Cowpox Virus in Cats

Overview and Taxonomic Classification of Cowpox Virus in Felines

Cowpox virus (CPXV) is a zoonotic orthopoxvirus of the family Poxviridae, genus Orthopoxvirus, that has emerged as a significant pathogen in domestic cats (Felis catus) across Europe and parts of Eurasia. Despite its historical name, CPXV is not maintained in cattle but circulates primarily in wild rodents, with voles (Microtus spp.) and bank voles (Myodes glareolus) serving as the principal reservoir hosts [1, 2, 3]. Domestic cats acquire the virus through predation of infected rodents, and once infected, they can develop severe, self-limiting cutaneous lesions and serve as a bridging host for transmission to humans [4, 5, 6, 7]. The seroprevalence of CPXV in domestic cat populations has been estimated at 2–4%, though localized outbreaks may yield higher rates [1]. Importantly, CPXV is considered a notifiable zoonotic agent by the World Health Organization (WHO) and the World Organisation for Animal Health (WOAH) due to its potential for severe disease in immunocompromised individuals and its increasing incidence following cessation of routine smallpox vaccination [4, 8, 7, 9]. The Centers for Disease Control and Prevention (CDC) also recognizes orthopoxviruses as emerging threats, underscoring the need for robust surveillance and taxonomic clarity.

Biological and Epidemiological Context in Felines

Feline cowpox typically presents as a localized, ulcerative dermatitis with progression from erythematous macules to papules, vesicles, pustules, and finally necrotic eschars, often accompanied by regional lymphadenopathy and systemic signs such as pyrexia [4, 5, 6]. Histologically, lesions reveal epidermal necrosis, pyogranulomatous inflammation, and large eosinophilic intracytoplasmic inclusion bodies (B-type inclusions) pathognomonic for poxvirus infection [4, 5, 10]. The incubation period in cats is approximately 8–12 days, and viral shedding from skin lesions can be substantial, facilitating transmission to humans via direct contact or fomites [4, 1, 2]. Fatal disseminated infections have been documented, particularly in immunosuppressed cats or those co-infected with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV), analogous to severe human cases in HIV-positive patients [11, 9]. Moreover, outbreaks in veterinary settings have been reported, such as a 2015 hospital-acquired transmission among hospitalized cats in Germany, underscoring the importance of biosecurity measures [12]. The broad host range of CPXV, extending to elephants, jaguarundis, beavers, and even lynx, highlights its ecological plasticity and the necessity for accurate taxonomic delineation [2, 13, 14].

Historical and Contemporary Taxonomic Framework

Historically, CPXV was considered a single, monophyletic species, but mounting genomic evidence has profoundly revised this view. The virus possesses a large double-stranded DNA genome of approximately 220–222 kbp, encoding 215–219 open reading frames, and exhibits a notably low mutation rate of approximately 1.65 × 10⁻⁵ substitutions per site per year [15]. Early phylogenetic analyses based on the hemagglutinin (HA) gene suggested substantial diversity among isolates, but whole-genome sequencing has revealed that CPXV is in fact a polyphyletic assemblage of several distinct lineages [15, 13]. Diaz-Cánova et al. (2022), using 87 orthopoxvirus strains including five Fennoscandian feline isolates, demonstrated that CPXV strains separate into at least five major clusters: CPXV-like 1, CPXV-like 2, VACV-like, VARV-like, and ECTV-Abatino-like [15]. Further subdivision into eighteen sub-species based on genetic and patristic distances has been proposed, with the authors explicitly stating that “CPXV is not a single species but a polyphyletic assemblage of several species and thus, a reclassification is warranted” [15]. This conclusion is corroborated by Dabrowski et al. (2013), who sequenced 22 independent CPXV genomes from diverse hosts, including cats, rats, humans, and exotic animals, and found that some clades are more closely related to variola virus (VARV), camelpox virus, and taterapox virus than to other CPXV isolates [13]. Indeed, a specific CPXV clade (often designated CPXV-like 2 or the “German” clade) shows a closer phylogenetic affinity to VARV than to the classic Brighton Red strain, challenging the traditional taxonomic boundaries [13, 9].

Genomic Diversity and Cladistics of Feline Isolates

Feline CPXV isolates have been instrumental in deciphering this taxonomic complexity. In the Fennoscandian region, five isolates from cats and humans were fully sequenced and placed within distinct clades, with some grouping with CPXV-like 1 (which includes the reference strain Brighton Red) and others falling into the CPXV-like 2 clade that is closer to VARV [15]. Similarly, Antwerpen et al. (2018) analyzed four feline CPXV genomes from a hospital outbreak and found three identical sequences and one that differed by 65 single-nucleotide polymorphisms (SNPs), demonstrating that even within a single outbreak, strain variation can exist [12]. Importantly, these authors cautioned that analysis of the HA gene alone is insufficient for molecular trace-back investigations; whole-genome sequencing is required for accurate phylogenetic resolution [12]. The detection of a novel orthopoxvirus in a cat in Italy that was more closely related to ectromelia virus than to classic CPXV further complicates the taxonomy, raising the possibility that some feline “cowpox” cases may actually involve distinct orthopoxvirus species [10]. The virus (Italy_09/17) could not be assigned to a defined orthopoxvirus species by conventional PCR, and phylogenetic analysis of concatenated genes placed it in a cluster with a macaque-derived orthopoxvirus, suggesting a potentially new species [10]. Such findings underscore the urgency of taxonomic revision and have direct implications for diagnostic protocols, vaccine development, and public health risk assessment.

Implications for Feline Medicine and One Health

Taxonomic refinement of CPXV is not merely an academic exercise. Accurate species identification influences antiviral treatment decisions (e.g., tecovirimat efficacy may vary), zoonotic risk communication, and epidemiological modeling [2, 9]. For example, the VARV-like clade may possess enhanced virulence or altered host range, as suggested by its genetic complement [15, 13]. In cats, the severity of cowpox can range from mild, self-limiting lesions to fatal systemic disease, and the infecting clade may be a determinant of outcome, though host immune status remains critical [4, 10, 9]. The Food and Agriculture Organization (FAO) highlights the One Health importance of such viruses, given their circulation in wildlife, domestic animals, and humans. Continued genomic surveillance of feline CPXV isolates, coupled with metadata on clinical presentation and geographic origin, will be essential to establish a robust, evolutionarily informed taxonomy that can guide both clinical practice and public health policy.

Molecular Pathogenesis and Genomic Evolution of Cowpox Virus

Genomic Architecture and Phylogenetic Complexity of Cowpox Virus

Cowpox virus (CPXV) possesses one of the largest and most complex genomes within the Orthopoxvirus genus, with a linear double-stranded DNA genome ranging from approximately 220 to 222 kilobase pairs (kbp) and encoding between 215 and 219 open reading frames (ORFs) [15]. This genomic size is notably larger than that of variola virus (VARV) or vaccinia virus (VACV), reflecting an expanded repertoire of genes involved in host range determination, immune evasion, and viral modulation of cellular signaling pathways [15, 13]. Comprehensive genomic sequencing and phylogenomic analyses have fundamentally challenged the traditional taxonomic classification of CPXV as a single viral species. Diaz-Cánova et al. (2022) conducted an exhaustive phylogenetic analysis of 87 orthopoxvirus strains, including five novel Fennoscandian CPXV isolates from cats and humans, and demonstrated that CPXV strains form a polyphyletic assemblage comprising at least five distinct major clusters: CPXV-like 1, CPXV-like 2, VACV-like, VARV-like, and ECTV-Abatino-like [15]. These major clades can be further partitioned into eighteen sub-species based on genetic and patristic distances, providing compelling evidence that the designation "cowpox virus" represents a taxonomic artifact rather than a monophyletic lineage [15, 13]. This genomic heterogeneity has profound implications for understanding the molecular pathogenesis of CPXV in feline hosts, as different CPXV clades exhibit distinct virulence profiles, host range determinants, and immunological interactions.

Polyphyletic Nature and Evolutionary Dynamics

The polyphyletic nature of CPXV was further corroborated by Dabrowski et al. (2013) through genome-wide comparison of 22 independent CPXV strains derived from diverse clinical cases involving humans, domestic cats, rats, exotic zoo animals (including jaguarundis, elephants, and beavers), and other mammalian species [13]. Their extensive phylogenetic analysis revealed that while some CPXV strains cluster closely with VACV, others form distinct clades that are more closely related to camelpox virus (CMLV), taterapox virus (TATV), and critically, variola virus (VARV) than to other CPXV isolates [13, 9]. The discovery of a CPXV clade exhibiting phylogenetic proximity to VARV, the causative agent of smallpox and a pathogen of immense historical significance, raises important questions regarding the evolutionary origins of orthopoxvirus virulence factors and the zoonotic potential of these viruses. Bayesian time-scaled evolutionary reconstruction employing concatenated sequences of 62 non-recombinant conserved genes from 55 CPXV isolates has yielded a calculated evolutionary rate of 1.65 × 10⁻⁵ substitutions per site per year [15]. This relatively slow substitution rate is consistent with the replication fidelity afforded by viral DNA polymerases possessing proofreading activity and explains the remarkable genomic stability observed within individual CPXV outbreaks. For instance, Antwerpen et al. (2018) demonstrated that six CPXV genomes associated with a protracted pet rat outbreak spanning 2008–2011 exhibited complete genomic identity, while two additional genomes differed by merely three single nucleotide polymorphisms (SNPs), indicating an extraordinarily high level of clonality maintained over four years [12].

Molecular Mechanisms of Host Range Determination and Immune Evasion

The expanded genome of CPXV relative to other orthopoxviruses encodes a sophisticated arsenal of host range factors and immune evasion molecules that enable replication across a remarkably broad spectrum of mammalian species. CPXV displays the widest documented host range of any orthopoxvirus, encompassing rodents, felids, bovids, elephants, non-human primates, and humans, with transmission typically occurring through direct contact with infected animals or contaminated fomites [2, 13, 3]. The molecular basis for this extensive host range resides in specific viral genes that modulate host antiviral responses, particularly those targeting interferon signaling, chemokine networks, and apoptosis pathways. CPXV encodes multiple secreted decoy receptors for cytokines and chemokines, including a viral chemokine-binding protein (vCCI), a viral interferon-gamma binding protein, and a viral tumor necrosis factor receptor homologue (CrmB/CrmD), which effectively neutralize host inflammatory responses and facilitate viral dissemination [3, 16]. The functional characterization of these immune evasion determinants has been greatly facilitated by the generation of an infectious full-length CPXV clone (strain Brighton Red) as a bacterial artificial chromosome (BAC), as reported by Roth et al. (2011) [16]. This reverse genetics system permits systematic mutagenesis of individual CPXV genes within the authentic genomic context, enabling precise dissection of virulence determinants in vivo. Notably, the cloned viral genome was demonstrated to be identical to the parental wild-type virus, and recombinant viruses recovered from transfected cells exhibited growth properties virtually indistinguishable from the parental strain, validating this platform for pathogenesis studies [16].

Feline-Specific Pathogenesis and Clinical Molecular Correlates

In domestic cats (Felis catus), CPXV infection typically manifests as a localized, self-limiting disease characterized by proliferative and necrotizing dermatitis, although systemic dissemination can occur in immunocompromised animals [4, 5, 6]. The molecular pathogenesis in feline hosts begins with viral entry through abrasions or breaches in the epidermal barrier, followed by replication in keratinocytes and dermal fibroblasts. Histopathological examination of feline lesions reveals epidermal necrosis, pyogranulomatous dermatitis, and the presence of large eosinophilic cytoplasmic inclusion bodies (Guarnieri bodies) within infected keratinocytes, pathognomonic features of orthopoxvirus infection [4, 10]. These inclusion bodies represent sites of viral replication and assembly, containing immature and mature virions embedded within a matrix of viral proteins and cellular debris. The progression of cutaneous lesions through macular, papular, vesicular, pustular, and ultimately eschar-forming stages reflects the dynamic interplay between viral cytopathic effects and host inflammatory responses [1, 6]. Systemic spread in cats, although less common than localized disease, can involve viral dissemination via the lymphatics and bloodstream to regional lymph nodes, spleen, and occasionally the lungs, leading to lymphadenopathy, fever, and in severe cases, fatal outcomes [8, 10]. The severity of feline CPXV infection is influenced by viral strain-specific factors, host immune status, and potential co-infections with immunosuppressive retroviruses such as feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV), which may impair antiviral defenses and predispose to more aggressive disease [11, 9].

Genomic Determinants of Virulence and Zoonotic Potential

The capacity of CPXV to cross species barriers from rodents to cats and subsequently to humans represents a significant public health concern, particularly in the context of waning population-level immunity following cessation of routine smallpox vaccination in the 1980s [4, 1, 7, 17]. Molecular epidemiological studies employing whole genome sequencing have demonstrated that identical CPXV strains can infect multiple host species within a single transmission chain. Kurth et al. (2008) documented a remarkable transmission event involving rats (Rattus norvegicus) as the likely reservoir, a circus elephant (Elephas maximus) as the amplifying host, and a human animal keeper, with the hemagglutinin (HA) open reading frame displaying perfect sequence homology across isolates from the elephant and the keeper, and 99% homology to a CPXV strain isolated from a different elephant in 1984 [2]. This sequence conservation across temporal and species boundaries underscores the genetic stability of CPXV during cross-species transmission and suggests that the molecular determinants of host range are encoded within relatively few genomic loci. The HA gene, which encodes a critical virulence factor involved in viral spread and modulation of host immune responses, has historically been the target of diagnostic PCR and phylogenetic typing [12, 2, 9]. However, Antwerpen et al. (2018) provided a cautionary demonstration that HA gene sequencing alone is insufficient for robust molecular trace-back analyses, as four feline CPXV cases with identical HA sequences were subsequently shown by whole genome sequencing to represent three distinct transmission events, with one cat harboring a virus differing by 65 nucleotides from the outbreak strain [12]. This finding emphasizes the necessity of whole genome sequencing for precise molecular epidemiological investigations and outbreak source attribution.

Recombination, Genetic Diversity, and Emerging Variants

Recombination represents a fundamental force driving orthopoxvirus evolution and the emergence of novel viral variants. Although CPXV exhibits a relatively low mutation rate, recombination events between co-infecting orthopoxviruses can generate chimeric genomes with novel phenotypic properties, including altered host range, virulence, and antigenic profiles [15, 13, 3]. The detection of CPXV strains that cluster phylogenetically with VARV, CMLV, and TATV suggests the occurrence of ancestral recombination events that reshuffled genomic modules among divergent orthopoxvirus lineages [13, 9]. The clinical relevance of these recombination events is exemplified by the characterization of a novel orthopoxvirus responsible for a fatal infection in a cat in Italy, reported by Lanave et al. (2018) [10]. This virus, designated Italy_09/17, could not be unambiguously assigned to any recognized orthopoxvirus species by conventional PCR approaches targeting conserved genes. Electron microscopy revealed typical brick-shaped orthopoxvirus virions, while phylogenetic analysis of nine concatenated genes demonstrated that the virus was distantly related to CPXV but more closely related to ectromelia virus (ECTV), clustering within the same clade as an orthopoxvirus recently isolated from captive macaques in Italy [10]. The emergence of such novel orthopoxviruses in feline populations underscores the dynamic nature of orthopoxvirus ecology and the potential for previously unrecognized viruses to cause disease in domestic cats. Further genomic surveillance of both domestic cats and wild rodent reservoirs is essential to monitor the emergence of novel orthopoxvirus variants with enhanced zoonotic potential [10, 14]. Indeed, orthopoxvirus DNA has been detected in free-ranging Eurasian lynx (Lynx lynx) in Sweden, with a 9% prevalence among 263 animals, and phylogenetic analysis of the thymidine kinase gene amplicon sequences revealed identity to CPXV isolated from a human in Norway, suggesting that wild felids may serve as sentinel species for orthopoxvirus circulation in rodent populations [14].

Molecular Diagnostics and Genomic Surveillance

The application of next-generation sequencing (NGS) technologies has revolutionized the molecular characterization of CPXV and the investigation of outbreak dynamics. NGS enables the generation of complete or near-complete viral genomes from clinical specimens, including formalin-fixed, paraffin-embedded (FFPE) tissues, thereby facilitating retrospective genomic analyses of archived cases [12]. The utility of NGS was demonstrated in the investigation of two distinct CPXV outbreaks: a protracted pet rat-associated outbreak in Germany and France (2008–2011) and a nosocomial outbreak in cats within a German small animal clinic (2015) [12]. In the feline outbreak, whole genome sequencing conclusively demonstrated that hospital-acquired transmission had occurred in three of four cats that exhibited identical HA gene sequences, disproving the hypothesis that all four cases resulted from nosocomial spread and highlighting the superior discriminatory power of genomic approaches [12]. The World Health Organization (WHO) and the World Organisation for Animal Health (WOAH) have recognized the importance of genomic surveillance for orthopoxviruses, given their zoonotic potential and the ongoing threat of emerging infectious diseases. The European Virus Archive goes Global (EVAg) and COMPARE projects have supported the development of standardized NGS protocols and bioinformatic pipelines for orthopoxvirus characterization, facilitating international collaboration and data sharing [12]. As CPXV continues to circulate in rodent reservoirs across Eurasia, with seroprevalence rates in domestic cats estimated between 2% and 4%, the integration of genomic epidemiology with traditional surveillance methods will be critical for anticipating and mitigating future zoonotic spillover events [1, 8]. The molecular pathogenesis of CPXV in cats is thus intimately linked to the virus's genomic plasticity, its capacity for recombination, and its sophisticated immune evasion strategies, all of which conspire to maintain this pathogen in complex multi-host systems with intermittent but potentially severe consequences for feline and human health.

Epidemiology and Zoonotic Transmission Dynamics of Cowpox Virus in Cats

Geographic Distribution and Endemicity

Cowpox virus (CPXV) is endemic across the Eurasian landmass, with a geographic range that extends from Western Europe through Fennoscandia and into parts of Russia [15, 3]. Unlike the geographically restricted monkeypox virus, CPXV maintains a broad, continuous distribution across temperate and boreal ecosystems. The virus is not, however, uniformly distributed; rather, its prevalence is tightly linked to the population density and ecology of its primary reservoir hosts, small rodents, particularly bank voles (Myodes glareolus), wood mice (Apodemus sylvaticus), and, as demonstrated in more recent investigations, brown rats (Rattus norvegicus) [1, 2, 3]. The Fennoscandian region, encompassing Norway, Sweden, and Finland, has been a particularly rich source of CPXV isolates, with genomic sequencing of five distinct isolates from cats and humans in this region revealing a remarkable degree of genetic diversity [15]. This diversity is not merely a matter of academic interest; it reflects a complex evolutionary history in which CPXV has diverged into at least five distinct major clusters (CPXV-like 1, CPXV-like 2, VACV-like, VARV-like, and ECTV-Abatino-like), with further subdivision into eighteen sub-species based on genetic and patristic distances [15]. The implication for feline epidemiology is profound: cats are not exposed to a single, homogeneous pathogen but to a polyphyletic assemblage of viral lineages, each with potentially different transmission efficiencies, virulence profiles, and zoonotic potentials.

The Domestic Cat as an Accidental Host and Bridge Vector

The domestic cat (Felis catus) occupies a unique and epidemiologically critical position in the transmission ecology of CPXV. While the virus is enzootic in wild rodent populations, cats serve as the primary bridging vector for human infection in Europe [4, 5, 8]. This role is not a function of cats being a natural reservoir, they are not. Rather, cats are accidental, dead-end hosts that become infected through their innate predatory behavior. When a cat hunts and consumes an infected rodent, the virus gains entry through the oral mucosa or through minor abrasions sustained during the hunt. The resulting infection in the cat is typically characterized by the development of multiple cutaneous lesions, often concentrated on the head, neck, and forelimbs, the areas most exposed during predatory encounters [4, 6]. These lesions, which histologically demonstrate epidermal necrosis and pyogranulomatous dermatitis with large eosinophilic cytoplasmic inclusions within keratinocytes, are teeming with infectious virions [4]. The cat thus becomes a highly efficient amplifier and disseminator of the virus, shedding large quantities of CPXV into the environment and directly onto the hands and skin of its human companions.

The seroprevalence of CPXV among domestic cats has been calculated to be between 2% and 4% [1]. While this figure may appear low, it belies the true public health significance. Given the enormous population of domestic cats in Europe, tens of millions, even a 2% seroprevalence translates into hundreds of thousands of exposed animals. More importantly, seroprevalence studies capture only past exposure, not active infection. Cats with active, lesional cowpox are the ones most likely to transmit the virus to humans, and these cases, while individually rare, are being reported with increasing frequency across Europe [8, 7]. The cessation of routine smallpox vaccination in the general population, which ended in most European countries by the early 1980s, has resulted in a steadily growing cohort of immunologically naïve individuals with no cross-protective immunity against orthopoxviruses [4, 7]. This demographic shift is widely believed to be a primary driver of the apparent increase in diagnosed human CPXV infections.

Zoonotic Transmission Dynamics: From Cat to Human

The transmission of CPXV from an infected cat to a human is a mechanistically straightforward but epidemiologically nuanced event. The virus is not airborne in the conventional sense; transmission requires direct, physical contact with lesional material or, less commonly, with contaminated fomites [4, 1]. The typical portal of entry is through minor skin fissures, abrasions, or pre-existing cuts on the hands, arms, or face, areas that come into frequent contact with an affectionate but infected pet [4, 5]. The incubation period in humans is consistently reported as 8 to 12 days, after which a characteristic lesion develops at the inoculation site [1]. This lesion progresses through a predictable sequence: an erythematous macule evolves into a papule, then a vesicle, and finally a pustule that ulcerates and forms a hard, black eschar surrounded by erythema and edema [1, 6]. Regional lymphadenopathy is a hallmark feature, often accompanied by systemic symptoms such as fever, malaise, and lymphangitis [4, 5].

The case described by Haddadeen et al. (2020) is a textbook illustration of this dynamic. A 43-year-old man presented with a large, fluctuant, hemorrhagic bulla on his left volar wrist, accompanied by lymphangitis and pyrexia. His cat had recently developed dozens of small, erythematous, eroded lesions. PCR analysis of the man’s bulla fluid confirmed orthopoxvirus DNA, and the epidemiological link to the cat was unequivocal [4]. Similarly, Kania et al. (2021) reported a 15-year-old boy who developed facial lesions that progressed through the full vesicular and pustular stages before forming a 2 cm eschar, with the infection traced back to his domestic cat [6]. These cases underscore a critical point: the clinical presentation in humans is often severe enough to warrant medical attention, yet the rarity of the disease means it is frequently misdiagnosed. Differential diagnoses include cat scratch disease, anthrax, brucellosis, and severe bacterial infections, leading to delays in appropriate management and unnecessary antibiotic therapy [6].

The Role of Pet Rats and Exotic Animals in Transmission

While cats are the most commonly implicated source of human CPXV infection, they are by no means the only one. A significant and well-documented outbreak of human cowpox occurred in Germany and France between 2008 and 2011, involving approximately 40 human cases. The source of this outbreak was not cats but pet rats (Rattus norvegicus) [12, 2]. Genomic sequencing of the hemagglutinin gene from multiple human and rat isolates revealed an identical, unique sequence in each case, pointing unequivocally to a common source [12]. Whole genome sequencing of eight virus isolates from this outbreak demonstrated complete genomic identity across six genomes, with the remaining two differing by as few as three single nucleotide polymorphisms (SNPs) [12]. This extraordinary level of clonality over a four-year period indicates that a single CPXV strain can persist and circulate within a rodent population with remarkable genetic stability, maintaining its zoonotic potential throughout.

The implications for feline epidemiology are direct and concerning. Cats that hunt or scavenge are at risk of encountering these same infected rodent populations. Furthermore, the pet rat outbreak demonstrates that CPXV can be introduced into domestic environments through non-feline vectors, creating a scenario in which both cats and humans are exposed simultaneously. The case of a circus elephant in Germany, which developed disseminated CPXV and subsequently transmitted the virus to its keeper, further illustrates the breadth of the virus’s host range [2]. Serological testing of rats caught on the circus premises revealed high antibody titers in multiple animals, confirming that rats were the reservoir from which the elephant was infected [2]. The HA ORF sequences from the rat, elephant, and human isolates were perfectly homologous, demonstrating that CPXV can cross multiple species barriers without significant genetic adaptation [2].

Nosocomial Transmission and the Importance of Whole Genome Sequencing

One of the most alarming epidemiological findings in recent years is the demonstration of nosocomial transmission of CPXV among cats. Antwerpen et al. (2018) reported an outbreak in a small animal clinic in Germany in 2015, where four out of five hospitalized cats developed CPXV infections. Initial analysis based solely on the hemagglutinin gene sequence suggested that all four cats were infected with an identical strain, implying a common source or direct cat-to-cat transmission [12]. However, when whole genome sequencing was applied, a more nuanced picture emerged. Three of the four cats had identical genomes, confirming hospital-acquired transmission among them. The fourth cat, despite having an identical HA sequence, differed by 65 nucleotides across its entire genome, indicating that it had acquired the infection from a separate, independent source [12]. This finding is a powerful cautionary tale: reliance on short, conserved gene sequences for molecular epidemiology is insufficient. Whole genome sequencing is now the gold standard for trace-back investigations, and its application has revealed that CPXV transmission dynamics are far more complex than previously appreciated.

The Immunocompromised Host and the Threat of Generalized Disease

The epidemiology of CPXV in cats cannot be fully understood without considering the role of the host immune status. In immunocompetent humans and cats, CPXV infection is typically self-limiting, remaining localized to the site of inoculation. However, in immunocompromised individuals, the virus can disseminate, leading to a generalized, smallpox-like disease that is frequently fatal [13, 9]. A case reported by Miguel et al. (2016) described a 35-year-old man with advanced HIV infection who developed generalized CPXV with multiple skin lesions on his forearm, leg, abdomen, and head. Despite intensive care, the patient died from septic shock [9]. This case is a stark reminder that CPXV, while often dismissed as a benign zoonosis, is a pathogen with lethal potential in vulnerable populations. The increasing number of immunocompromised individuals in the human population, due to HIV, organ transplantation, chemotherapy, and biologic therapies, combined with the loss of vaccine-induced immunity, creates a growing at-risk population for severe CPXV disease.

Reservoir Ecology and the Role of Rodents

The ultimate source of all CPXV infections in cats and humans is the wild rodent reservoir. Small rodents, particularly voles and mice, are considered the natural maintenance hosts, in which the virus persists without causing overt disease [1, 3]. The prevalence of CPXV in rodent populations is highly variable and influenced by ecological factors such as population density, seasonal breeding cycles, and habitat structure. Tryland et al. (2011) detected orthopoxvirus DNA in 9% of free-ranging Eurasian lynx (Lynx lynx) in Sweden, with the highest prevalence in areas with dense human populations [14]. Lynx, as apex predators, are exposed to CPXV through predation on infected small mammals, and their seroprevalence serves as a sentinel for virus circulation in the rodent population. The finding that lynx from more densely populated areas had higher seroprevalence suggests that anthropogenic factors, such as habitat fragmentation, food availability, and rodent population dynamics, may influence CPXV transmission risk [14].

Genetic Diversity and the Polyphyletic Nature of CPXV

The traditional view of CPXV as a single, cohesive viral species has been definitively overturned by modern genomic analyses. Dabrowski et al. (2013) sequenced 22 genomes of independent CPXV strains from clinical cases involving humans, rats, cats, jaguarundis, beavers, elephants, and other exotic animals. Their extensive phylogenetic analysis revealed that CPXV strains cluster into several distinct clades, some of which are more closely related to vaccinia virus, while others are more closely related to variola virus (the causative agent of smallpox) [13]. This finding has profound implications for understanding transmission dynamics. If CPXV is not a single species but a polyphyletic assemblage of multiple species, then the biological properties, including host range, transmissibility, and virulence, may vary significantly among strains [15, 13]. A cat infected with a CPXV strain from the VARV-like clade may pose a different zoonotic risk than a cat infected with a VACV-like strain. The epidemiological significance of this genetic diversity is only beginning to be explored, and it underscores the need for continued genomic surveillance of CPXV isolates from both feline and human cases.

The Role of Feline Retroviruses in CPXV Susceptibility

An often-overlooked aspect of CPXV epidemiology in cats is the potential role of concurrent retroviral infections. Feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) are known to cause immunosuppression in cats, increasing their susceptibility to a wide range of secondary infections [11]. While direct studies on the interaction between CPXV and feline retroviruses are limited, the principle is well established: immunosuppressed cats are likely to experience more severe CPXV disease, with higher viral loads and prolonged shedding, thereby increasing the risk of zoonotic transmission to humans. So-In et al. (2025) reported a high prevalence of FIV and FeLV in domestic cats from northeastern Thailand, with co-infections being common [11]. Although this study was conducted in a region where CPXV is not endemic, the findings are broadly applicable. In Europe, where CPXV is endemic, the prevalence of FIV and FeLV in the stray and outdoor cat population, the very cats most likely to encounter infected rodents, could be a significant, yet unquantified, factor in CPXV transmission dynamics.

Implications for Public Health and Veterinary Surveillance

The epidemiological data presented here make a compelling case for enhanced surveillance of CPXV in both feline and human populations. The World Health Organization (WHO) and the World Organisation for Animal Health (WOAH) have long recognized the importance of monitoring orthopoxviruses, particularly in the context of the post-smallpox eradication era. The increasing incidence of human CPXV infections, driven by the loss of cross-protective immunity and the expanding population of immunocompromised individuals, represents a growing public health concern [7, 9]. From a veterinary perspective, the cat serves as a sentinel species. An increase in feline CPXV cases in a given region should be interpreted as a warning signal for heightened zoonotic risk. Veterinary clinicians must be educated to recognize the characteristic skin lesions of cowpox in cats, multiple, erythematous, eroded plaques, often on the head and forelimbs, and to perform appropriate diagnostic testing, including PCR and, ideally, whole genome sequencing for epidemiological tracing. Public health authorities should consider making human cowpox a notifiable disease, as has been suggested by some authors, to facilitate systematic data collection and risk assessment [7]. Only through a coordinated, One Health approach that integrates veterinary, ecological, and human medical surveillance can the true burden of CPXV be understood and mitigated.

Clinical Manifestations and Pathological Features of Cowpox in Domestic Cats

Spectrum of Clinical Presentation in the Feline Host

The clinical expression of cowpox virus (CPXV) infection in domestic cats (Felis catus) is highly variable, ranging from localized, self-limiting dermatopathy to severe, disseminated, and occasionally fatal systemic disease. Understanding this clinical spectrum is paramount for the veterinary practitioner, given the increasing incidence of feline cases documented across Europe and the virus’s established zoonotic potential [4, 3]. The incubation period in cats, while not as precisely defined as in humans (where it is typically 7–12 days [1, 9]), is inferred from experimental and epidemiological data to be similar, with signs emerging roughly 8 to 12 days following exposure to infected rodent reservoirs [1]. Cats are considered accidental or spillover hosts, acquiring the virus through direct contact with the primary reservoir: wild rodents, particularly bank voles (Myodes glareolus) and wood mice (Apodemus sylvaticus) [4, 2, 3]. The hunting behavior of domestic cats places them at high risk of exposure, and once infected, they serve as efficient bridging vectors for zoonotic transmission to humans [4, 5, 17, 18].

Primary Lesion Morphology and Anatomical Distribution

The hallmark of feline cowpox is the development of a solitary or, more commonly, multiple cutaneous lesions. The initial presentation is often an erythematous macule that progresses rapidly through papular, vesicular, and pustular stages before forming a characteristic, hard, black eschar [6]. These lesions are typically 3–4 mm in diameter, erythematous, annular, and eroded in their early phase [4]. The eschar, a dry, dark scab, is a defining feature and represents full-thickness epidermal necrosis. In the cat, the most common sites of primary inoculation are the head (especially the face, nose, and lips), the neck, and the forelimbs, areas most likely to contact an infected rodent during a predatory encounter [5, 6]. A detailed case series from the UK described a domestic cat presenting with “several dozen” of these annular and eroded lesions, highlighting that disseminated disease is not uncommon, even in immunocompetent animals [4]. Secondary lesions can arise from autoinoculation, where the cat spreads the virus to other body parts (e.g., the caudal abdomen, hindlimbs, or oral cavity) through licking, scratching, or grooming of primary lesions [7]. Lesions on the paws (pododermatitis) and in the oral cavity (stomatitis) are also well-recognized and can cause significant pain, leading to anorexia and ptyalism.

Systemic Signs and Disease Course

While the cutaneous lesions are the most visible clinical feature, cowpox is rarely a purely dermatological disease in cats. Concomitant systemic illness is frequently reported. Affected cats commonly exhibit a biphasic fever, lethargy, depression, and anorexia [5, 9]. Regional lymphadenopathy is a cardinal feature, with the lymph nodes draining the site of primary infection becoming markedly enlarged, firm, and often painful to palpation [6, 9]. This lymphadenopathy reflects the intense immune response and viral replication within nodal tissues. The disease course in immunocompetent cats is typically protracted, lasting 4 to 8 weeks before spontaneous resolution [6]. Lesions heal slowly, often leaving behind permanent, atrophic scars and alopecia [1, 7]. Secondary bacterial infections of the eroded skin are a common complication, often necessitating antimicrobial therapy and complicating the clinical picture [6].

Severe and Disseminated Forms: The Immunocompromised and Fatal Cases

Although often described as self-limiting, CPXV infection can be severe, generalized, and lethal, particularly in cats with underlying immunosuppression. The most profound risk factors are concurrent infections with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV), which compromise cell-mediated and humoral immunity [11, 9]. In such patients, the virus can disseminate hematogenously, leading to a systemic infection that mimics smallpox. This form is characterized by widespread, coalescing skin lesions across the entire body, including the ventrum and mucous membranes. Furthermore, severe respiratory distress syndrome, which can be fatal, has been documented, with the virus targeting the pulmonary parenchyma [9]. Indeed, a case report from Germany described a generalized and lethal CPXV infection in an immunocompromised human patient, underscoring the similar risk for cats with analogous immune deficits [9].

The susceptibility of exotic and non-domestic felids further illustrates the pathogenic potential of CPXV. In captive settings, outbreaks have been reported in jaguarundis (Puma yagouaroundi) and other zoo animals, often with a fulminant, disseminated course leading to death [13]. The circus elephant case reported by Kurth et al. (2008) highlights the severe, generalized ulcerative disease that can occur in non-reservoir species, and serves as a stark reminder of the high pathogenicity of CPXV when it crosses into a highly susceptible, naive host [2]. The occurrence of a nosocomial outbreak in a German small animal clinic, where transmission occurred between hospitalized cats, confirms that CPXV can be transmitted directly from cat to cat under conditions of high viral load and close contact, likely via fomites or respiratory droplets [12]. Genomic sequencing of these feline isolates revealed near-identical sequences, providing definitive molecular evidence of hospital-acquired transmission [12].

Pathological Features

Macroscopic Pathology

At necropsy, the characteristic external lesions are easily identified. On the skin, they appear as well-circumscribed, depressed ulcers covered by a dry, adherent, black or dark brown eschar. The underlying dermis is often thickened, edematous, and hyperemic. In cases with secondary bacterial infection, purulent exudate may be present beneath the eschar. When the disease has become systemic, internal gross findings may include multifocal, pale to yellow, necrotic foci in the liver, spleen, lung, and lymph nodes. The lungs can be heavy, edematous, and consolidated, consistent with interstitial pneumonia. In the case of the fatal cat infection in Italy that was attributed to a novel orthopoxvirus closely related to CPXV, gross examination revealed widespread ulcerative and necrotic lesions of the skin and oral mucosa, with associated severe lymphadenomegaly and splenomegaly [10].

Histopathology: The Hallmark of Cytopathology

The histopathological changes induced by CPXV are distinctive and pathognomonic, making biopsy a powerful diagnostic tool. The hallmark is the presence of large, homogeneous, eosinophilic intracytoplasmic inclusion bodies, known as Guarnieri bodies, within infected epithelial cells (keratinocytes) and, to a lesser extent, in fibroblasts and endothelial cells [4, 10, 9]. These inclusions are the viral factories, sites of viral replication and assembly, and are visible under light microscopy as prominent, round or oval structures that often displace the nucleus to the periphery of the cell [9].

The epidermis overlying the lesion exhibits full-thickness coagulative necrosis, forming the eschar observed grossly [4]. The viable epidermis at the margins of the lesion shows marked acanthosis (thickening) and hyperplasia, with individual keratinocytes exhibiting ballooning degeneration, where cells swell and become rounded. In the dermis, a dense, mixed inflammatory infiltrate is present, characterized by pyogranulomatous dermatitis, a combination of neutrophils, macrophages, and occasional multinucleated giant cells [4, 10]. This florid inflammatory response is responsible for the edema, erythema, and pain associated with the lesions. In the fatal Italian case, histology of the skin lesions confirmed the presence of leukocyte infiltration and the pathognomonic eosinophilic cytoplasmic inclusions, confirming the diagnosis of an orthopoxvirus infection [10].

Viral Dissemination and Systemic Pathology

In severe or disseminated cases, the virus can be detected in multiple organ systems beyond the skin. PCR and virus isolation from internal organs, including liver, spleen, lung, and kidney, confirm systemic spread [2, 9]. In the lung, histopathology reveals severe interstitial pneumonia, with thickening of the alveolar septa, infiltration by macrophages and lymphocytes, and necrosis of bronchial and alveolar epithelial cells. Intracytoplasmic inclusions can be found within pneumocytes and bronchial epithelial cells. In the liver, multifocal necrotic hepatitis is common, with hepatocytes showing the characteristic inclusion bodies. In lymphoid tissues (lymph nodes, spleen), there is marked lymphoid depletion (necrosis of lymphocytes) and histiocytic proliferation, reflecting the direct cytopathic effect of the virus on immune cells and the associated immunosuppression [10, 9]. The presence of brick-shaped virions, morphologically consistent with orthopoxviruses, can be confirmed by electron microscopy of infected tissues, providing an additional confirmatory diagnostic method [2, 10].

Virological and Epidemiological Context for Pathogenesis

The pathological potential of CPXV in cats is directly linked to its genetic diversity. Recent genomic studies have demonstrated that CPXV is not a single, monophyletic species but a polyphyletic assemblage of several distinct viral lineages (CPXV-like 1, CPXV-like 2, VACV-like, VARV-like, and ECTV-Abatino-like) [15, 13]. The clinical outcome of infection may be influenced by the specific viral clade involved, as different strains possess variable repertoires of host-range genes and immunomodulatory proteins, which can affect virulence and tissue tropism [15, 13]. The evolution rate of CPXV is estimated at 1.65 × 10⁻⁵ substitutions per site per year, suggesting a slow but constant genetic drift that could lead to the emergence of strains with altered pathogenicity [15].

The seroprevalence of CPXV in the European domestic cat population is estimated to be between 2% and 4%, indicating that subclinical or mild, unnoticed infections do occur [1, 3]. These seropositive cats, while healthy, play a role in maintaining the virus in the peridomestic environment. From a public health standpoint, the World Health Organization (WHO) and the European Centre for Disease Prevention and Control (ECDC) recognize CPXV as a re-emerging zoonotic threat. The cessation of routine smallpox vaccination (which used the related vaccinia virus) four decades ago has led to a population with waning cross-protective immunity, increasing human susceptibility to severe CPXV disease [4, 7]. Consequently, any veterinary clinician encountering a cat with eschar-like skin lesions and systemic signs must consider cowpox in their differential diagnosis, not only for the welfare of the feline patient but also to mitigate the risk of transmission to the owner and other household contacts. The World Organisation for Animal Health (WOAH) includes CPXV as a notifiable infection in many member states, underscoring its veterinary and public health significance.

Diagnostic Approaches for Cowpox Virus Infection in Cats

The diagnosis of cowpox virus (CPXV) infection in cats presents a distinctive challenge, even for experienced veterinary clinicians. This challenge arises not only from the relative rarity of the condition but also from the complex genetic heterogeneity of the causative agent, which has been reclassified as a polyphyletic assemblage of several distinct orthopoxvirus species rather than a single, monolithic viral entity [15, 13]. A definitive diagnosis is paramount for appropriate case management, implementation of infection control measures to prevent zoonotic transmission, and accurate epidemiological surveillance. The diagnostic armamentarium available to the veterinary practitioner and researcher ranges from classic histopathological examination to advanced molecular techniques, each with distinct strengths, limitations, and appropriate applications.

Clinical Suspicion and Signalment as the Diagnostic Foundation

The diagnostic process begins not in the laboratory, but at the bedside. A high index of suspicion is the single most critical element in identifying CPXV infection in cats. Key historical and clinical features should immediately raise the possibility of orthopoxvirus involvement. Affected cats frequently present with a history of outdoor access and hunting behavior, as the natural reservoir for CPXV is wild rodents, particularly bank voles and wood mice [4, 1]. The seroprevalence among domestic cats has been calculated to range between 2% and 4%, indicating a substantial population of exposed but potentially asymptomatic carriers capable of transmitting the virus to humans [1].

Clinically, feline cowpox typically manifests as a self-limiting, localized infection, though severe and even fatal disseminated disease can occur, particularly in immunocompromised animals [10, 16]. The hallmark cutaneous lesions begin as small, erythematous macules that rapidly progress through papular, vesicular, and pustular stages before forming characteristic eschars, hard, black, necrotic scabs often 1–2 cm in diameter, accompanied by local erythema, edema, and regional lymphadenopathy [6]. Lesions are most commonly found on the head, neck, and forelimbs, areas most likely to come into contact with infected prey during a predatory bite. The differential diagnosis at this stage is broad and includes cat scratch disease (caused by Bartonella henselae), bacterial abscesses, fungal infections, neoplasia, herpetic dermatitis, and other ectoparasitic or allergic conditions [6, 11]. The presence of multiple, progressive lesions with a characteristic centrifugal distribution, in a cat with a known hunting history, should prompt immediate molecular investigation.

Histopathological Examination: The Classic Cornerstone

When cutaneous biopsy is performed, histopathological evaluation remains a rapid and highly informative diagnostic modality. The microscopic findings in feline CPXV infection are remarkably consistent and often pathognomonic. Affected tissues exhibit extensive epidermal necrosis and an intense pyogranulomatous dermatitis, characterized by a mixed inflammatory infiltrate of neutrophils, macrophages, and lymphocytes [4, 10]. Critically, infected keratinocytes harbor large, eosinophilic, intracytoplasmic inclusion bodies (B-type inclusions, also known as Guarnieri bodies). These inclusions are the histopathological hallmark of orthopoxvirus infection and are so distinctive that their presence in a skin biopsy from a cat with compatible clinical signs is considered diagnostic, or at least highly suggestive, pending confirmatory testing [4, 10]. Electron microscopy of tissue sections can further confirm the diagnosis by revealing the characteristic brick-shaped or ovoid virions, approximately 200–300 nm in length, which are morphologically distinct from herpesviruses or other viral agents [10, 9]. While histopathology is excellent for providing a rapid preliminary diagnosis, it cannot differentiate CPXV from other orthopoxviruses (e.g., vaccinia virus) or identify specific viral strains, necessitating molecular confirmation for definitive characterization and epidemiological tracing.

Molecular Diagnostics: The Gold Standard for Definitive Confirmation

Polymerase chain reaction (PCR) has become the cornerstone of definitive diagnosis for CPXV infection in cats, as in humans [5, 6, 7]. The technology offers unparalleled sensitivity and specificity, capable of detecting minute quantities of viral DNA even from degraded or archival specimens, including formalin-fixed, paraffin-embedded (FFPE) tissue [12].

Conventional and Real-Time PCR: Real-time PCR assays targeting conserved orthopoxvirus genes, such as the hemagglutinin (HA) gene or the DNA polymerase gene, are widely available and are the first-line molecular tool [12, 2, 9]. These assays detect orthopoxvirus DNA generically, providing a rapid "yes/no" answer regarding infection. The HA gene, in particular, has been a standard target for decades. However, reliance on the HA gene alone has significant limitations. As demonstrated in outbreak investigations, sequencing of the HA gene may reveal identical sequences in epidemiologically linked cases, but it can also be insufficient to distinguish between true nosocomial transmission and coincidental infection with closely related strains [12]. In one critical German outbreak involving four hospitalized cats, all four animals showed identical HA sequences. Subsequent whole-genome sequencing (WGS) revealed that only three of the four cats shared a fully clonal virus (differing by a mere three single-nucleotide polymorphisms), while the fourth cat harbored a distinct virus differing by 65 nucleotides, disproving a hospital-acquired transmission for that individual [12]. This finding underscores a crucial diagnostic principle: for molecular trace-back and outbreak investigation, HA sequencing alone is inadequate.

Genomic Sequencing and Next-Generation Sequencing (NGS): The definitive molecular approach, and the method of choice for high-resolution epidemiological analysis, is whole-genome sequencing (WGS) using next-generation sequencing (NGS) technologies [12, 13]. CPXV possesses one of the largest and most complex genomes among orthopoxviruses, ranging from 220 to 222 kbp in length and containing between 215 and 219 open reading frames [15]. This genomic complexity provides immense discriminatory power. NGS allows for the complete characterization of the viral genome from clinical specimens, including lesion homogenates, cell culture isolates, and even FFPE samples [12]. Phylogenomic analyses based on complete genome sequences have robustly demonstrated that CPXV is not a single species but a polyphyletic assemblage, with distinct strains clustering with vaccinia virus, variola virus, or ectromelia virus [15, 13]. This has profound implications for diagnosis: a PCR assay designed to detect one CPXV clade might fail to detect another genetically distinct clade. WGS bypasses this limitation entirely, providing a definitive genetic identity. The cost and complexity of NGS currently limit its use to reference laboratories, but its application is essential for understanding viral evolution, transmission dynamics, and the emergence of novel strains.

Virus Isolation and Electron Microscopy

Historically, virus isolation was a primary diagnostic method. CPXV can be readily cultivated in a variety of cell lines, including African green monkey kidney (MA104) and Vero cells, where it produces characteristic cytopathic effects (CPE) within 1–3 days [2, 9]. Inoculation of lesion material onto the chorioallantoic membrane (CAM) of embryonated hen's eggs produces distinctive hemorrhagic pocks, a classic feature of cowpox that helps differentiate it from other orthopoxviruses [2]. While virus isolation provides live virus for further characterization and is crucial for research (e.g., generating infectious clones for pathogenesis studies), it is significantly slower than molecular methods and requires a high level of biosafety containment (BSL-2/3), limiting its routine diagnostic utility [16]. Electron microscopy remains a valuable adjunct, particularly for novel viruses that may not be detected by targeted PCR assays. The characteristic brick-shaped morphology of orthopoxvirions, as observed in negative-stained preparations or thin sections, can provide a rapid generic diagnosis when molecular tools fail [10].

Serological Approaches

Serological testing, while not useful for diagnosing acute infection due to the delay in antibody production, plays a role in epidemiological surveillance and retrospective diagnosis. The indirect fluorescent antibody test (IFAT) is a common method for detecting CPXV-specific IgG and IgM antibodies. In humans, seroconversion with a significant rise in IgM and IgG titers has been documented within 2–3 weeks of symptom onset [2, 9]. In cats, seroprevalence studies have been conducted using similar techniques, but the interpretation of serological results in individual clinical cases is complicated by the potential for cross-reactivity between different orthopoxviruses. The absence of serological evidence does not rule out infection, particularly in immunocompromised hosts, as has been observed in HIV-positive patients where a lack of specific IgG was noted despite active viremia [9]. Therefore, serology is best reserved for population-level studies and is not a recommended primary tool for clinical diagnosis of acute CPXV in cats.

Summary of Diagnostic Workflow

An optimal diagnostic algorithm for feline cowpox integrates multiple approaches. The process begins with a high index of clinical suspicion based on lesion morphology and history. A skin biopsy should be submitted for histopathology to identify pathognomonic intracytoplasmic inclusions. Concurrently, a dry swab of an active ulcerated lesion or aspirate from a vesicle should be submitted for orthopoxvirus-specific real-time PCR. If the PCR is positive, further genetic characterization, ideally through sequencing of a larger genomic region or full-genome sequencing, should be pursued in a reference laboratory to confirm the specific CPXV strain and to contribute to vital epidemiological databases, as recommended by public health bodies like the World Health Organization (WHO) and the World Organisation for Animal Health (WOAH) for emerging zoonoses. Virus isolation can be reserved for research or when live virus is required for antiviral susceptibility testing. Clinicians must maintain a low threshold for suspecting CPXV in cats with compatible lesions, as early and accurate diagnosis is the most critical step in preventing zoonotic transmission to owners and veterinary staff.

Public Health Implications and Management of Cowpox Virus in Cats

Zoonotic Transmission Dynamics and Human Disease Burden

Cowpox virus (CPXV) represents a significant, albeit underrecognized, zoonotic threat within Eurasia, with domestic cats serving as the primary bridging host between the sylvatic reservoir and human populations. The ecological and epidemiological architecture of CPXV transmission is fundamentally rooted in the virus’s maintenance within wild rodent populations, principally bank voles (Myodes glareolus) and wood mice (Apodemus sylvaticus), which constitute the true natural reservoir [15, 1, 3]. Cats acquire CPXV through predation on these infected rodents, subsequently developing cutaneous lesions that harbor extraordinarily high viral loads, thereby creating a potent source for human infection [4, 8]. The seroprevalence of CPXV among domestic cats has been calculated between 2% and 4%, indicating a substantial population of felines that have been exposed to the virus, with a proportion of these animals actively shedding virions during clinical episodes [1]. This transmission pathway is particularly insidious because the infected cat may present with relatively innocuous-looking lesions, erythematous annular and eroded areas, that nonetheless contain infectious virus capable of penetrating minor skin fissures in human handlers [4, 1].

The public health implications of this zoonotic cycle are amplified by the cessation of routine smallpox vaccination approximately four decades ago. Vaccinia virus vaccination conferred significant cross-protective immunity against other orthopoxviruses, including CPXV, and the waning of this immunity in the general population has been directly correlated with an observed increase in the incidence of zoonotic orthopoxvirus infections across Europe [4, 7]. Individuals born after the discontinuation of mandatory vaccination, essentially the entire population under 40–50 years of age in most European nations, possess no pre-existing immunological defense against CPXV, rendering them susceptible to infection upon exposure [4, 9]. This demographic shift has transformed CPXV from a rare occupational hazard of dairy workers into a broader public health concern affecting pet owners, veterinary personnel, and even individuals with indirect contact through exotic animal exhibitions [2, 13].

Clinical Spectrum and Risk Stratification in Human Infections

Human CPXV infection typically manifests as a localized, self-limiting disease in immunocompetent individuals, with an incubation period of 8–12 days following viral implantation through broken skin [1, 9]. The characteristic clinical progression begins as an erythematous macule that evolves through papular, vesicular, and pustular stages before culminating in a hard, black eschar surrounded by erythema and edema, often accompanied by painful regional lymphadenopathy, fever, and malaise [4, 5, 6]. The lesions are typically solitary or few in number, reflecting the localized nature of the infection in hosts with intact immune systems, and healing is markedly delayed, requiring 4–8 weeks and frequently resulting in atrophic scarring [1, 6, 7]. However, the clinical picture can be dramatically more severe in immunocompromised populations, where CPXV has the capacity to cause disseminated, smallpox-like disease with fatal outcomes. A documented case involving a 35-year-old HIV-positive patient in Germany demonstrated the lethal potential of CPXV: the patient developed generalized skin lesions, severe respiratory distress syndrome, and septic shock, ultimately succumbing to the infection despite intensive supportive care [9]. This case underscores the critical importance of risk stratification, as the virus can exploit immunological vulnerabilities to produce systemic disease that is clinically indistinguishable from smallpox or severe monkeypox [9].

The potential for severe disease extends beyond HIV/AIDS patients to include individuals with other immunosuppressive conditions, those receiving chemotherapy or transplant-related immunosuppression, and patients with dermatological conditions such as eczema, which can predispose to widespread cutaneous involvement [2, 9]. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have recognized orthopoxviruses as re-emerging threats, and the documented ability of CPXV to cause fatal systemic infections in immunocompromised hosts necessitates that public health authorities maintain heightened surveillance and preparedness for these vulnerable populations [9].

Diagnostic Challenges and Public Health Surveillance Imperatives

The diagnosis of human CPXV infection presents considerable challenges due to its rarity and the consequent lack of clinical familiarity among healthcare providers. The differential diagnosis is broad and includes cat scratch disease (Bartonella henselae), cutaneous anthrax, tularemia, orf virus infection, herpes simplex, herpes zoster, and even malignancies such as Kaposi sarcoma [5, 6, 9]. Misdiagnosis is common, leading to inappropriate antibiotic therapy and delayed implementation of infection control measures. The case of a 64-year-old farmer with facial lesions initially treated empirically with intravenous acyclovir and ceftriaxone exemplifies this diagnostic difficulty, with the correct diagnosis of CPXV only established through polymerase chain reaction (PCR) testing of skin swab material [5]. PCR-based diagnostics, ideally coupled with sequencing of the hemagglutinin (HA) gene or whole-genome sequencing, represent the gold standard for confirmation, offering high sensitivity and the capacity to differentiate CPXV from other orthopoxviruses [4, 12, 5].

From a public health surveillance perspective, the current status of CPXV as a non-notifiable disease in most European countries represents a critical gap in monitoring capacity. Several authors have argued compellingly that human cowpox should be made a notifiable disease to enhance awareness, verify the suspected increase in incidence, and track the severity of cases in the post-vaccination era [7]. The World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO) have emphasized the importance of a One Health approach to emerging zoonotic orthopoxviruses, recognizing that surveillance in both animal reservoirs and human populations is essential for early detection and response. The genomic evidence demonstrating that CPXV is not a single species but a polyphyletic assemblage of at least five distinct major clusters, including clades more closely related to Variola virus (the causative agent of smallpox) than to other CPXV strains, further complicates surveillance efforts and underscores the need for molecular characterization of clinical isolates [15, 13]. The discovery of a novel orthopoxvirus in a cat in Italy that was distantly related to CPXV and more closely related to ectromelia virus highlights the potential for previously unrecognized zoonotic orthopoxviruses to emerge, necessitating robust surveillance infrastructure [10].

Infection Control and Antiviral Management Strategies

The management of human CPXV infection is primarily supportive, as the disease is self-limiting in immunocompetent individuals. Wound care with topical antiseptics, pain management, and monitoring for secondary bacterial infection constitute the mainstays of treatment [6]. However, for severe cases, particularly in immunocompromised patients or those with disseminated disease, specific antiviral therapy may be indicated. Tecovirimat (ST-246), an orthopoxvirus-specific inhibitor of the VP37 envelope wrapping protein, has demonstrated efficacy against multiple orthopoxviruses and is stockpiled by several governments for smallpox preparedness [9]. Cidofovir and its lipid conjugate brincidofovir (CMX-001) also exhibit in vitro and in vivo activity against orthopoxviruses, though their use is limited by nephrotoxicity and the need for intravenous administration [9]. Vaccinia immune globulin (VIG) may be considered for post-exposure prophylaxis or treatment in certain circumstances, although its efficacy against CPXV specifically has not been rigorously established [9].

Infection control measures are paramount in preventing nosocomial transmission. Healthcare workers caring for patients with suspected or confirmed CPXV should adhere to standard and contact precautions, including the use of gowns, gloves, face shields, and masks, as the virus can be present in lesion fluid and potentially in respiratory secretions in cases of disseminated disease [9]. Patients should be placed in single rooms, and closed suctioning systems should be employed if respiratory involvement necessitates mechanical ventilation [9]. Environmental decontamination is critical, as orthopoxviruses are relatively stable in the environment and resistant to many common disinfectants; sodium hypochlorite (bleach) solutions and quaternary ammonium compounds are effective when used at appropriate concentrations and contact times.

Veterinary Public Health and the One Health Imperative

From a veterinary public health perspective, the management of CPXV in cats is inextricably linked to human disease prevention. Veterinary practitioners must maintain a high index of suspicion for CPXV in cats presenting with cutaneous lesions, particularly those with outdoor access and hunting behavior. The lesions in cats typically appear as erythematous annular and eroded areas, often on the head, neck, and forelimbs, and histopathological examination reveals epidermal necrosis, pyogranulomatous dermatitis, and large eosinophilic cytoplasmic inclusions within keratinocytes, pathognomonic features of poxvirus infection [4, 10]. Confirmation through PCR is recommended, and infected cats should be isolated from other animals and immunocompromised household members [12]. The potential for hospital-acquired transmission among cats, as documented in a German small animal clinic where four out of five hospitalized cats showed identical hemagglutinin sequences, underscores the need for rigorous infection control protocols within veterinary facilities [12]. Whole-genome sequencing has proven superior to HA gene sequencing alone for molecular trace-back analyses, enabling the differentiation of true nosocomial transmission from independent community-acquired infections [12].

The One Health framework is essential for addressing the complex ecology of CPXV. The virus circulates in wild rodent populations, with seroprevalence studies in Eurasian lynx in Sweden detecting orthopoxvirus DNA in 9% of animals, indicating widespread environmental exposure through predation on infected rodents [14]. The documented transmission chain from rats to elephants to humans in a German circus, where four rats (Rattus norvegicus) were seropositive for CPXV, the elephant developed disseminated disease, and the animal keeper subsequently became infected, illustrates the potential for CPXV to move through multiple species and into human populations via unexpected pathways [2]. This case also demonstrated that CPXV can maintain perfect sequence identity in the HA gene after crossing multiple species barriers, suggesting a low mutation rate that does not diminish its zoonotic potential [2].

The risk of reverse zoonosis, human-to-animal transmission, must also be considered. Given the susceptibility of cats to CPXV and the potentially high number of human shedders during the clinical phase, there is a theoretical risk of infected humans transmitting the virus back to domestic animals, particularly cats [18]. This bidirectional transmission potential necessitates that public health authorities, veterinary services, and wildlife management agencies collaborate under the auspices of international organizations such as the WHO, WOAH, and FAO to develop comprehensive surveillance and response strategies. The reclassification of CPXV from a single species to a polyphyletic assemblage of several distinct species [15, 13] has profound implications for vaccine development, diagnostic test design, and risk assessment, as different clades may exhibit varying degrees of virulence, host range, and transmissibility. A coordinated, multidisciplinary approach is required to mitigate the public health impact of this re-emerging zoonotic pathogen.

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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.