPV-associated cancers, particularly FOSCC. A multi-institutional epidemiologic study identified clumping clay cat litter and flea collar use as significant risk factors for FOSCC, with odds ratios of 1.66 and 4.48, respectively [38]. Crystalline silica, a known carcinogen, may be present in clay-based litters, and tetrachlorvinphos, a carcinogenic organophosphate, is a common active ingredient in flea collars. These findings suggest that chronic exposure to environmental carcinogens may act synergistically with FcaPV infection to promote oral carcinogenesis, potentially through mechanisms involving DNA damage, oxidative stress, or local immunosuppression within the oral mucosa.

Tobacco smoke exposure is another well-documented risk factor for FOSCC, with 35.2% of affected cats in one review having a history of such exposure [34]. This parallels the strong association between tobacco use and human head and neck squamous cell carcinoma (HNSCC), reinforcing the value of feline oral cancer as a comparative model for human disease [39]. The consumption of canned food and the use of deworming collars have also been associated with increased risk, although these findings are based on limited data [34]. The presence of oral comorbidities, including periodontal disease and feline chronic gingivostomatitis, has been reported in 6.4% of cats with FOSCC, suggesting that chronic inflammation may create a permissive microenvironment for FcaPV infection and neoplastic progression [34].

Ultraviolet radiation exposure is a well-established risk factor for cutaneous SCC in cats, particularly in lightly pigmented individuals and in anatomical sites with sparse hair coverage, such as the nasal planum and pinnae. The observation that FcaPV-2 E6/E7 RNA is detectable in SCCs from UV-protected sites but not from UV-exposed sites suggests that these two carcinogenic pathways, viral oncogenesis and UV-induced DNA damage, may be mutually exclusive in feline cutaneous SCC [18]. This dichotomy has important implications for risk stratification and prevention, as cats with predominantly UV-associated SCCs may benefit from sun avoidance measures, while those with FcaPV-associated SCCs may require different management strategies.

Molecular Epidemiological Insights: Viral Integration and Oncogene Expression

The transition from asymptomatic FcaPV infection to neoplastic disease is marked by specific molecular events that can be exploited for epidemiological surveillance and risk assessment. The integration of FcaPV-2 DNA into the host genome, demonstrated by whole genome sequencing in feline MCC, represents a critical step in viral carcinogenesis, as it allows for the unregulated expression of E6 and E7 oncogenes and the consequent degradation of p53 and inactivation of Rb [15, 16]. The detection of integrated viral DNA in tumor cells, but not in adjacent normal tissue, provides strong evidence for a causal role of FcaPV-2 in MCC development. The pattern of p16, pRb, and p53 immunoreactivity in FcaPV-positive lesions, characterized by intense p16 upregulation, loss of pRb, and loss of p53, serves as a reliable surrogate marker for viral oncogene activity and can be used to classify lesions as papillomavirus-associated [16, 25]. This immunohistochemical signature has been validated in FcaPV-2-positive MCC, BISC, and cutaneous SCC, and its presence in a lesion should prompt consideration of an underlying viral etiology.

The expression of FcaPV-2 E6 and E7 mRNA, detectable by RNAscope in situ hybridization, provides the most direct evidence of active viral oncogene transcription within neoplastic cells. In FcaPV-2-positive MCC, E6/E7 RNA is detected in the majority of tumor cells, and its expression correlates with the loss of p53 and pRb proteins [15]. In cutaneous SCC, the hybridization pattern shifts from the intense nuclear signals characteristic of productive viral replication in hyperplastic epidermis to the punctate signals indicative of integrated, non-productive infection in the invasive neoplasm [18]. This transition mirrors the pattern observed in human papillomavirus-induced cancers and provides a molecular signature of malignant progression. The detection of FcaPV-2 DNA and mRNA in the blood of healthy cats raises the intriguing possibility that hematogenous dissemination of the virus may contribute to the development of multifocal lesions or to the infection of anatomically distant epithelial sites [17]. The clinical significance of this finding for disease transmission and pathogenesis warrants further investigation.

Comparative and Evolutionary Perspectives

The epidemiology of FcaPV infection must be understood within the broader context of papillomavirus evolution and host adaptation. Phylogenetic analyses have demonstrated long-term virus-host co-speciation among feline papillomaviruses, with the evolutionary relationships between feline PVs perfectly mirroring those of their feline hosts [7]. This co-divergence, estimated to have occurred over millions of years, indicates that FcaPVs are highly adapted to their feline hosts and have co-evolved mechanisms to persist within the feline epithelial microenvironment without causing disease in the majority of infected individuals. The overall evolutionary rate for feline PV coding genomes has been estimated at 1.95 × 10⁻⁸ nucleotide substitutions per site per year, a rate that is consistent with other slowly evolving DNA viruses [7]. This ancient association explains the near-universal carriage of FcaPV-2 DNA on the skin of healthy cats and suggests that the virus has evolved strategies to avoid immune clearance and to maintain a commensal relationship with its host [24].

The identification of FcaPV DNA on the skin of a human house cat owner, assembled from metagenomic sequencing data, raises questions about the potential for cross-species transmission [10, 11]. While this finding likely represents incidental contamination rather than true infection, it underscores the ubiquity of these viruses in the domestic environment and the potential for human exposure. The detection of feline sarcoid-associated papillomavirus (FeSarPV) co-infecting bovine cutaneous warts further illustrates the capacity of some PV types to cross species barriers, although the clinical significance of such events remains unclear [9]. The recent discovery of a novel papillomavirus in a tree ocelot (Leopardus wiedii) suggests that the diversity of feline PVs extends well beyond the domestic cat and that wild felid populations may harbor additional viral types with unknown pathogenic potential [8].

In conclusion, the epidemiology of FcaPV infection is characterized by a high prevalence of subclinical carriage, marked geographic variation in viral type distribution, and a strong association between specific viral types, particularly FcaPV-2, and neoplastic disease. Host factors such as advanced age and immunosuppression, along with environmental exposures to carcinogens like tobacco smoke, crystalline silica, and organophosphate pesticides, modulate the risk of progression from asymptomatic infection to malignancy. The integration of viral DNA into the host genome and the consequent dysregulation of cell cycle control pathways represent the

Clinical Manifestations and Histopathology of Feline Papillomavirus-Associated Lesions

The clinical spectrum of feline papillomavirus (FcaPV) infection is remarkably diverse, ranging from benign, self-limiting cutaneous papillomas to highly aggressive malignant neoplasms such as Merkel cell carcinoma (MCC) and squamous cell carcinoma (SCC). The histopathological features of these lesions reflect the complex interplay between viral oncogene expression, host cellular differentiation programs, and immune surveillance. A thorough understanding of these manifestations is essential for accurate diagnosis, prognostication, and the identification of feline models for human papillomavirus (HPV)-associated cancers. The World Organisation for Animal Health (WOAH) recognizes the significance of understanding PV-associated diseases in companion animals, given their role as sentinels for environmental carcinogens and as comparative models for human oncology.

Cutaneous Viral Plaques and Bowenoid In Situ Carcinoma

Viral plaques represent the earliest clinically recognizable manifestation of FcaPV infection in the feline skin. These lesions typically present as well-demarcated, slightly raised, hyperpigmented or erythematous plaques that may be solitary or multifocal. They occur most frequently on the head, neck, and proximal limbs, though distribution can be widespread in immunosuppressed individuals. Histopathologically, viral plaques are characterized by focal to multifocal areas of epidermal hyperplasia with orthokeratotic or parakeratotic hyperkeratosis. The hallmark cytopathic effect is the presence of koilocytes, enlarged keratinocytes with perinuclear halos, pyknotic or irregularly shaped nuclei, and basophilic intranuclear inclusion bodies, predominantly within the granular and spinous layers [2, 1, 37]. These changes are most prominent in productive infections where the virus completes its life cycle in terminally differentiating keratinocytes.

Bowenoid in situ carcinoma (BISC) represents a neoplastic progression of viral plaques and is now recognized as a precursor lesion to invasive SCC. Clinically, BISC lesions appear as well-circumscribed, scaling, crusted plaques or patches with variable hyperpigmentation, often mistaken for actinic keratosis or inflammatory dermatoses. Multiple lesions are common, and they can coalesce into larger, irregularly shaped plaques. The biological behavior of BISC is notable for its potential for slow progression over months to years, though spontaneous regression is exceptionally rare.

The histopathology of BISC is distinctive and parallels high-grade vulvar intraepithelial neoplasia (VIN) in humans. Full-thickness epidermal dysplasia is observed, with loss of normal stratification, cellular pleomorphism, and an increased nuclear-to-cytoplasmic ratio. Mitotic figures, including atypical forms, are present at all levels of the epidermis. The basement membrane remains intact, distinguishing BISC from invasive SCC. A critical diagnostic feature is the preservation of PV-induced cytopathic changes, including koilocytosis and nuclear atypia in the superficial layers, which may be less prominent than in viral plaques owing to the integration of viral DNA and disruption of the productive viral life cycle [19, 1]. Chromogenic in situ hybridization (CISH) and fluorescence in situ hybridization (FISH) have demonstrated that FcaPV-2 DNA localizes predominantly within the nuclei of koilocytes in the upper epidermal strata, often distributed multifocally near the periphery of BISC lesions [19, 26]. This pattern is consistent with a nonproductive, transforming infection, analogous to HPV-induced high-grade squamous intraepithelial lesions in humans.

Immunohistochemically, BISC lesions exhibit intense, diffuse nuclear and cytoplasmic labeling for p16CDKN2A protein (p16), a surrogate biomarker for high-risk PV E7-mediated retinoblastoma protein (pRb) degradation [16, 25]. This p16 overexpression is a consistent finding across FcaPV-associated BISC and is used diagnostically to differentiate PV-driven lesions from other dysplastic conditions. In contrast, pRb and p53 protein expression are markedly reduced or absent in lesional cells, reflecting the functional inactivation of these tumor suppressors by viral oncoproteins [14, 15, 16].

Cutaneous Squamous Cell Carcinoma

Feline cutaneous squamous cell carcinoma (SCC) is one of the most common malignant skin tumors in cats, and a substantial subset is etiologically linked to FcaPV, particularly FcaPV-2. Clinically, PV-associated SCCs occur in two distinct epidemiological contexts: those arising on UV-protected sites (e.g., trunk, proximal limbs) and those on UV-exposed sites (e.g., nasal planum, pinnae, eyelids). PV DNA is detected in a significantly higher proportion of SCCs from UV-protected sites (up to 56%) compared to UV-exposed sites, where actinic damage is the dominant carcinogen [18, 40]. Grossly, these tumors present as exophytic, ulcerated, or crusted masses that may be invasive into underlying tissues. A critical clinical observation is the frequent development of SCC in cats with concurrent BISC or viral plaques, suggesting a field cancerization effect mediated by PV infection [14, 16].

Histopathologically, PV-associated SCC is characterized by invasive cords, nests, and islands of neoplastic squamous epithelial cells that breach the basement membrane and infiltrate the dermis and subcutis. Marked cellular pleomorphism, dyskeratosis, and abnormal keratinization (including keratin pearls) are typical. The stroma often exhibits a desmoplastic reaction with variable lymphoplasmacytic inflammation. A distinguishing feature from UV-induced SCC is the presence of residual PV-induced cytopathic changes, koilocytes, nuclear enlargement, and basophilic intranuclear inclusions, in the non-invasive, in situ component of the tumor or in adjacent epidermis [18, 2].

The molecular pathogenesis of FcaPV-driven SCC is elegantly elucidated by RNAscope in situ hybridization (ISH) studies. In hyperplastic perilesional skin, FcaPV-2 E6/E7 transcripts exhibit a pattern consistent with productive infection: intense nuclear signals in the superficial, terminally differentiating keratinocytes and sparse, punctate signals in the basal layers. However, within the SCC proper, the hybridization pattern shifts to exclusively punctate nuclear signals distributed throughout all epidermal layers, with progressive loss of the intense superficial signal [18]. This transition from a productive to a nonproductive, transforming infection is pathognomonic for PV-induced carcinogenesis and mirrors the pattern observed in HPV-driven human cervical and head and neck cancers. The punctate pattern reflects the presence of integrated viral genomes and the constitutive expression of E6 and E7 oncogenes, which drive genomic instability and clonal expansion.

FcaPV-2-positive SCCs display a characteristic immunophenotype: diffuse, strong p16 overexpression, absent or markedly reduced pRb, and low-to-absent p53 expression [16, 13]. This pattern is in stark contrast to UV-induced SCCs, which often exhibit p53 mutations and variable p16 expression. The E6 oncoprotein of FcaPV-2 directly binds to the ubiquitin ligase E6AP, facilitating the poly-ubiquitination and proteasomal degradation of p53, a mechanism functionally analogous to high-risk HPV E6 [13]. Similarly, the E7 oncoprotein binds and degrades pRb, leading to the release of E2F transcription factors and subsequent activation of p16 via a negative feedback loop [14, 12]. Notably, telomerase reverse transcriptase (TERT) expression and telomerase activity are elevated in FcaPV-2-positive SCC cell lines, but E6 knockdown does not alter TERT levels, suggesting that telomerase activation in feline SCC is mediated through pathways distinct from those in HPV-positive human cancers [29].

Merkel Cell Carcinoma

Feline Merkel cell carcinoma (MCC) is a rare but highly aggressive cutaneous neuroendocrine tumor that has emerged as a paradigm for PV-induced carcinogenesis in cats. Over 90% of feline MCC cases harbor FcaPV-2 DNA, a proportion that exceeds the association of human MCC with Merkel cell polyomavirus (MCV; approximately 80%) [14, 16]. Clinically, feline MCC typically presents as a solitary, rapidly growing, firm, dermal or subcutaneous nodule in aged animals (median age >12 years). The head, neck, and limbs are most commonly affected. Concurrent skin lesions, including BISC, viral plaques, and SCC, are frequently observed in the same patient, underscoring the field effect of FcaPV-2 infection [14, 16]. The tumors are highly aggressive, with a propensity for local recurrence and metastasis to regional lymph nodes and distant organs.

Histopathologically, feline MCC is composed of sheets, trabeculae, and nests of small, round, monomorphic cells with scant cytoplasm, finely stippled nuclear chromatin, and inconspicuous nucleoli, a classic "small blue cell tumor" morphology. The mitotic index is invariably high, and areas of necrosis are common. The tumor cells infiltrate the dermis and subcutis, often with a trabecular or organoid growth pattern. Immunohistochemically, the neoplastic cells co-express cytokeratin 18 (CK18), cytokeratin 20 (CK20), synaptophysin, CD56, and SOX2, while being uniformly negative for cytokeratin 14 (CK14), p40, and p63 [14, 32]. This immunophenotype is distinct from that of mature feline Merkel cells, which are p73-negative, and basal cells, which are p40/p63-positive and SOX2-negative. The expression of SOX2 in MCC suggests an origin from a primitive, undifferentiated precursor cell rather than from mature Merkel cells [32]. Furthermore, p73, a member of the p53 family, is consistently expressed in feline MCC but not in normal feline Merkel cells, indicating a tumor-specific activation of this pathway [32].

The molecular pathogenesis of FcaPV-2-positive MCC is now well characterized. Whole genome sequencing has confirmed the integration of FcaPV-2 DNA into the host genome, and RNAscope ISH detects E6/E7 transcripts in the vast majority of cases [15]. Immunohistochemically, these tumors exhibit the hallmark p16-high, pRb-low, p53-low phenotype, consistent with oncoprotein-mediated degradation of tumor suppressors [14, 16]. Interestingly, p53 missense mutations are also detected in a high proportion of both FcaPV-2-positive and -negative MCCs, suggesting that p53 mutagenesis may cooperate with viral oncogene expression in tumorigenesis [15]. In FcaPV-2-negative MCC, the tumor cells are p16-negative, pRb-positive, and exhibit strong p53 immunoreactivity, indicative of p53 mutation or stabilization by other mechanisms [16]. Cell line models recapitulate these features: the FcaPV-2-positive FMX-MCC01 line shows constitutive E6/E7 expression, p16 overexpression, and absent p53, while the FcaPV-2-negative AS-MCC01 line lacks p16 and expresses mutant p53 [15].

Oral In Situ Carcinoma and Oral Squamous Cell Carcinoma

Oral papillomavirus lesions in cats are increasingly recognized as distinct clinical entities with unique biological behavior. Feline oral in situ carcinoma (OISC) presents as flat or slightly raised, velvety, erythematous or white plaques on the dorsal surface of the tongue, with less frequent involvement of the gingiva and buccal mucosa. Affected cats typically exhibit chronic drooling (ptyalism), oral pain, and dysphagia for over six months prior to diagnosis [21]. Multiple oral lesions are common, and concurrent cutaneous lesions (viral plaques or BISC) may be present in a subset of patients. Critically, OISC demonstrates an indolent clinical course: survival times of six months or longer are common with supportive care, and progression to invasive SCC is exceptionally rare [21]. This is in stark contrast to conventional feline oral squamous cell carcinoma (FOSCC), which is one of the most aggressive malignancies in cats, with median survival times of less than two months.

Histopathologically, OISC lesions are morphologically indistinguishable from cutaneous BISC. Full-thickness epithelial dysplasia with loss of stratification, cellular pleomorphism, and atypical mitoses is present, confined by an intact basement membrane. PV-induced cytopathic changes, including koilocytosis and nuclear atypia, are visible in the majority of cases [21]. Intense, diffuse p16 immunolabeling is present in all lesions, confirming PV etiology. FcaPV-3 DNA is detected in the majority of OISC cases (5/7), with FcaPV-1 DNA found in the remaining two, suggesting that these PV types may have a particular tropism for oral mucosa [21]. This viral distribution contrasts with cutaneous lesions, where FcaPV-2 predominates.

The relationship between FcaPV and conventional FOSCC is more complex and remains an area of active investigation. Detection rates of FcaPV DNA in FOSCC vary widely by geographic region and methodological approach, ranging from 5–7% for FcaPV-2 in European studies to over 30% in some cohorts [22, 28, 34]. FcaPV-2 is the most prevalent type in FOSCC, but FcaPV-1, -3, -4, and -5 are also detected at lower frequencies [22]. Importantly, viral gene expression (E6, E7, E2, and L1 mRNA) has been demonstrated in a proportion of FcaPV-2 DNA-positive FOSCCs, suggesting active, possibly productive, infection [28]. However, the overall viral load in FOSCC is generally lower than in cutaneous SCC, and the p16/pRb/p53 immunophenotype in FOSCC is heterogeneous: only a small subset (approximately 14%) exhibit the p16-high, pRb-low pattern characteristic of high-risk PV infection, while the majority show low p16 and variable p53/pRb expression [25]. This suggests that FcaPV may play a causal role in only a minority of FOSCCs, with the majority being driven by other factors such as tobacco smoke exposure, chronic oral inflammation, and genetic mutations (e.g., TP53) [33, 38, 34].

Other Cutaneous Neoplasms: Basal Cell Carcinoma and Papillomas

Feline basal cell carcinoma (BCC) is a rare neoplasm, and the role of PV in its pathogenesis has only recently been appreciated. A seminal case report described a flank BCC in a 10-year-old cat that exhibited unusual histological features: a biphasic morphology with superficial epithelial nests and deep spindle-shaped cells, accompanied by prominent PV-induced cytopathic changes (koilocytes and large cytoplasmic bodies) in approximately 40% of tumor cells [4]. A novel PV sequence, most similar to FcaPV-6, was amplified from this lesion, while a concurrent thoracic BCC with typical histology was PV-negative. This observation suggests that a subset of feline BCCs may be PV-driven and that novel PV types may be associated with distinct histological variants. Similarly, a second novel PV type (FcaPV-7) has been identified in a BCC with intense p16 immunostaining [3]. These findings expand the spectrum of PV-associated lesions in cats and highlight the continued discovery of novel PV types with oncogenic potential.

Cutaneous papillomas are the least common PV-associated lesion in cats, in contrast to dogs. Clinically, they present as small, exophytic, pedunculated or sessile, cauliflower-like masses, often on the head or nasal planum [5, 1]. Histopathologically, they are characterized by papillary epidermal hyperplasia with pronounced orthokeratotic hyperkeratosis, prominent koilocytosis, and basophilic intranuclear inclusions. A novel PV sequence of putative new genus was recently identified in a

Diagnostic Approaches for Feline Papillomavirus Infection and Disease

The diagnostic investigation of feline papillomavirus (FcaPV) infection and its associated neoplastic sequelae demands a multi-modal, integrated strategy that transcends mere pathogen detection to establish causal relationships between viral presence and clinical disease. The inherent complexity of FPV biology, characterized by asymptomatic carriage, latent infection, and the requirement for active viral oncogene expression to drive carcinogenesis, necessitates a nuanced approach that combines histopathological evaluation, immunohistochemical profiling, nucleic acid detection, and increasingly, transcript localization and genomic integration analysis. The diagnostic armamentarium must therefore be deployed not in isolation but as a coordinated, hierarchical system, with each modality providing complementary evidence that collectively builds a compelling case for viral aetiology in a given lesion.

Histopathological and Cytological Evaluation

The foundational step in diagnosing FPV-associated disease remains thorough histopathological examination of biopsy specimens. While cytology can identify characteristic koilocytic change, enlarged epithelial cells with perinuclear halos, pyknotic nuclei, and frequently, basophilic intranuclear inclusion bodies, definitive characterization requires tissue architecture assessment. Specific lesion types demonstrate distinct histomorphological signatures. Viral plaques and Bowenoid in situ carcinomas (BISCs) exhibit full-thickness epidermal dysplasia with loss of maturation, cellular pleomorphism, and dyskeratosis, often with a “windblown” appearance of keratinocytes [2, 1]. The presence of large, glassy, amphophilic cytoplasmic bodies has been reported in a unique FcaPV-associated basal cell carcinoma (BCC), a finding not typical of conventional BCC and potentially pathognomonic for a novel viral association [4]. Similarly, feline Merkel cell carcinoma (MCC) displays a characteristic immunophenotype that distinguishes it from other cutaneous neuroendocrine tumours; importantly, greater than 90% of feline MCCs harbour FcaPV-2 DNA, establishing this histotype as a sentinel lesion for viral investigation [14, 16]. Oral in situ carcinomas, recently characterized as a distinct entity, histologically resemble cutaneous BISC but arise on the dorsal lingual mucosa, often with visible PV-induced cytopathic change in adjacent epithelium [21]. Histopathology alone cannot, however, discriminate between productive infection, where viral particles are assembled in differentiated keratinocytes, and non-productive oncogenic infection, where viral genome integration disrupts the normal viral life cycle. This distinction is critical, as the pathobiology, and therefore the diagnostic significance, differs fundamentally between these states.

Immunohistochemical Surrogate Marker Analysis

Immunohistochemistry (IHC) provides an indirect but powerful surrogate for FPV oncogenic activity, principally through the assessment of p16CDKN2A, retinoblastoma protein (pRb), and p53 expression. The mechanistic basis for this approach is well-established: high-risk PV E7 oncoprotein binds and degrades pRb, relieving negative feedback on p16 transcription and causing dramatic p16 overexpression [14, 16]. Simultaneously, E6 oncoprotein binds p53 via the E6AP ubiquitin ligase complex, targeting p53 for proteasomal degradation and resulting in markedly reduced p53 protein levels [15, 13]. Thus, the IHC triad of intense, diffuse p16 positivity, coupled with loss of pRb and p53 immunolabelling, has become a reliable hallmark of FPV-driven carcinogenesis across multiple lesion types, including cutaneous SCC [18], BISC [19], and notably, MCC [14, 32]. In one seminal series, 20 of 21 feline MCCs exhibited this exact pattern, while the single FcaPV-2-negative case demonstrated strong p53 immunoreactivity, consistent with a p53 mutation-driven, virus-independent pathway [16]. This pattern is not absolute, however. In feline oral squamous cell carcinoma (FOSCC), the relationship is less consistent: a subset of tumours display high p16 expression, but only a minority of these harbour detectable FPV DNA [25], suggesting that alternative mechanisms, such as CDKN2A promoter hypomethylation or Rb pathway disruption, can phenocopy the viral signature. Conversely, p16 overexpression is not universally observed in FPV-positive lesions; in one study of FcaPV-4-associated cutaneous SCC, p16 positivity was demonstrable [20], but the intensity and distribution may vary by viral type and lesion chronicity. The specificity of p16 IHC as a stand-alone test is therefore suboptimal, and it is best interpreted as part of a broader panel. The expression of additional markers, including p63, p73, and SOX2, further refines diagnostic precision: FcaPV-2-positive MCC is consistently p63-negative, p73-positive, and SOX2-positive, a profile that distinguishes it from basal cell carcinoma (p63-positive, p73-positive, SOX2-negative) and from normal Merkel cells [32]. These differential expression patterns underscore the value of multiplex IHC panels in classifying FPV-associated neoplasms.

Nucleic Acid Detection and Typing

Polymerase chain reaction (PCR) remains the most widely employed method for direct FPV DNA detection, owing to its high sensitivity, rapid turnaround, and ability to identify specific viral types. Both consensus PCR, using degenerate primers such as FAP59/FAP64 targeting the L1 open reading frame, and type-specific PCR are utilized. The latter is essential given the expanding diversity of feline PVs; at least seven FcaPV types (1–7) have now been fully sequenced [3, 5], and novel types continue to be identified through metagenomic approaches [10, 11, 4]. Type-specific PCR targeting E1, E6, E7, or L1 genes has revealed striking variation in viral prevalence across geographic regions and lesion types. For instance, FcaPV-2 is detected in 46.9% of cutaneous SCCs from Taiwan versus only 8.6% from Japan [23], while FcaPV-3 and FcaPV-4 predominate in certain Japanese cohorts [20]. In FOSCC, a multicentric European study detected FcaPV-2 in 7.5%, FcaPV-1 in 6.2%, and FcaPV-3 in 5.3% of cases, with marked inter-country variation [22]. These data emphasize that a single PCR assay targeting one virus may yield false negatives; comprehensive typing panels are mandatory for epidemiological accuracy and for establishing viral aetiology in individual cases.

However, PCR has a critical limitation: it detects DNA irrespective of biological activity. The high genoprevalence of FcaPV-2 on the skin of clinically healthy cats, reported as 98% in one Swiss study [24], means that PCR positivity alone does not distinguish incidental contamination or latent carriage from active, pathogenic infection. Quantitative PCR (qPCR) can provide some resolution; viral load in FOSCCs expressing the E6E7 oncogene transcript was higher than in non-expressing tumours and ulcerative lesions, although the difference did not reach statistical significance in one study [28]. More importantly, qPCR cannot localize the signal to neoplastic cells versus adjacent normal epithelium or infiltrating inflammatory cells. This spatial ambiguity is resolved by in situ hybridization techniques.

In Situ Hybridization: Localizing Viral Nucleic Acids

Chromogenic in situ hybridization (CISH) and fluorescence in situ hybridization (FISH) have emerged as indispensable tools for definitively associating FPV with specific cells within a lesion. By visualizing viral DNA or RNA directly within histological sections, these methods provide the spatial resolution that PCR lacks. In BISC, FISH using FcaPV-2-specific probes demonstrated intralesional DNA in 35.7% of lesions that were PCR-positive, with signal predominantly within koilocyte nuclei in the upper epidermal strata [26]. CISH for FcaPV-2 and FcaPV-4 achieved 100% concordance with qPCR in BISC, with signal appearing as discrete nuclear dots within grouped neoplastic keratinocytes [19]. The pattern of signal is diagnostically informative: in productive infections of hyperplastic epidermis, diffuse nuclear signal is observed in superficial differentiated layers, reflecting active viral replication; in neoplastic progression, the signal shifts to a punctate, scattered distribution across all epidermal layers, consistent with integrated, non-productive viral genomes [18]. This transition from a productive to a non-productive transcription pattern, documented by RNAscope in situ hybridization for FcaPV-2 E6/E7 transcripts in cutaneous SCC, provides powerful evidence of a causative role for the virus [18]. RNAscope is particularly valuable because it detects actively transcribed viral oncogenes rather than latent DNA. In feline MCC, RNAscope for FcaPV-2 E6/E7 identified viral mRNA in 18 of 21 FcaPV-2-positive cases, with signal confined to tumour cells [15]. Integration of FcaPV-2 into the host genome has been confirmed by whole genome sequencing (WGS) in MCC [14, 15], and the oncogenic significance of this integration is underscored by the resulting deregulation of E6/E7 expression. Detection of integrated viral sequences is not yet routine in clinical diagnostics but may become increasingly relevant as targeted antiviral or immunomodulatory therapies are developed.

Serology and Hematogenous Detection

Serological assays, while not part of routine diagnostic workup, have provided critical insights into the natural history of infection. Antibodies against the FcaPV-2 major capsid protein L1 are detected in only 22% of healthy cats, despite a genoprevalence of 98%, suggesting that viral replication is often confined to the skin and poorly exposed to the immune system [24]. In contrast, cats with BISC mount robust antibody responses, indicating that lesional progression is associated with enhanced viral antigen presentation or productive replication [24]. Interestingly, FcaPV-2 DNA and mRNA, including L1, E2, E6, and E7 transcripts, have been detected in the peripheral blood of 25% of healthy cats [17], raising the possibility of hematogenous dissemination as a route of intra-individual spread. The diagnostic utility of blood-based PCR for active infection remains uncertain, however, given that blood positivity may reflect transient viraemia from cutaneous sites rather than systemic infection.

Integration of Diagnostic Modalities and Prognostic Implications

The ultimate diagnostic challenge lies not in detecting FPV, but in determining whether the detected virus is driving the disease. A hierarchical diagnostic algorithm is therefore recommended. Initial histopathological assessment identifies lesions with characteristic morphology. IHC for p16, pRb, and p53 provides rapid, cost-effective screening; the classic “p16-high/pRb-low/p53-low” pattern strongly suggests viral oncogenesis. Confirmatory molecular testing using type-specific PCR or, ideally, CISH or RNAscope, localizes the virus to neoplastic cells and, in the case of RNAscope, demonstrates active oncogene transcription. WGS may be reserved for cases where integration status is required for research or therapeutic trial eligibility.

The prognostic value of these diagnostic approaches is emerging. In BISC, CISH evaluation of surgical margins did not consistently predict recurrence; some margin-positive cases healed without relapse, while margin-negative cases recurred, suggesting that viral clearance or re-infection dynamics, rather than residual neoplastic cells, may govern outcomes [41]. This finding cautions against over-interpretation of margin status in virally-induced lesions. Conversely, in FOSCC, the detection of FcaPV-2 E6/E7 expression may identify a subset with distinct biology, potentially less dependent on p53 mutation and more amenable to immune checkpoint inhibition, though this remains to be validated in clinical trials [28, 33]. The increasing recognition that FcaPV types exhibit tissue tropism (e.g., FcaPV-3 in oral lesions [21], FcaPV-7 in BCC [3]) further emphasizes the need for type-specific diagnostics to inform both prognosis and potential therapeutic strategies. As the field moves toward personalized veterinary oncology, the diagnostic framework for FPV will continue to evolve, integrating transcriptomic, epigenomic, and immune microenvironment profiling to refine patient stratification and guide targeted intervention.

Comparative Pathology: Feline Versus Human Papillomavirus-Associated Merkel Cell Carcinoma

The comparative pathology of Merkel cell carcinoma (MCC) across species presents a fascinating and clinically critical divergence in viral etiology, despite striking similarities in histomorphology, clinical behavior, and molecular dysregulation. In humans, the overwhelming majority of MCC cases, greater than 80%, are driven by the Merkel cell polyomavirus (MCPyV), a double-stranded DNA virus belonging to the Polyomaviridae family [14]. In stark contrast, the feline counterpart is predominantly associated with a papillomavirus, specifically Felis catus papillomavirus type 2 (FcaPV2), a member of the Papillomaviridae family [14, 16]. This fundamental difference in viral classification underscores a remarkable example of convergent oncogenesis, wherein two distinct viral families have evolved analogous molecular strategies to subvert host tumor suppressor pathways, culminating in a nearly identical neoplastic phenotype. Understanding these parallel yet distinct pathways is essential not only for advancing veterinary oncology but also for refining comparative models that inform human cancer biology.

Viral Etiology and Genomic Integration

The most salient point of divergence between human and feline MCC lies in the causative agent. Human MCC is defined by the clonal integration of MCPyV DNA into the host genome, with viral replication requiring a truncating mutation in the large T-antigen (LT) that abrogates replicative function while retaining the N-terminal domain responsible for transforming activity [14]. The virus is ubiquitous in the human population, yet only a minute fraction of infected individuals develop MCC, suggesting a critical interplay between viral integration, host immunity, and additional somatic mutations.

In the feline model, the landscape is dominated by FcaPV2. Polymerase chain reaction (PCR) analyses have demonstrated that over 90% of feline MCC cases harbor FcaPV2 DNA, with type-specific primers confirming the presence of the virus in 20 of 21 cases in a seminal study [16]. The complete FcaPV2 genome has been characterized from feline MCC lesions, and whole genome sequencing (WGS) has unequivocally demonstrated the integration of FcaPV2 DNA into the feline host genome, mirroring the integration event seen in human MCPyV-positive MCC [14, 15]. This integration is not a passive event; it is accompanied by the active transcription of viral oncogenes. RNAscope in situ hybridization (ISH) has localized FcaPV2 E6 and E7 mRNA transcripts within the nuclei of neoplastic Merkel cells in 18 of 21 FcaPV2-positive feline MCC cases, providing direct evidence that the viral oncogenes are being actively expressed within the tumor microenvironment [15]. This pattern of punctate nuclear signals is analogous to the expression pattern of MCPyV T-antigen transcripts in human MCC and is considered a hallmark of virus-driven oncogenesis [14, 18].

Molecular Mechanisms of Tumor Suppressor Dysregulation

The molecular pathogenesis of both human and feline MCC converges on the inactivation of two critical tumor suppressor pathways: the retinoblastoma (Rb) pathway and the p53 pathway. In human MCPyV-positive MCC, the viral large T-antigen binds to and inhibits pRb, while the small T-antigen contributes to cellular transformation through interactions with the MYC and mTOR pathways. p53 is often inactivated through indirect mechanisms, including overexpression of the p53 inhibitor MDM2, though direct mutation of TP53 is less common in the polyomavirus-driven subset [14].

FcaPV2 achieves the same functional outcome through its E6 and E7 oncoproteins, which are structurally and functionally analogous to the high-risk human papillomavirus (HPV) E6 and E7 proteins [12, 13]. FcaPV2 E7 binds to and promotes the degradation of pRb, leading to the release of E2F transcription factors and the subsequent upregulation of p16CDKN2A, a surrogate biomarker of Rb pathway inactivation [14, 16]. Immunohistochemical analyses of FcaPV2-positive feline MCC have consistently demonstrated intense, diffuse nuclear and cytoplasmic p16 immunoreactivity, coupled with a marked reduction or complete loss of pRb protein expression [16]. This p16 overexpression is a reliable surrogate marker for Rb inactivation and is routinely used in both human HPV-associated cancers and feline FcaPV2-associated neoplasms.

The p53 pathway is similarly targeted. FcaPV2 E6 binds to the cellular ubiquitin ligase E6AP, forming a ternary complex that recruits p53, leading to its poly-ubiquitination and subsequent proteasomal degradation [13]. This mechanism is virtually identical to that employed by high-risk HPV E6 in human cervical and oropharyngeal cancers. In FcaPV2-positive feline MCC, immunohistochemistry reveals a consistent loss of p53 protein expression, consistent with enhanced degradation [14, 16]. However, a critical nuance emerges from comparative genomic studies: missense mutations in the TP53 gene have been identified in 8 of 10 FcaPV2-positive feline MCC cases, as well as in the single FcaPV2-negative case examined [15]. This suggests that while FcaPV2 E6 drives p53 degradation, additional selective pressure for TP53 mutation may occur, potentially in a subset of tumor cells that escape E6-mediated degradation or as a secondary event during tumor progression. This dual mechanism, viral oncoprotein-mediated degradation coupled with somatic mutation, is less commonly observed in human MCPyV-positive MCC, where p53 mutations are infrequent, but is highly reminiscent of the landscape seen in HPV-positive human cancers [12].

Immunophenotype and Cell of Origin

The histogenesis of MCC has been a subject of considerable debate. In humans, the tumor expresses both epithelial (cytokeratin 20, CK20) and neuroendocrine markers (synaptophysin, chromogranin A, CD56), leading to hypotheses of derivation from either epidermal Merkel cells or a pluripotent stem cell. Feline MCC shares this dual phenotype, consistently expressing CK18, CK20, synaptophysin, and CD56 [14, 32]. However, detailed immunohistochemical profiling has revealed important differences that shed light on the cell of origin and the impact of viral oncogenesis.

A comprehensive study by Sumi et al. [32] examined the expression of basal cell markers p40, p63, and p73, as well as the stem cell marker SOX2 and cytokeratin 14 (CK14), in feline MCC compared to normal fetal, infant, and adult feline skin. Normal mature Merkel cells in adult and infant skin were found to be CK14-negative, CK18-positive, CK20-positive, SOX2-positive, synaptophysin-positive, and CD56-positive, but notably p73-negative [32]. In striking contrast, all 32 FcaPV2-positive feline MCC cases examined were immunopositive for p73, CK18, and SOX2, and were uniformly negative for p40, p63, and CK14 (31 of 32 cases) [32]. This immunophenotype, p73-positive, p63-negative, p40-negative, CK14-negative, is fundamentally different from both normal Merkel cells (which are p73-negative) and normal basal keratinocytes (which are p40-positive, p63-positive, and SOX2-negative) [32].

The loss of p63 expression in feline MCC is particularly intriguing. p63 is a master regulator of epithelial stem cell maintenance and is essential for the life cycle of papillomaviruses, which require the transcription factor for viral replication in basal keratinocytes [32]. The absence of p63 in FcaPV2-positive MCC suggests that the virus has hijacked a cell that has already undergone or is undergoing a distinct differentiation program, or that the viral oncogenes themselves drive the loss of p63 expression as part of the transformation process. This unique cytokeratin profile (CK14-negative, CK18-positive, CK20-positive) and the aberrant expression of p73 and SOX2 suggest that feline MCC may arise from a progenitor cell that is distinct from both the classical Merkel cell and the basal keratinocyte, potentially a neuroendocrine-committed precursor that becomes permissive for FcaPV2 infection and transformation [32]. This contrasts with the human paradigm, where MCPyV is thought to target dermal fibroblasts or pre-existing Merkel cells, and where p63 expression is often retained in a subset of tumors.

Clinical and Pathological Parallels

Despite the divergent viral etiology, the clinical and pathological features of feline and human MCC are remarkably congruent. Both diseases occur predominantly in aged individuals, with a median age of onset in cats of approximately 12–15 years, mirroring the elderly human population [14]. Both are highly aggressive, with a propensity for local recurrence, regional lymph node metastasis, and distant dissemination. Histologically, both are characterized by sheets and nests of small, round, blue cells with scant cytoplasm, finely granular chromatin, and a high mitotic rate, often exhibiting a trabecular or organoid growth pattern [14].

A particularly compelling comparative feature is the frequent association of MCC with concurrent papillomavirus-associated skin lesions. In cats, FcaPV2-positive MCC often arises in the context of other FcaPV2-driven lesions, such as viral plaques, Bowenoid in situ carcinoma (BISC), and squamous cell carcinoma (SCC) [14, 16]. This suggests a field cancerization effect, wherein the same viral infection drives multifocal neoplastic transformation across the cutaneous epithelium. In humans, while MCPyV-positive MCC is not typically associated with concurrent HPV-driven lesions, the concept of viral field cancerization is well established in HPV-related anogenital and oropharyngeal cancers. The feline model thus provides a unique opportunity to study the evolution of a single virus across a spectrum of neoplastic phenotypes, from benign viral plaques to in situ carcinoma to fully malignant MCC and SCC.

From an epidemiological perspective, the prevalence of FcaPV2 in feline MCC appears to be geographically consistent, with studies from Japan, Europe, and North America all reporting positivity rates exceeding 90% [14, 16]. This contrasts with the geographic variability observed in FcaPV2 prevalence in feline SCC, which ranges from 8.6% in Japan to 46.9% in Taiwan, suggesting that MCC may be a more obligate viral tumor in cats [23]. The World Organisation for Animal Health (WOAH) recognizes the importance of understanding viral oncogenesis in companion animals, as these models can inform both veterinary and human public health strategies. Similarly, the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have long emphasized the role of viral infections in human carcinogenesis, and the feline MCC model offers a powerful, naturally occurring system to study papillomavirus-driven neuroendocrine carcinogenesis in a manner that is directly translatable to human HPV-associated malignancies.

Genomic Characterization and Strain Diversity of Feline Papillomaviruses

The genomic architecture of feline papillomaviruses (FcaPVs) reveals a sophisticated molecular organization that mirrors the canonical papillomavirus blueprint while harboring distinctive features that underpin their pathogenic potential. The complete genome of FcaPV2, the most extensively characterized and clinically consequential type, is a circular double-stranded DNA molecule of approximately 7,500 to 8,000 base pairs, with the prototypical strain P20 measuring precisely 8,069 bp and exhibiting a GC content of 54.38% [10, 11]. This genome displays the classical organization typical of feline papillomaviruses, comprising six annotated protein-coding regions that encompass both early and late gene functions [11]. The early region encodes the regulatory and oncogenic proteins E1, E2, E6, and E7, while the late region encodes the structural capsid proteins L1 and L2, arranged in a configuration that is evolutionarily conserved across the Papillomaviridae family [10, 11, 7]. The genomic organization of FcaPV7, a recently identified type, similarly contains five predicted early proteins and two late proteins, with an L1 open reading frame (ORF) showing 77% similarity to that of FcaPV6, confirming its placement within the Tau-papillomavirus genus [3].

Phylogenetic Classification and Evolutionary Relationships

The taxonomic landscape of feline papillomaviruses has expanded considerably, with six fully sequenced types, FcaPV1 through FcaPV6, now recognized in domestic cats, plus the putative FcaPV7 [2, 3]. Phylogenetic analysis has been instrumental in resolving the evolutionary relationships among these viruses, revealing that FcaPV3, FcaPV4, FcaPV5, and FcaPV6 cluster together within a novel genus alongside FcaPV7, supported by robust bootstrap values [3, 6]. This grouping is significant because it suggests a shared evolutionary trajectory and potentially similar biological properties among these types. The complete genome sequencing of FcaPV6 from a nasal biopsy of a cat with recurrent squamous cell carcinoma (SCC) of the nasal planum demonstrated a 7,453 bp genome with closest homology to FcaPV3, yet sufficiently distinct to warrant classification as a new viral type [6]. Such phylogenetic resolution has profound implications for understanding the evolutionary dynamics of feline papillomaviruses and their host adaptation.

Perhaps the most elegant demonstration of virus-host coevolution in the feline papillomavirus system comes from the seminal work of Rector and colleagues, who sequenced complete papillomavirus genomes from four additional wild feline species, Lynx rufus (bobcat), Puma concolor (mountain lion), Panthera leo persica (Asiatic lion), and Uncia uncia (snow leopard), and demonstrated that the evolutionary relationships among these feline papillomaviruses perfectly mirror those of their feline hosts, despite a complex and dynamic phylogeographic history [7]. All feline papillomaviruses belong to the Lambdapapillomavirus genus and share a distinctive genomic feature: an unusual second noncoding region situated between the early and late protein regions, which is present exclusively in members of this genus [7]. By applying host species divergence times, this landmark study provided the first precise estimates for papillomavirus evolutionary rates, calculating an overall rate of 1.95 × 10⁻⁸ nucleotide substitutions per site per year for the viral coding genome, with a 95% confidence interval of 1.32 × 10⁻⁸ to 2.47 × 10⁻⁸ [7]. This rate, calibrated across millions of years of feline evolution, establishes a fundamental temporal framework for understanding the molecular clock of papillomaviruses and their long-term association with felid hosts.

However, subsequent discoveries have complicated this co-speciation narrative. The identification of Leopardus wiedii papillomavirus type 1 (LwiePV1), the first papillomavirus type within Lambdapapillomavirus in a Leopardus host, provided evidence for a polyphyletic origin of feline lambda papillomaviruses and represents an exception to the strict codivergence between feline lambda papillomaviruses and their feline hosts [8]. This finding suggests that while co-speciation is a dominant evolutionary force shaping feline papillomavirus diversity, host-independent evolution and cross-species transmission events may also contribute to the phylogenetic complexity observed in this viral family [8]. Such evolutionary plasticity has direct implications for understanding the pathogenic potential of these viruses and their capacity to adapt to new hosts.

Genomic Integration and Oncogene Expression

The molecular pathogenesis of FcaPV2-associated neoplasia is fundamentally linked to the genomic integration of viral DNA into the host genome and the consequent dysregulated expression of viral oncogenes. Whole genome sequencing of FcaPV2-positive Merkel cell carcinomas (MCC) has definitively demonstrated the integration of FcaPV2 DNA into the feline host genome, representing a critical step in malignant transformation [14, 15]. This integration event is not merely a stochastic occurrence but appears to be mechanistically linked to the oncogenic process, as evidenced by the consistent detection of FcaPV2 E6/E7 mRNA within tumor cells using RNAscope in situ hybridization, which identified viral transcripts in 18 of 21 FcaPV2-positive MCC cases while no signal was detected in FcaPV2-negative controls [15].

The transcriptional patterns observed during neoplastic progression provide compelling evidence for the causative role of FcaPV2 in feline cutaneous SCC. In hyperplastic skin adjacent to SCC lesions, RNAscope in situ hybridization reveals intense nuclear signals within the superficial epidermis, consistent with productive viral replication, whereas within the SCC itself, punctate signals are distributed across all epidermal layers with progressive loss of the intense nuclear signals characteristic of productive infection [18]. This hybridization pattern is pathognomonic for unregulated E6 and E7 transcription and decreased viral replication, mirroring precisely the pattern observed in human papillomavirus-induced cancers as they transition from hyperplastic lesions containing productive infections to nonproductive neoplasms [18]. Such transcriptional reprogramming indicates that viral oncogene expression becomes uncoupled from the normal differentiation-dependent viral life cycle, a hallmark of papillomavirus-mediated carcinogenesis.

The molecular consequences of FcaPV2 integration and oncogene expression are profound and mechanistically analogous to those of high-risk human papillomaviruses. FcaPV2 E6 binds to the cellular ubiquitin ligase E6AP, promoting the formation of an E6AP/p53 ternary complex that facilitates p53 poly-ubiquitination and subsequent proteasomal degradation [13]. This mechanism was experimentally confirmed in both artificial transfection models and spontaneous feline SCC cell lines expressing FcaPV2 E6, where p53 protein levels and poly-ubiquitination degree were directly proportional to E6 mRNA levels [13]. Simultaneously, FcaPV2 E7 mediates inhibition of the retinoblastoma protein (pRb), leading to the characteristic overexpression of p16CDKN2A through loss of the negative feedback loop normally maintained by functional pRb [14, 16]. The coordinated inactivation of these two critical tumor suppressor pathways, p53 degradation via E6 and pRb inhibition via E7, creates a cellular environment permissive for genomic instability, unchecked proliferation, and malignant progression. In FcaPV2-positive MCC, increased p16 and decreased pRb and p53 protein levels were consistently observed, whereas FcaPV2-negative MCC cases exhibited strong p53 immunoreactivity suggesting alternative mutational mechanisms of p53 inactivation [16].

Strain Diversity and Geographic Variation

The genetic diversity of feline papillomaviruses extends beyond inter-type variation to encompass significant intra-type strain heterogeneity that varies across geographic regions. A multicentric study investigating the presence of multiple FcaPV types in feline oral squamous cell carcinoma (FOSCC) across Italy and Austria revealed variable circulation rates of different viral types, with FcaPV2 detected in 7.5% of specimens, FcaPV1 in 6.2%, FcaPV3 in 5.3%, FcaPV4 in 0.9%, and FcaPV5 in 1.8%, while FcaPV6 was not detected in any sample [22]. These prevalence patterns suggest that different FcaPV types have distinct ecological niches and transmission dynamics that may be influenced by host genetics, environmental factors, or viral fitness characteristics.

Striking geographic variation in FcaPV2 prevalence has been documented through comparative studies between Asian populations. While 46.9% (23/49) of feline SCC cases from Taiwan were PCR-positive for FcaPV2, only 8.6% (3/35) of cases from Japan harbored detectable FcaPV2 DNA [23]. This dramatic disparity cannot be attributed to methodological differences, as both cohorts were analyzed using identical conventional PCR reactions targeting E1 and E7 genes [23]. Intriguingly, earlier Japanese studies had detected FcaPV3 and FcaPV4, but not FcaPV2, in feline SCC samples, suggesting that the apparent absence of FcaPV2 in some regions may reflect genuine differences in viral circulation rather than detection failure [23, 20]. The deletion of the 334th tryptophan residue in the L1 ORF of FcaPV4 detected in Japanese samples further underscores the potential for region-specific genomic variations that may influence viral fitness, antigenicity, or pathogenic potential [20].

The discovery of novel FcaPV types continues to expand the known diversity of feline papillomaviruses, with implications for understanding both the evolutionary breadth and pathogenic spectrum of these viruses. FcaPV6 was identified through total DNA sequencing of a nasal biopsy from a cat with recurrent SCC of the nasal planum, with the complete 7,453 bp genome showing closest phylogenetic affinity to FcaPV3 [6]. FcaPV7 was amplified from a basal cell carcinoma containing unusual histological evidence of papillomavirus infection and intense p16CDKN2A immunostaining, with its 7,467 bp genome classified within the Tau-papillomavirus genus based on greater than 60% similarity to FcaPV3, FcaPV4, FcaPV5, and FcaPV6 [3]. Despite its association with neoplastic lesions, FcaPV7 appears to be a rare infection, as specific primers failed to amplify FcaPV7 DNA from any of 60 samples from the mouth and skin of cats [3]. This sporadic detection pattern raises intriguing questions about whether FcaPV7 represents an emerging pathogen, a highly host-restricted virus with limited transmission potential, or a virus that primarily circulates in reservoir populations with occasional spillover into domestic cats.

Genomic Signatures of Viral Adaptation and Host Tropism

The genomic features of feline papillomaviruses reveal signatures of adaptation to the feline epithelial microenvironment and provide molecular explanations for their tissue tropism and pathogenic specificity. The complete genome of strain P20, assembled through metagenomic sequencing from human skin of a house cat owner, demonstrated approximately 75% sequence similarity to other feline papillomavirus genomes, consistent with the species-specific nature of papillomavirus infection [10, 11]. This degree of sequence divergence is typical for papillomaviruses infecting different host species and reflects the co-evolutionary pressures that shape viral genome evolution. The detection of FcaPV2 DNA in the blood of 25% (26/103) of healthy cats, with evidence of active viral gene expression including L1, E2, E6, and E7 transcripts, suggests that hematogenous dissemination may represent an additional route of viral spread that contributes to the establishment of multifocal lesions and potentially to viral persistence [17].

The genomic integration patterns observed in FcaPV2-associated MCC provide mechanistic insights into viral oncogenesis that parallel human Merkel cell polyomavirus-associated MCC. Whole genome sequencing of two FcaPV2-positive MCC cases revealed integration of viral genes into the host genome, with the integrated viral DNA retaining the capacity for oncogene expression [15]. This integration event appears to be a prerequisite for malignant transformation, as it allows for sustained expression of E6 and E7 in the absence of the viral replication cycle, thereby driving continuous cell cycle progression and inhibiting apoptosis. The demonstration that FcaPV2 E6/E7 mRNA is detectable in xenograft tissues from FcaPV2-positive MCC cell lines, coupled with the absence of p53 protein and the presence of only occasional pRb-positive cells, confirms that the integrated viral oncogenes maintain their transforming activity even in the context of heterotopic transplantation [15]. These findings establish feline MCC as a robust spontaneous animal model for studying papillomavirus-mediated carcinogenesis and underscore the translational relevance of feline papillomavirus research to human oncology.

References

[1] Munday J, Sharp C, Beatty J. Novel viruses: Update on the significance of papillomavirus infections in cats. Journal of feline medicine and surgery. 2018. DOI: https://doi.org/10.1177/1098612X18808105

[2] Medeiros-Fonseca B, Faustino-Rocha A, Medeiros R, Oliveira PA, Costa RMGd. Canine and feline papillomaviruses: an update. Frontiers in Veterinary Science. 2023. DOI: https://doi.org/10.3389/fvets.2023.1174673

[3] Munday J, Gedye K, Knox M, Pfeffer H, Lin X. Genetic characterisation of Felis catus papillomavirus type 7, a rare infection of cats that may be associated with skin cancer.. Veterinary Microbiology. 2023. DOI: https://doi.org/10.1016/j.vetmic.2023.109813

[4] Munday J, Hunt H, Orbell G, Pfeffer H. Detection of a Novel Papillomavirus Type within a Feline Cutaneous Basal Cell Carcinoma. Veterinary Sciences. 2022. DOI: https://doi.org/10.3390/vetsci9120671

[5] Munday J, Wong A, Julian A. Cutaneous papilloma associated with a novel papillomavirus sequence in a cat. Journal of Veterinary Diagnostic Investigation. 2022. DOI: https://doi.org/10.1177/10406387221107152

[6] Carrai M, Brussel KV, Shi M, Li C, Chang W, Munday J, et al.. Identification of a Novel Papillomavirus Associated with Squamous Cell Carcinoma in a Domestic Cat. Viruses. 2019. DOI: https://doi.org/10.3390/v12010124

[7] Rector A, Lemey P, Tachezy R, Mostmans S, Ghim S, Doorslaer Kv, et al.. Ancient papillomavirus-host co-speciation in Felidae. Genome Biology. 2007. DOI: https://doi.org/10.1186/gb-2007-8-4-r57

[8] Dolz G, Lecis R, Solorzano-Morales A, Aguilar-Vargas F, Solorzano-Scott T, Peña R, et al.. Leopardus wiedii Papillomavirus type 1, a novel papillomavirus species in the tree ocelot, suggests Felidae Lambdapapillomavirus polyphyletic origin and host-independent evolution.. Infection, Genetics and Evolution. 2020. DOI: https://doi.org/10.1016/j.meegid.2020.104239

[9] Silva MAd, Carvalho CCR, Coutinho LCA, Reis MC, Batista MVdA, Castro RSd, et al.. Co-infection of Bovine Papillomavirus and feline-associated Papillomavirus in bovine cutaneous warts.. Transboundary and Emerging Diseases. 2012. DOI: https://doi.org/10.1111/j.1865-1682.2012.01307.x

[10] Graham EH, Adamowicz MS, Angeletti P, Clarke JL, Fernando S, Herr JR. Genome Sequence of Feline Papillomavirus Strain P20 Assembled from Metagenomic Data from the Skin of a House Cat Owner. Microbiology Resource Announcements. 2022. DOI: https://doi.org/10.1128/mra.01070-21

[11] Graham EH, Adamowicz MS, Angeletti P, Clarke JL, Fernando S, Herr JR. Feline Papillomavirus Strain P20 Assembled from Metagenomic Data Isolated from the Human Skin of a House Cat Owner. bioRxiv. 2021. DOI: https://doi.org/10.1101/2021.11.01.466825

[12] Cruz-Gregorio A, Aranda-Rivera AK, Pedraza-Chaverri J. Pathological Similarities in the Development of Papillomavirus-Associated Cancer in Humans, Dogs, and Cats. Animals. 2022. DOI: https://doi.org/10.3390/ani12182390

[13] Altamura G, Power K, Martano M, Uberti Bd, Galiero G, Luca Gd, et al.. Felis catus papillomavirus type-2 E6 binds to E6AP, promotes E6AP/p53 binding and enhances p53 proteasomal degradation. Scientific Reports. 2018. DOI: https://doi.org/10.1038/s41598-018-35723-7

[14] Chambers J, Ito S, Uchida K. Feline papillomavirus-associated Merkel cell carcinoma: a comparative review with human Merkel cell carcinoma. Journal of Veterinary Medical Science. 2023. DOI: https://doi.org/10.1292/jvms.23-0322

[15] Ito S, Chambers J, Sumi A, Omachi T, Haritani M, Nakayama H, et al.. Genomic integration and expression of Felis catus papillomavirus type 2 oncogenes in feline Merkel cell carcinoma. Veterinary Pathology-Supplement. 2022. DOI: https://doi.org/10.1177/03009858221139197

[16] Ito S, Chambers J, Sumi A, Yamashita-Kawanishi N, Omachi T, Haga T, et al.. Involvement of Felis catus papillomavirus type 2 in the tumorigenesis of feline Merkel cell carcinoma. Veterinary Pathology-Supplement. 2021. DOI: https://doi.org/10.1177/03009858211045440

[17] Altamura G, Jebara G, Cardeti G, Borzacchiello G. Felis catus papillomavirus type-2 but not type-1 is detectable and transcriptionally active in the blood of healthy cats.. Transboundary and Emerging Diseases. 2018. DOI: https://doi.org/10.1111/tbed.12732

[18] Hoggard N, Munday J, Luff J. Localization of Felis catus papillomavirus type 2 E6 and E7 RNA in feline cutaneous squamous cell carcinoma. Veterinary Pathology-Supplement. 2018. DOI: https://doi.org/10.1177/0300985817750456

[19] Vascellari M, Mazzei M, Zanardello C, Melchiotti E, Albanese F, Forzan M, et al.. Felis catus Papillomavirus Types 1, 2, 3, 4, and 5 in Feline Bowenoid in Situ Carcinoma: An In Situ Hybridization Study. Veterinary Pathology-Supplement. 2019. DOI: https://doi.org/10.1177/0300985819859874

[20] Yamashita-Kawanishi N, Sawanobori R, Matsumiya K, Uema A, Chambers J, Uchida K, et al.. Detection of felis catus papillomavirus type 3 and 4 DNA from squamous cell carcinoma cases of cats in Japan. Journal of Veterinary Medical Science. 2018. DOI: https://doi.org/10.1292/jvms.18-0089

[21] Munday J, Bell C, Gulliver E. Feline oral in situ carcinoma associated with papillomavirus infection: A case series of 7 cats. Veterinary Pathology-Supplement. 2025. DOI: https://doi.org/10.1177/03009858251352594

[22] Altamura G, Cuccaro B, Eleni C, Strohmayer C, Brandt S, Borzacchiello G. Investigation of multiple Felis catus papillomavirus types (-1/-2/-3/-4/-5/-6) DNAs in feline oral squamous cell carcinoma: a multicentric study. Journal of Veterinary Medical Science. 2022. DOI: https://doi.org/10.1292/jvms.22-0060

[23] Yamashita-Kawanishi N, Chang C, Chambers J, Uchida K, Sugiura K, Kukimoto I, et al.. Comparison of prevalence of Felis catus papillomavirus type 2 in squamous cell carcinomas in cats between Taiwan and Japan. Journal of Veterinary Medical Science. 2021. DOI: https://doi.org/10.1292/jvms.21-0153

[24] Geisseler M, Lange C, Favrot C, Fischer N, Ackermann M, Tobler K. Geno- and seroprevalence of Felis domesticus Papillomavirus type 2 (FdPV2) in dermatologically healthy cats. BMC Veterinary Research. 2016. DOI: https://doi.org/10.1186/s12917-016-0776-7

[25] Supsavhad W, Dirksen WP, Hildreth B, Rosol T. p16, pRb, and p53 in Feline Oral Squamous Cell Carcinoma. Veterinary Sciences. 2016. DOI: https://doi.org/10.3390/vetsci3030018

[26] Demos LE, Munday J, Lange C, Bennett M. Use of fluorescence in situ hybridization to detect Felis catus papillomavirus type 2 in feline Bowenoid in situ carcinomas. Journal of feline medicine and surgery. 2018. DOI: https://doi.org/10.1177/1098612X18795919

[27] Massimini M, Crisi P, Borzacchiello G, Altamura G, Salda LD, Rinaldi V, et al.. Unusual tongue metastasis from lung adenocarcinoma in a cat with feline lung-digit syndrome.. Journal of Comparative Pathology. 2023. DOI: https://doi.org/10.1016/j.jcpa.2023.10.007

[28] Altamura G, Cardeti G, Cersini A, Eleni C, Cocumelli C, Pino LEBD, et al.. Detection of Felis catus papillomavirus type-2 DNA and viral gene expression suggest active infection in feline oral squamous cell carcinoma.. Veterinary and Comparative Oncology. 2020. DOI: https://doi.org/10.1111/vco.12569

[29] Altamura G, Martano M, Licenziato L, Maiolino P, Borzacchiello G. Telomerase Reverse Transcriptase (TERT) Expression, Telomerase Activity, and Expression of Matrix Metalloproteinases (MMP)-1/-2/-9 in Feline Oral Squamous Cell Carcinoma Cell Lines Associated With Felis catus Papillomavirus Type-2 Infection. Frontiers in Veterinary Science. 2020. DOI: https://doi.org/10.3389/fvets.2020.00148

[30] Țuțu P, Altamura G, Bocaneti FD, Hritcu O, Pașca A, Dascalu M, et al.. Immunohistochemical and western blot expression of MMPs, TIMPs, and cytokeratin 10 in feline squamous cell carcinoma. Frontiers in Veterinary Science. 2026. DOI: https://doi.org/10.3389/fvets.2026.1787600

[31] Kummer S, Klang A, Strohmayer C, Walter I, Jindra C, Kneissl SM, et al.. Feline SCCs of the Head and Neck Display Partial Epithelial-Mesenchymal Transition and Harbor Stem Cell-like Cancer Cells. Pathogens. 2023. DOI: https://doi.org/10.3390/pathogens12111288

[32] Sumi A, Chambers J, Ito S, Kojima K, Omachi T, Doi M, et al.. Different expression patterns of p63 and p73 in Felis catus papillomavirus type 2-associated feline Merkel cell carcinomas and other epidermal carcinomas. Journal of Veterinary Medical Science. 2023. DOI: https://doi.org/10.1292/jvms.23-0293

[33] Țuțu P, Bocaneti FD, Altamura G, Dascalu M, Horodincu L, Soreanu O, et al.. Feline oral squamous cell carcinoma: recent advances and future perspectives. Frontiers in Veterinary Science. 2025. DOI: https://doi.org/10.3389/fvets.2025.1663990

[34] Sequeira I, Pires M, Leitão J, Henriques J, Viegas C, Requicha J. Feline Oral Squamous Cell Carcinoma: A Critical Review of Etiologic Factors. Veterinary Sciences. 2022. DOI: https://doi.org/10.3390/vetsci9100558

[35] Loft K, Soohoo J, Simon B, Lange C. Feline cystadenomatosis affecting the ears and skin of 57 cats (2011–2019). Journal of feline medicine and surgery. 2021. DOI: https://doi.org/10.1177/1098612X211024498

[36] Kessell A, McNair D, Munday J, Savory R, Halliday C, Malik R. Successful treatment of multifocal pedal Prototheca wickerhamii infection in a feline immunodeficiency virus-positive cat with multiple Bowenoid in situ carcinomas containing papillomaviral DNA sequences. JFMS open reports. 2017. DOI: https://doi.org/10.1177/2055116916688590

[37] Layne EA. Papillomavirus: Clinical Presentations and Treatment Approaches.. The Veterinary clinics of North America. Small animal practice. 2024. DOI: https://doi.org/10.1016/j.cvsm.2024.11.007

[38] Noall LG, Lee S, Burton J, Marquardt TM, Cermak J, Thombs LA, et al.. A multi-institutional epidemiologic study evaluating environmental risk factors for feline oral squamous cell carcinoma.. Veterinary and Comparative Oncology. 2023. DOI: https://doi.org/10.1111/vco.12914

[39] Wypij J. A Naturally Occurring Feline Model of Head and Neck Squamous Cell Carcinoma. Pathology Research International. 2013. DOI: https://doi.org/10.1155/2013/502197

[40] Teh A, Krockenberger M. Do papillomaviruses cause feline cutaneous squamous cell carcinoma?. Veterinary Evidence. 2021. DOI: https://doi.org/10.18849/ve.v6i3.402

[41] Abramo F, Mazzei M, Forzan M, Giannetti G, Albanese F, Melchiotti E, et al.. Using colorimetric in situ hybridisation method for FcaPV-2 to estimate postsurgical prognosis in feline Bowenoid in situ carcinoma.. Veterinary dermatology (Print). 2024. DOI: https://doi.org/10.1111/vde.13297 *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.