Pseudocowpox Virus

Overview and Taxonomy of Pseudocowpox Virus

Taxonomic Hierarchy and Classification

Pseudocowpox virus (PCPV) is an enveloped, double-stranded DNA virus belonging to the family Poxviridae, subfamily Chordopoxvirinae, genus Parapoxvirus [1, 10]. The genus Parapoxvirus comprises several closely related species that infect a diverse range of mammalian hosts, including ruminants, camelids, and pinnipeds. Within this genus, PCPV is one of the recognized species, alongside orf virus (ORFV), bovine papular stomatitis virus (BPSV), and the tentative species red deerpox virus, seal parapoxvirus, and camel contagious ecthyma virus [10, 20]. The classification of PCPV as a distinct species is based on biological, antigenic, and molecular criteria, though the delineation between parapoxvirus species has historically been challenging due to their high degree of genetic and antigenic similarity.

The virion morphology of PCPV is characteristic of the Parapoxvirus genus. Unlike the brick-shaped orthopoxviruses, parapoxvirus particles are ovoid to cylindrical, measuring approximately 260–300 nm in length and 160–190 nm in width, with a distinctive criss-cross pattern of surface tubules visible under negative-stain electron microscopy [5]. This unique morphology, often described as resembling a ball of yarn, is a key diagnostic feature. The viral genome consists of linear double-stranded DNA, approximately 130–140 kbp in length, with high guanine-cytosine (G+C) content typical of poxviruses. PCPV encodes a multitude of genes involved in viral replication, host immune modulation, and pathogenesis, including homologues of interleukin-10 (vIL-10) and granulocyte-macrophage colony-stimulating factor/interleukin-2 inhibition factor (GIF) [7, 22].

Genetic and Molecular Characterization

The molecular taxonomy of PCPV has been greatly refined through sequencing and phylogenetic analysis of conserved viral genes. The most frequently targeted locus for species identification is the B2L gene, which encodes a major envelope protein (42 kDa) and is highly conserved across the Parapoxvirus genus [4, 8, 9]. Comparative sequence analysis of the B2L gene allows for robust differentiation between PCPV, ORFV, and BPSV, with interspecies nucleotide identities typically ranging from 80% to 96% [15, 16]. For instance, a study in Brazil reported that the minimum genetic distance between a PCPV isolate from water buffalo and ORFV was 6.7%, and from BPSV was 18.4%, based on B2L sequences [9].

Beyond B2L, other genetic markers have proven valuable for molecular epidemiological studies and intra-genus differentiation. The GIF gene (encoding the GM-CSF/IL-2 inhibition factor) has been used to characterize PCPV strains in Iraq, revealing clustering patterns that align with geographically distinct isolates from New Zealand and Finland [7]. The ORF11 and ORF32 genes, corresponding to vaccinia virus orthologs, have also been employed to assess genetic diversity among Japanese PCPV isolates, with ORF32 exhibiting greater variability and thus greater utility for fine-scale molecular epidemiology [11]. The vIL-10 gene, a homologue of host interleukin-10, is another important target; functional studies have shown that PCPV vIL-10 exhibits reduced receptor binding and anti-inflammatory activity compared to ORFV vIL-10, suggesting species-specific adaptations in immune evasion strategies [22].

Phylogenetic analyses consistently demonstrate that PCPV isolates form a monophyletic clade distinct from ORFV and BPSV, though the genetic distance between PCPV and ORFV is relatively narrow, reflecting their shared evolutionary ancestry within the genus [4, 18]. Importantly, isolates from different host species and geographic regions often cluster together, indicating that host origin does not strictly correlate with genetic lineage. For example, a PCPV isolate from a camel in Sudan may cluster more closely with bovine PCPV strains from Europe than with ORFV strains from the same geographic region [25]. This intermingling of host-derived sequences suggests that PCPV possesses a broad host range and that cross-species transmission events are not uncommon.

Relationship to Other Parapoxviruses and Taxonomic Challenges

The taxonomic distinction between PCPV and its closest relatives, BPSV and ORFV, is supported by both clinical and molecular evidence. BPSV is primarily associated with oral lesions in calves, while PCPV is classically linked to teat and udder lesions in lactating cattle, though this clinical delineation is not absolute [24]. Coinfections with PCPV and BPSV have been documented in the same animal, as reported in a Japanese calf that sequentially shed both viruses over a 22-day period [6]. Similarly, dual infections with PCPV and lumpy skin disease virus (LSDV) have been identified in Nigerian cattle, highlighting the potential for mixed poxvirus infections in livestock populations [17].

The classification of parapoxviruses infecting camelids and pinnipeds remains an area of active research and taxonomic debate. Camel contagious ecthyma (CCE) is caused by a parapoxvirus that has been provisionally designated as camel parapoxvirus (CPPV). However, phylogenetic analyses based on the B2L gene have shown that CPPV isolates can be classified into two genetic lineages: one clustering with ORFV and the other with PCPV [25]. This finding suggests that CCE may be caused by multiple parapoxvirus species, or that the causative agent represents a distinct but closely related lineage that is not yet fully resolved. The World Organisation for Animal Health (WOAH) and the International Committee on Taxonomy of Viruses (ICTV) continue to evaluate the taxonomic status of these isolates. Similarly, parapoxviruses isolated from seals and sea lions form a separate clade within the genus, provisionally designated as seal parapoxvirus, and are genetically distinct from PCPV and other terrestrial parapoxviruses [20].

From an evolutionary perspective, the parapoxviruses are thought to have coevolved with their respective mammalian hosts over millennia, with PCPV likely having originated in bovid ancestors. The detection of PCPV DNA in ticks infesting cattle in Burkina Faso raises intriguing questions about the potential role of arthropod vectors in viral maintenance and transmission, though mechanical transmission via houseflies (Musca domestica) has been experimentally supported [3, 19]. The genus Parapoxvirus is generally considered to exhibit a narrow host range relative to orthopoxviruses, but the expanding host spectrum of PCPV, now documented in American bison, water buffalo, and dromedary camels, challenges this paradigm [5, 9, 25].

Zoonotic and Agricultural Significance

PCPV is a zoonotic pathogen of global importance, recognized by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) as the causative agent of milker's nodule (paravaccinia) in humans [1, 10, 12, 13]. Human infection occurs through direct contact with infected cattle, often during milking, and typically results in self-limiting, papular-vesicular lesions on the hands and forearms. The virus is capable of inducing erythema multiforme as a rare complication, analogous to that observed with ORFV [12]. The zoonotic potential of PCPV underscores the need for accurate differential diagnosis from other zoonotic poxviruses, such as vaccinia virus and cowpox virus, which can produce clinically indistinguishable lesions [21, 23].

From an agricultural perspective, PCPV is endemic in cattle populations worldwide, causing significant economic losses due to reduced milk production, mastitis, and labor costs associated with management of affected animals [1, 23, 24]. The virus is classified under the WOAH Terrestrial Animal Health Code as a disease of zoonotic and economic relevance, though it is not subject to official control measures in most countries. The development of recombinant PCPV vectors for cancer immunotherapy represents a novel and unexpected dimension of this virus's relevance, highlighting its potential utility beyond conventional veterinary medicine [2, 14].

Molecular Pathogenesis and Host Immune Evasion

The molecular pathogenesis of Pseudocowpox virus (PCPV) represents a sophisticated paradigm of host-parasite co-evolution, wherein the virus has acquired a remarkable arsenal of immunomodulatory genes that enable it to establish productive infection, persist in the face of robust host defenses, and facilitate transmission across multiple mammalian species. As a member of the genus Parapoxvirus within the family Poxviridae, PCPV shares the hallmark features of poxviral replication, cytoplasmic replication, large double-stranded DNA genome, and expression of numerous virulence factors, yet exhibits distinct pathogenic strategies that differentiate it from orthopoxviruses and even from other parapoxviruses such as Orf virus (ORFV) and Bovine papular stomatitis virus (BPSV) [1, 10]. Understanding the molecular underpinnings of PCPV pathogenesis is essential not only for appreciating its zoonotic potential but also for recognizing the virus as an emerging platform for immunotherapeutic vector development [2, 14].

Molecular Determinants of Cellular Tropism and Entry

PCPV exhibits a pronounced tropism for keratinocytes and mucosal epithelial cells, which is consistent with its clinical presentation as a mucocutaneous pathogen that produces proliferative and then necrotic lesions on the teats, udder, muzzle, and oral mucosa of cattle [1, 26]. The initial steps of infection are mediated by the major envelope protein encoded by the B2L gene (ORF011), a homologue of the vaccinia virus H3L protein that functions as a heparin-binding surface protein facilitating viral attachment to glycosaminoglycans on the host cell surface [4, 8, 9]. The B2L gene is highly conserved among parapoxviruses, and its sequence has been the primary target for molecular characterization and phylogenetic studies of PCPV isolates worldwide, with nucleotide identities ranging from 95–100% among PCPV strains and approximately 83–98% identity with ORFV and BPSV [5, 9, 15, 25]. This conservation underscores the critical role of the envelope protein in mediating entry into susceptible cells, which include epithelial cells of the skin and mucous membranes, as well as certain immune cell populations.

Interestingly, PCPV demonstrates a restricted cellular tropism in vitro that may explain aspects of its in vivo pathogenesis. Early studies using bovine macrophage cultures and tracheal-ring organ cultures revealed that, unlike cowpox virus, PCPV does not replicate in bovine macrophages, suggesting that the virus has evolved to avoid replication in professional phagocytes, a strategy that may limit early innate immune recognition [27]. This is in stark contrast to ORFV, which can infect macrophages and modulate their function. The inability of PCPV to replicate in macrophages may represent an immune evasion strategy that reduces the production of pro-inflammatory cytokines from these cells, thereby blunting the early inflammatory response and facilitating the establishment of infection in epithelial cells. Furthermore, PCPV has been shown to infect and replicate in primary human immune cells, including monocyte-derived dendritic cells (moDCs) and macrophages, albeit with differential outcomes compared to ORFV, highlighting species-specific differences in cellular permissiveness [14].

Cytopathology and the Delayed-Onset Lesion Phenotype

The pathogenesis of PCPV infection is remarkable for its protracted and asynchronous clinical course. Experimental inoculation of calves with PCPV via intranasal or transdermal routes initially results in virus replication and shedding up to day 13 post-infection (pi) without any discernible local or systemic signs; yet, between days 28 and 34 pi, a delayed and severe clinical phase ensues, characterized by the eruption of papulo-pustular, erosive-fibrinous, and scabby lesions on the muzzle that can coalesce to form extensive necrotic plaques [26]. This biphasic presentation is unprecedented among parapoxviruses and suggests that PCPV possesses unique molecular mechanisms that permit prolonged subclinical replication followed by an explosive and delayed cytopathic effect. Histopathological examination of these delayed lesions reveals marked epidermal hyperplasia with prominent rete pegs, severe orthokeratotic and parakeratotic hyperkeratosis, and multifocal keratinocyte necrosis with ballooning degeneration and the formation of typical 3–5 µm eosinophilic intracytoplasmic inclusion bodies (Bollinger bodies) [5, 24, 26]. These inclusion bodies represent sites of viral replication and assembly, and their presence is a diagnostic hallmark of parapoxvirus infection [5, 24].

The delayed lesion phenotype likely stems from the combined effects of viral immune evasion proteins that suppress early host responses, followed by an eventual loss of viral control that permits widespread keratinocyte destruction. The detection of infectious virus and viral DNA in lesions as late as 34–42 days pi indicates that PCPV can persist in the epidermis for extended periods, evading or subverting the host immune response [26]. This prolonged viral persistence is facilitated by the expression of a repertoire of immunomodulatory genes that directly antagonize key components of the innate and adaptive immune systems, including interleukin-10 (IL-10) homologues, a GM-CSF/IL-2 inhibition factor (GIF), and other yet-to-be-characterized virulence determinants.

Subversion of Cytokine Networks: The Parapoxvirus IL-10 Homologue

One of the most extensively characterized immune evasion mechanisms among parapoxviruses is the expression of a viral homologue of interleukin-10 (vIL-10). PCPV, like ORFV and BPSV, encodes an IL-10-like protein that exhibits both anti-inflammatory and immunostimulatory activities, but with quantitative and qualitative differences that may shape the unique pathogenesis of PCPV [22]. Recombinant PCPV vIL-10 binds to the IL-10 receptor 1 (IL-10R1) and suppresses the production of the pro-inflammatory chemokine monocyte chemoattractant protein-1 (MCP-1/CCL2) in lipoteichoic acid-activated THP-1 monocytes, though with reduced potency compared to ORFV vIL-10 [22]. PCPV vIL-10 also inhibits the production of IL-8 and IL-1β, key mediators of neutrophil recruitment and pyrexia, respectively. Moreover, PCPV vIL-10 retains the capacity to stimulate mast cell proliferation, suggesting that it can modulate both the innate and adaptive arms of the immune response [22].

The differential activity of PCPV vIL-10 compared to ORFV vIL-10 may explain species-specific differences in lesion severity and duration. While ORFV vIL-10 is a potent, near-equivalent mimic of cellular IL-10 that greatly enhances infection, PCPV vIL-10 appears to have evolved a more balanced immunomodulatory profile, sufficiently anti-inflammatory to suppress early innate responses and permit viral replication, yet not so potent as to completely paralyze the immune system, which could lead to disseminated disease [22]. This is consistent with the observation that PCPV infections typically remain localized and self-limiting, even while the virus exhibits prolonged persistence in the epidermis [12, 26]. The presence of vIL-10 in PCPV has been confirmed at the genetic level, with ORF32 (the vIL-10 gene) showing greater genetic diversity among PCPV strains compared to the conserved B2L gene, suggesting that immunomodulatory genes are under selective pressure from host immune responses [11].

The GIF Protein: Targeting GM-CSF and IL-2

Another critical immune evasion strategy employed by PCPV is the expression of the GM-CSF/IL-2 inhibition factor (GIF), a secreted viral protein that binds with high affinity to and neutralizes the activity of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2) [7]. These cytokines are essential for the proliferation, differentiation, and activation of multiple immune cell lineages: GM-CSF is central to the development and function of dendritic cells, macrophages, and granulocytes, while IL-2 is a key T-cell growth factor that drives clonal expansion and effector function. By sequestering these cytokines, GIF effectively blunts the generation of productive adaptive immune responses and impairs the recruitment and activation of antigen-presenting cells at the site of infection.

The GIF gene (ORF117) is unique to parapoxviruses and has been a target for molecular detection and phylogenetic analysis of PCPV. Studies in Iraq employed PCR targeting a 408-bp fragment of the GIF gene to confirm PCPV infection in dairy cows with milker’s nodule lesions, and phylogenetic analysis of the GIF sequences revealed clustering of Iraqi PCPV strains with isolates from New Zealand and Finland, demonstrating the global distribution of genetically related PCPV strains [7]. The GIF protein likely plays a pivotal role in the delayed lesion phenotype observed in experimental infections: by suppressing GM-CSF and IL-2 activity during the early phase of infection, PCPV prevents the robust recruitment and activation of dendritic cells and T cells that would normally clear the virus, allowing the virus to replicate and spread laterally within the epidermis for weeks before the adaptive response eventually overcomes this blockade [7, 26].

Interferon Modulation and Innate Immune Evasion

PCPV possesses a paradoxical relationship with the type I interferon (IFN) system that sets it apart from other poxviruses. While many poxviruses encode multiple proteins that directly inhibit IFN induction or signaling, PCPV has been shown to be a potent inducer of IFN-α in human peripheral blood mononuclear cells (PBMCs), exhibiting a 1000-fold higher induction of IFN-α compared to the extensively studied Modified Vaccinia Ankara (MVA) strain [14]. This robust IFN induction may seem counterintuitive for a virus that establishes persistent infection; however, a nuanced interpretation suggests that PCPV may exploit the IFN response to shape the local immune environment in a manner that favors viral persistence, possibly by inducing a state of “trained immunity” or by selectively activating certain antiviral pathways while suppressing others.

Indeed, when compared to other poxviruses in a screen of nine different species, PCPV uniquely increased the expression of CD86 on human monocyte-derived dendritic cells and on CD163+CD206+ M2-type macrophages derived from CD14+ monocytes, suggesting that PCPV can reprogram immunosuppressive macrophages toward an antigen-presenting phenotype [14]. In these M2 macrophages, PCPV significantly elevated the secretion of IL-18, IL-6, and IP-10 (CXCL10), cytokines and chemokines that are associated with a shift away from a suppressive tumor-associated macrophage (TAM) phenotype and toward a more pro-inflammatory, anti-tumorigenic state. This remarkable ability to modulate macrophage polarization may be a key component of PCPV pathogenesis in vivo, as it could redirect the local immune response away from a purely antiviral state and toward a more balanced, wound-healing response that allows viral persistence and eventual resolution without severe tissue destruction.

Furthermore, co-culture experiments demonstrated that PCPV could overcome the immunosuppressive effects of myeloid-derived suppressor cells (MDSCs) on human autologous CD8+ T cells, indicating that the virus can directly or indirectly antagonize the suppressive functions of MDSCs [14]. This property is of particular interest because it suggests that PCPV has evolved to counteract multiple layers of host immunosuppression, both from regulatory cells and from its own vIL-10, to maintain a permissive environment for replication while avoiding the immunopathology that would result from a completely uncontrolled infection.

Evasion of Adaptive Immunity and Co-Infection Dynamics

The ability of PCPV to infect and persist in the presence of an adaptive immune response is evidenced by the detection of viral DNA in lesions weeks after the onset of clinical signs, as well as the demonstration that infected animals seroconvert with only partial virus neutralization at low serum dilutions [26]. This partial neutralization suggests that PCPV may employ mechanisms to reduce the accessibility of neutralizing epitopes on the virion surface, a strategy common among poxviruses that produce both intracellular mature virions (IMV) and extracellular enveloped virions (EEV). While not directly demonstrated for PCPV, the presence of an EEV form in other poxviruses allows the virus to spread cell-to-cell and disseminate within the host while being partially shielded from antibody-mediated neutralization.

The clinical and molecular co-detection of PCPV with other poxviruses, including BPSV, vaccinia virus (VACV), and lumpy skin disease virus (LSDV), in the same animal or even the same lesion, provides further evidence for the sophisticated interplay between PCPV and the host immune system [6, 17, 23, 24]. Sequential detection of PCPV followed by BPSV in the same calf demonstrates that infection with one parapoxvirus does not necessarily confer cross-protective immunity against another, and that co-infection or sequential infection may occur when animals are exposed to multiple viral species in the same environment [6]. Co-infections between PCPV and LSDV have been documented in Nigerian cattle, where 11.9% of samples positive for LSDV were also positive for PCPV [17]. Similarly, co-infection of PCPV with VACV and BPSV has been reported in Brazilian cattle, with zoonotic transmission to milkers observed for all three viruses [23, 24]. These findings

Epidemiology and Global Distribution in Cattle

The global epidemiological landscape of pseudocowpox virus (PCPV) in cattle is characterized by a paradox of ubiquity and underrecognition. While the virus is acknowledged as a cosmopolitan pathogen of bovines, the true extent of its distribution, incidence, and economic impact remains obscured by diagnostic challenges, clinical similarities to other poxviral diseases, and a historical lack of systematic surveillance in many regions. PCPV, a member of the genus Parapoxvirus within the family Poxviridae, is the etiological agent of pseudocowpox, a disease primarily affecting dairy cattle worldwide [1, 10]. The virus is endemic in virtually all cattle-rearing regions, yet its presence is often only recognized during overt outbreaks or when zoonotic transmission to humans, manifesting as milker's nodule, occurs [10, 12, 13]. This section provides an exhaustive analysis of the epidemiological patterns, global distribution, transmission dynamics, and host range of PCPV in cattle, drawing upon molecular, serological, and ecological evidence from diverse geographical settings.

Global Distribution and Regional Prevalence

PCPV exhibits a truly global distribution, with confirmed reports spanning every continent where cattle are raised. The virus has been molecularly characterized and phylogenetically analyzed in cattle populations across Europe, Asia, Africa, the Americas, and Oceania. However, the reported prevalence varies dramatically based on the diagnostic methods employed, the target population (dairy versus beef), and the clinical presentation under investigation. In many regions, PCPV is likely enzootic, circulating at low levels within herds and causing sporadic, often mild disease that goes unreported. The virus is frequently detected in dairy cattle, where the milking process facilitates both transmission and human exposure [10, 23].

In Africa, PCPV has been documented in several countries, though data remain sparse for many nations. A seminal study in Burkina Faso detected PCPV DNA in 8.2% of tick pools collected from cattle, highlighting the potential for vector-borne transmission and the virus's circulation in West African livestock systems [3]. This finding is particularly significant as it demonstrates that PCPV can be harbored by ticks, specifically Amblyomma variegatum and Hyalomma spp., which are abundant in the region. The first molecular confirmation of PCPV in Zambia was reported in 2020, where an outbreak initially suspected to be lumpy skin disease (LSD) was identified as pseudocowpox through high-resolution melting (HRM) analysis and B2L gene sequencing [4]. This case underscores the critical need for differential diagnosis, as PCPV and LSDV can present with similar cutaneous lesions, yet their control measures differ substantially. In Nigeria, a 2023 investigation of suspected LSD outbreaks revealed that 11.9% of samples were co-infected with LSDV and PCPV, marking the first documented co-infection in the country [17]. This finding has profound epidemiological implications, suggesting that PCPV may be more widespread in African cattle than previously appreciated and that its presence may be masked by more clinically dramatic capripoxvirus infections. Similarly, in Botswana, PCPV was detected in a single cattle sample during a broader poxvirus surveillance study, with the isolate clustering phylogenetically between camel and cattle PCPV strains, indicating potential cross-species transmission events [18].

In Asia, PCPV has been confirmed in Japan, Iraq, and Bangladesh. Japan provides a particularly instructive case study. For decades, PCPV was not isolated in the country, leading to uncertainty regarding its epidemiological status. The first isolation occurred in 2016 from a calf in Yamaguchi Prefecture presenting with oral vesicles, and subsequent phylogenetic analysis confirmed the virus as PCPV [8]. Since then, molecular surveillance has revealed that PCPV is circulating in multiple prefectures, including Hokkaido, Japan's largest cattle-producing region [11]. A 2019 study in Japan documented the sequential detection of PCPV and bovine papular stomatitis virus (BPSV) in the same calf, demonstrating that co-infections and sequential infections with different parapoxviruses can occur within individual animals and on the same farm [6]. In Iraq, the first phylogenetic characterization of PCPV from dairy cows in Al-Qadisiyah province was reported in 2019, with isolates showing close genetic relationships to strains from New Zealand and Finland, suggesting international viral movement, likely through livestock trade [7]. In Bangladesh, PCPV DNA was detected in scab and oral swab samples from dairy cows and calves in the Rangpur district, representing the first molecular confirmation of zoonotic poxviruses in the country [32].

South America, particularly Brazil, has emerged as a hotspot for PCPV research. A retrospective study in Goiás State from 2010 to 2018 identified PCPV in six out of 25 confirmed poxvirus outbreaks in cattle, with cases predominantly occurring in dairy herds during the dry season [23]. In the Distrito Federal region, a similar retrospective analysis from 2015 to 2018 found PCPV in nine out of 52 confirmed poxvirus cases, with an additional five cases of co-infection with BPSV [24]. These studies collectively demonstrate that PCPV is a significant component of the bovine poxvirus landscape in Brazil, often co-circulating with vaccinia virus (VACV) and BPSV. The detection of PCPV DNA in milk samples from naturally infected cows in Brazil [30] represents a critical epidemiological finding, as it suggests a potential route for non-contagious transmission and contamination of dairy products, with implications for both animal and public health. Furthermore, PCPV has been documented in water buffalo (Bubalus bubalis) in Brazil, expanding the known host range and indicating that the virus can infect multiple bovine species within the same ecological niche [9].

In North America, PCPV has been reported in both domestic cattle and captive bison. A 2020 report described PCPV infection in an American bison (Bison bison) in the United States, presenting with characteristic proliferative and ulcerative lesions on the udder, perineum, and thighs [5]. This case is particularly noteworthy as it represents the first documented infection in this species, suggesting that PCPV may have a broader host range than previously recognized and that bison herds could serve as a reservoir for the virus. In Colombia, PCPV was detected in dairy cattle and farmworkers in the Amazon region during a 2014 outbreak investigation, highlighting the zoonotic risk in tropical livestock systems [21].

Transmission Dynamics and Risk Factors

The epidemiology of PCPV is fundamentally shaped by its transmission biology. The virus is primarily transmitted through direct contact with infected animals, particularly via lesions on the teats, udder, and oral mucosa [1, 10]. The milking process is a potent amplifier of transmission, as contaminated hands, milking machines, and cloths can mechanically transfer the virus to susceptible animals and humans [10, 28]. This explains the higher prevalence of PCPV in dairy herds compared to beef herds, as documented in Brazilian studies [23, 24]. The virus can also be transmitted indirectly through contaminated fomites, including bedding, feeding equipment, and environmental surfaces [19].

A critical and underappreciated aspect of PCPV epidemiology is the role of mechanical vectors. The detection of PCPV DNA in ticks infesting cattle in Burkina Faso [3] raises the possibility of arthropod-borne transmission. While the virus is not considered an arbovirus in the classical sense, ticks may serve as mechanical vectors, carrying the virus from infected to susceptible animals during feeding. This could be particularly important in transhumance systems, where cattle move across large geographic areas and come into contact with diverse tick populations. The study in Burkina Faso found that BPSV-positive ticks were associated with herds sharing water and pasture resources and with a history of seasonal transhumance, suggesting that common grazing practices facilitate viral spread [3]. Similarly, houseflies (Musca domestica) have been implicated as potential mechanical vectors for parapoxviruses, including PCPV. A Japanese study detected PCPV DNA on the body surface and in the feces of houseflies collected from cattle and sheep farms, with viral sequences identical to those found on cattle and in barn environments [19]. This finding suggests that flies could transport the virus from contaminated environments to susceptible animals, particularly during the summer months when fly populations peak.

The role of subclinical carriers in PCPV transmission is another critical epidemiological factor. PCPV can be detected in the oral cavity and on the body surface of cattle without overt clinical signs, as demonstrated in Japanese studies [19]. These subclinically infected animals may serve as silent reservoirs, shedding virus into the environment and perpetuating transmission cycles within herds. The detection of PCPV in environmental samples, including barn surfaces and water sources, further supports the notion that contaminated environments play a significant role in viral persistence and spread [6, 19].

Host Range and Species Susceptibility

While cattle are the primary reservoir and most economically significant host for PCPV, the virus exhibits a surprisingly broad host range. Beyond domestic cattle (Bos taurus and Bos indicus), PCPV has been documented in water buffalo (Bubalus bubalis) in Brazil [9], American bison (Bison bison) in the United States [5], and dromedary camels (Camelus dromedarius) in Sudan [25]. In camels, PCPV is one of the causative agents of camel contagious ecthyma (CCE), a disease that causes significant economic losses in camel-rearing regions of Africa and Asia [25]. Phylogenetic analyses of camel PCPV isolates have revealed two distinct genetic lineages, an Asian lineage and an African lineage, indicating that the virus has undergone geographic diversification within camel populations [25]. The detection of PCPV in both cattle and camels in the same geographic regions (e.g., Sudan) raises the possibility of cross-species transmission, although the direction and frequency of such events remain unclear.

The zoonotic potential of PCPV is well-established, with human infections, termed milker's nodule, occurring through direct contact with infected cattle, particularly during milking [10, 12, 13, 28]. Human cases have been reported globally, including in Australia [12], Germany [13], Russia [28], Colombia [21], and Georgia [31]. The clinical presentation in humans is typically benign and self-limiting, but complications such as erythema multiforme can occur [12]. The World Health Organization (WHO) and the Food and Agriculture Organization (FAO) recognize PCPV as a zoonotic pathogen of occupational importance, particularly for dairy farmers, milkers, and veterinarians. The high incidence of PCPV infections during Eid al-Adha, the Islamic feast of sacrifice, has been noted, likely due to increased contact with livestock during this period [13].

Molecular Epidemiology and Phylogenetic Insights

The molecular epidemiology of PCPV has been greatly advanced by the application of PCR-based assays and phylogenetic analysis of conserved genes, particularly the B2L gene encoding the major envelope protein [4, 7-9, 11, 17, 18, 32, 33]. The B2L gene is highly conserved among parapoxviruses, but sufficient genetic variation exists to differentiate PCPV from BPSV and ORFV and to identify geographic and host-specific lineages. Phylogenetic studies have consistently shown that PCPV isolates from cattle form a distinct clade within the Parapoxvirus genus, separate from BPSV and ORFV [9, 15]. However, within the PCPV clade, there is considerable genetic diversity, with isolates from different geographic regions often clustering separately.

For example, PCPV strains from Japan have been shown to form distinct clusters based on ORF11 and ORF32 gene sequences, with some strains grouping closely with isolates from other Asian countries and others showing unique genetic signatures [11]. Similarly, PCPV isolates from Zambia and Botswana have been found to cluster with isolates from cattle and reindeer in other parts of Africa, suggesting a regional circulation pattern [4, 18]. The detection of PCPV in ticks in Burkina Faso [3] and the subsequent phylogenetic characterization of these strains will be crucial for understanding the role of vectors in viral dispersal and evolution.

The use of high-resolution melting (HRM) assays has emerged as a powerful tool for the rapid detection and differentiation of PCPV from other poxviruses, including LSDV, ORFV, and BPSV [4, 17, 18, 29]. This is particularly important in regions where multiple poxviruses co-circulate and produce similar clinical signs, such as in Africa and South America. The HRM assay developed by Gelaye et al. (2017) can simultaneously detect and differentiate eight poxviruses from three genera, making it an invaluable tool for surveillance and outbreak response [29].

Economic and Agricultural Impact

The economic impact of PCPV on the cattle industry is difficult to quantify due to underreporting, but it is likely substantial, particularly in dairy operations. The disease causes lesions on the teats and udder, leading to pain during milking, reduced milk yield, and increased susceptibility to secondary bacterial mastitis [1, 10, 23]. In severe cases, the lesions can become extensive and necrotic, requiring veterinary intervention and potentially leading to culling of affected animals. The zoonotic potential of PCPV also has economic implications, as human infections can result in lost labor and medical costs. The World Organisation for Animal Health (WOAH) includes PCPV in its list of notifiable diseases for some member countries, reflecting its significance for international trade and animal health.

In conclusion, the epidemiology and global distribution of PCPV in cattle is characterized by widespread enzootic circulation, significant underdiagnosis, and a complex interplay of transmission routes involving direct contact, fomites, and potential mechanical vectors. The virus is present in all major cattle-rearing regions, with molecular evidence confirming its presence in Africa, Asia, the Americas, and Oceania. The broad host range, including cattle, buffalo, bison, and camels, coupled with the zoonotic risk to humans, underscores the need for enhanced surveillance, improved diagnostic capacity, and a One Health approach to disease management. The continued application of molecular epidemiological tools will be essential for tracking viral evolution, identifying transmission pathways, and informing control strategies.

Zoonotic Transmission and Public Health Implications

Pseudocowpox virus (PCPV) represents a significant, yet frequently underdiagnosed, zoonotic pathogen of global occupational and public health concern. As a member of the genus Parapoxvirus within the family Poxviridae, PCPV is the etiological agent of pseudocowpox in cattle and the corresponding human disease, milker’s nodule [1, 10, 13]. The zoonotic potential of PCPV is inextricably linked to the close contact between humans and livestock, particularly within the dairy industry, and the virus's capacity for efficient cross-species transmission. The World Health Organization (WHO) and the World Organisation for Animal Health (WOAH) recognize parapoxviruses as important zoonotic agents, underscoring the need for robust surveillance and differential diagnosis, as their clinical presentation can mimic more severe diseases such as anthrax or orthopoxvirus infections [10, 12, 31].

Mechanisms and Routes of Zoonotic Transmission

The primary route of PCPV transmission to humans is through direct, percutaneous inoculation of the virus from infected cattle. This occurs most frequently during occupational activities such as manual milking, where milkers' hands come into direct contact with active lesions on the teats and udders of dairy cows [10, 12, 28]. The virus exploits pre-existing abrasions, cuts, or microtrauma in the human epidermis to establish infection [13]. Beyond direct contact, atypical transmission events have been documented, including transmission via a calf bite, which introduces the virus through contaminated saliva or lesional material into the wound [13]. The detection of PCPV DNA in milk from naturally infected cows further expands the potential exposure routes, raising the possibility of viral contamination of raw milk and unpasteurized dairy products, which could serve as a fomite for human infection [30].

The role of mechanical vectors in the epidemiology of PCPV is an emerging area of investigation. Parapoxviruses, including PCPV, have been detected on the body surface and in the feces of houseflies (Musca domestica) collected from cattle and sheep farms [19]. The viral sequences obtained from flies were genetically identical to those found on the body surfaces of cattle and within barn environments, providing strong circumstantial evidence for mechanical transmission by these synanthropic insects [19]. This mechanism may facilitate viral spread between animals and potentially to humans, especially in environments where hygiene is compromised. Furthermore, PCPV has been molecularly identified in ticks (Amblyomma variegatum and Hyalomma spp.) infesting cattle in Burkina Faso, with a prevalence of 8.2% in tick pools [3]. While the biological significance of this finding requires further elucidation, ticks may act as mechanical vectors or, less likely, as biological reservoirs, it highlights an unexplored transmission pathway that complicates control efforts, particularly in regions with high tick burdens and transhumance practices [3].

Clinical Manifestations and Complications in Humans

In immunocompetent individuals, PCPV infection typically manifests as milker’s nodule, a self-limiting, benign skin condition. Following an incubation period of approximately 5 to 14 days, a solitary or few erythematous, papular, or nodular lesions develop, most commonly on the hands, fingers, and forearms [10, 13]. The lesions progress through characteristic stages: a maculopapular stage, a target-like stage with a red center and white halo, an acute nodular stage, and a regressive papillomatous stage, ultimately resolving without scarring over a period of 4 to 8 weeks [10, 12]. The clinical course is generally uncomplicated, and the infection confers immunity, albeit of short duration, lasting only several months [28].

However, PCPV infection can be associated with significant complications. A well-documented but rare sequela is erythema multiforme (EM), an acute, immune-mediated hypersensitivity reaction. A case report from regional Australia described an 18-year-old female who developed EM following a confirmed PCPV infection from milking infected cattle [12]. The presentation featured the characteristic target-like lesions of EM, which typically appear 1 to 3 weeks after the initial viral lesion. This phenomenon, more commonly associated with orf virus, underscores the systemic immunological impact of PCPV and the need for clinicians to recognize this potential complication [12]. The pathogenic mechanism is hypothesized to involve a cell-mediated immune response to viral antigens, leading to keratinocyte apoptosis [12]. In immunosuppressed individuals, PCPV infection can take a severe and protracted course, with the potential for large, persistent, and disfiguring lesions that require aggressive surgical or antiviral intervention [13].

Public Health Burden and Diagnostic Challenges

The true global incidence of milker’s nodule is almost certainly underestimated. The disease is not reportable in most countries, and its self-limiting nature often leads to underdiagnosis or misdiagnosis. As noted by the US Centers for Disease Control and Prevention (CDC), the differential diagnosis of vesicular and pustular skin lesions on the hands of livestock workers must include milker’s nodule, but also orf, cowpox, vaccinia virus, cutaneous anthrax, tularemia, and herpes simplex [12, 13, 31]. In regions like Georgia, where anthrax is endemic, retrospective screening of anthrax-negative clinical specimens revealed that 64 out of 148 human samples were positive for zoonotic poxviruses, including PCPV [31]. This finding powerfully illustrates that PCPV infections occur far more frequently than recognized and are often mistaken for other diseases of public health importance.

Further complicating the diagnostic picture is the co-circulation of PCPV with other poxviruses in livestock populations. Co-infections with bovine papular stomatitis virus (BPSV) and vaccinia virus (VACV) are well-documented in cattle, and these mixed infections can be transmitted to humans [6, 23, 24, 30]. In Brazil, a study of dairy farms experiencing exanthematic disease outbreaks detected PCPV DNA in milk, and in 8 of 12 positive milk samples, there was also co-contamination with VACV DNA [30]. This dual contamination of milk represents a novel public health hazard, as both viruses can infect humans through direct contact or potentially through consumption of unpasteurized products. The simultaneous circulation of multiple poxvirus species in a single herd necessitates the use of sophisticated molecular diagnostic tools, such as high-resolution melting (HRM) assays, to accurately identify the causative agent and guide appropriate public health responses [18, 24, 29].

Immunomodulation and Viral Pathogenesis at the Human-Animal Interface

The biological underpinnings of PCPV’s zoonotic success lie in its arsenal of immunomodulatory proteins. Of particular significance is the viral homolog of interleukin-10 (vIL-10), which is encoded by PCPV and other parapoxviruses [22]. The PCPV IL-10, while showing reduced activity compared to the ORFV IL-10, retains the capacity to bind the human IL-10 receptor and exert anti-inflammatory effects, such as inhibiting the production of monocyte chemoattractant protein-1 (MCP-1) [22]. This suppression of the early innate immune response at the site of infection likely facilitates viral replication and contributes to the characteristic persistence of PCPV lesions, both in cattle and in humans [22, 26]. This immune evasion strategy not only promotes transmission from the bovine host but also explains the relatively indolent clinical course of milker’s nodule, as the virus dampens the inflammatory signals that would otherwise trigger rapid clearance.

Broader Host Range and Implications for Emerging Zoonotic Threats

PCPV is not restricted to cattle; its host range extends to other domestic and wild ruminants, broadening the potential for human exposure. Infections have been documented in water buffalo (Bubalus bubalis) in Brazil, American bison (Bison bison) in the United States, and dromedary camels in Sudan [5, 9, 25]. The detection of PCPV in camel contagious ecthyma lesions in Sudan, where it clusters with African PCPV lineages, indicates that camels may serve as a reservoir for human infection in regions where close human-camel contact is common [25]. The case of a bison infected with PCPV, presenting with vulvar and udder lesions, highlights that both captive and free-ranging wildlife can harbor the virus, creating opportunities for spillover to veterinarians, hunters, and wildlife handlers [5]. The genetic plasticity of PCPV is further demonstrated by the presence of unique PCPV strains in Botswana that cluster between camel and cattle isolates, suggesting ongoing cross-species transmission and viral adaptation [18].

The public health implications of this broad host range are profound. In regions with transhumance practices, such as West Africa, the movement of cattle across borders facilitates the dissemination of PCPV, and the presence of the virus in ticks adds a layer of complexity to its epidemiology [3]. The first documented co-infection of PCPV with lumpy skin disease virus (LSDV) in Nigeria underscores the potential for syndemic interactions that could exacerbate disease severity in livestock and, by extension, increase the zoonotic risk to humans [17]. A coordinated One Health approach is essential, emphasizing the need for cross-sectoral surveillance linking veterinary, medical, and entomological services to monitor the circulation of PCPV and related parapoxviruses.

Clinical Manifestations and Differential Diagnosis

Introduction to Pseudocowpox Virus Disease

Pseudocowpox virus (PCPV), a member of the genus Parapoxvirus within the family Poxviridae, is the etiological agent of pseudocowpox, a globally distributed zoonotic disease primarily affecting cattle. The clinical manifestations of PCPV infection in both its primary bovine hosts and incidental human hosts are highly characteristic yet often pose a formidable diagnostic challenge due to the striking clinical overlap with other poxviral and non-poxviral vesicular, pustular, and erosive conditions [1, 10, 17]. Understanding the full spectrum of clinical presentation, from the classical teat lesions in cattle to the rare but severe manifestations in atypical hosts and humans, is paramount for accurate field diagnosis, appropriate management, and timely implementation of control measures. This section provides an exhaustive analysis of the clinical disease in cattle, other susceptible species, and humans, followed by a comprehensive differential diagnosis that integrates clinical, epidemiological, and molecular perspectives.

Clinical Manifestations in the Primary Bovine Host

In cattle, pseudocowpox is often a mild, self-limiting infection, but its economic impact stems from lost milk production, increased risk of secondary mastitis, and labor costs associated with managing affected animals. The hallmark lesions are typically confined to the teats and udder of lactating cows, a distribution intrinsically linked to transmission via milking machines, contaminated fomites, and the hands of milkers [1, 10, 28]. The incubation period in natural and experimental infections is approximately 3 to 7 days, though experimental studies have revealed a far more complex and protracted pathogenesis than previously appreciated [26].

Classical Teat and Udder Lesions: The disease begins with the formation of small, erythematous papules that rapidly progress to vesicles and then to characteristic thick, dark red to brown scabs or crusts. A pathognomonic feature is the development of a "ring" or "horseshoe-shaped" scab, where the center of the lesion begins to heal and re-epithelialize while the periphery remains raised, crusted, and inflamed [1]. This distinct morphology, often referred to as "ring scab," can persist for 2 to 4 weeks before spontaneous resolution, typically without scarring. The lesions are often painful, leading to milk let-down failure, agitation during milking, and increased susceptibility to bacterial mastitis, particularly Staphylococcus aureus and Streptococcus agalactiae.

Atypical and Severe Manifestations: While classical teat lesions are the most common presentation, recent experimental and field investigations have dramatically expanded our understanding of the clinical spectrum. In a landmark experimental study by Ebling et al. (2020), calves inoculated intranasally or transdermally with PCPV developed a delayed and surprisingly severe clinical course [26]. No local or systemic signs were observed until 28 to 34 days post-infection (pi), at which point the animals developed a papulo-pustular, erosive-fibrinous, and scabby dermatitis of the muzzle. In some calves, these lesions coalesced into extensive fibrinonecrotic plaques covering almost the entire muzzle, extending to the lips and gingiva. This severe disease lasted 8 to 15 days and spontaneously resolved only after day 42 pi [26]. This finding is critical, as it reveals that PCPV can induce a severe, protracted, and erosive mucocutaneous disease in young animals, a presentation previously underappreciated and easily confused with other viral or nutritional conditions.

Oral and Mucosal Lesions: Infection of the oral mucosa is increasingly recognized in calves. Affected animals may present with anorexia, frothy salivation, and erosion of the mucosa of the lip and tongue [6, 8]. In a case from Japan, a calf exhibited white vesicles and hyperemia under the tongue surface without concurrent teat lesions, demonstrating that oral PCPV infection can occur independently of the more typical udder presentation [8]. The virus has been detected in the oral cavity of calves even in the absence of overt clinical signs, highlighting the potential for subclinical shedders to perpetuate the outbreak [6, 19].

Lesions in Bulls and Non-Lactating Animals: Pseudocowpox is not exclusively a disease of lactating females. Ulcerative penile lesions in beef bulls have been attributed to PCPV infection, representing a potentially emergent or underreported manifestation of the disease [34]. These lesions can impact fertility and are likely transmitted venereally or through contact with contaminated fomites. Lesions have also been documented on the perineum, udder, inguinal region, and medial thighs of bison (see below), indicating that any area of skin trauma or friction may serve as a portal of entry [5].

Clinical Manifestations in Non-Bovine Species

PCPV demonstrates a broad host range beyond cattle, infecting a variety of domestic and wild ruminants and, rarely, other mammals.

American Bison (Bison bison): A detailed case report in a seven-year-old female bison described multifocal, raised, keratinized plaques (0.5–2 cm) localized to the ventral tail, perineum, caudoventral abdomen, udder, and inguinal recesses [5]. Histopathology revealed marked epidermal hyperplasia, parakeratotic hyperkeratosis, and the presence of characteristic 3–5 μm eosinophilic intracytoplasmic inclusion bodies (Bollinger bodies) in epithelial cells, along with typical parapoxvirus particles on negative-staining electron microscopy [5]. This case underscores the potential for PCPV to cause significant clinical disease in novel hosts and highlights the importance of including PCPV in the differential diagnosis for bovine-like lesions in bison.

Water Buffalo (Bubalus bubalis): Outbreaks in young water buffaloes have been reported, with calves less than six months old presenting with widespread ulcers and peeling of the tongue epithelium [9]. The absence of vesicular disease in co-housed pigs or horses helped rule out foot-and-mouth disease (FMD) in that investigation, emphasizing the critical role of clinical epidemiology in differential diagnosis.

Camels: Pseudocowpox virus has been molecularly identified as a significant cause of camel contagious ecthyma (CCE) in dromedaries, particularly in Sudan and the Arabian Peninsula [25]. Lesions in camels are similar to orf virus infections in sheep and goats, presenting as proliferative, crusting dermatitis around the lips, nostrils, and mouth. The genetic diversity of camel PCPV strains is substantial, with some clustering closely with bovine isolates and others forming distinct lineages [25].

Pinnipeds (Seals) and Other Wildlife: Parapoxviruses closely related to, but genetically distinct from, classical PCPV cause nodular skin lesions in pinnipeds (seal pox) [20]. These lesions are typically found on the head, flippers, and trunk. Currently, seal parapoxvirus is considered a tentative species within the genus, but the zoonotic potential and host range of these marine strains remain an area of active investigation.

Zoonotic Clinical Manifestations: Milker's Nodule

Human infection with PCPV, known as milker's nodule (or paravaccinia), is an occupational zoonosis primarily affecting dairy farmers, milkers, and veterinarians. Transmission occurs through direct contact with infected cattle, typically during milking, or through contact with contaminated fomites, including calf bites [10, 12, 13, 28]. The virus gains entry through pre-existing abrasions or cuts on the skin.

Incubation and Lesion Progression: The incubation period in humans is 3 to 7 days. The lesion begins as a solitary, or occasionally multiple, erythematous, pruritic or mildly painful maculopapule, typically on the fingers, hands, or forearm. The lesion progresses through six characteristic clinical stages: maculopapular, target (or bull's-eye), acute weeping, nodular (or proliferative), papillomatous, and finally, regressive with a dry crust [10, 12, 13]. The mature nodule is often firm, bluish-red, and ranges from 0.5 to 2 cm in diameter. The lesion is typically painless but may be surrounded by erythema and mild edema. Regional lymphadenopathy is common. The disease is self-limiting, with spontaneous resolution occurring within 4 to 6 weeks, typically without scarring.

Complications: While typically benign, milker's nodule can lead to significant complications. A rare but well-documented sequela is the development of erythema multiforme (EM), a hypersensitivity reaction occurring 1 to 3 weeks after the onset of the cutaneous lesion [12]. In such cases, patients develop symmetric, target-like macules, papules, and bullae, typically on the extremities and trunk. This presentation is clinically indistinguishable from erythema multiforme triggered by other infections (e.g., herpes simplex, mycoplasma) or drugs. Although rare, it is important to recognize that PCPV can trigger this systemic reaction, and a history of animal contact is crucial for diagnosis [12].

Immunocompromised Hosts: In immunocompromised individuals, PCPV infection can take a severe, protracted, and destructive course. Giant, persistent, and weeping proliferative nodules may develop, requiring surgical excision or antiviral therapy. This is analogous to the severe orf virus infections documented in immunosuppressed patients and underscores the importance of vigilance in this population [13].

Differential Diagnosis

The clinical presentation of pseudocowpox is notoriously non-specific and overlaps considerably with a range of other poxviral infections and non-poxviral vesicular diseases affecting the skin and mucous membranes of cattle and humans. Reliance on clinical signs alone is insufficient for a definitive diagnosis, and molecular confirmation (e.g., PCR, HRM analysis) is strongly recommended by the World Organisation for Animal Health (WOAH) and the Centers for Disease Control and Prevention (CDC) for accurate diagnosis and outbreak management.

1. Bovine Papular Stomatitis Virus (BPSV): This is perhaps the most challenging differential, as BPSV is a closely related parapoxvirus that produces nearly identical clinical signs in cattle. Both cause papules, vesicles, and scabs on the teats and udder of cows and erosive lesions in the oral cavity of calves. Indeed, mixed infections (PCPV and BPSV) and sequential infections in the same animal are well-documented [6, 24]. A key epidemiological clue is that BPSV lesions in calves are more commonly observed on the muzzle and oral mucosa, while PCPV is more classically associated with teat lesions in adults, though this distinction is unreliable. Molecular differentiation using the B2L or GIF genes is essential [6, 29].

2. Orf Virus (Contagious Ecthyma): Orf virus primarily affects sheep and goats but can also cause lesions in cattle and humans (orf, or ecthyma contagiosum). Human orf is clinically indistinguishable from milker's nodule, presenting as a single, large, proliferative nodule on the hand. The only reliable way to differentiate the two is through a detailed history of animal contact (sheep/goats vs. cattle) and molecular analysis [13, 16, 29, 31]. A study in Lao goats found that orf virus, not PCPV, was the primary cause of lip and facial dermatitis [16].

3. Vaccinia Virus (Bovine Vaccinia): In Brazil and other regions, vaccinia virus (VACV) causes exanthematous lesions on the teats and udder of dairy cows that are grossly indistinguishable from pseudocowpox [23, 24]. Bovine vaccinia is a significant zoonotic concern. Coinfections of VACV and PCPV have been reported in both cattle and milk [23, 30]. Differentiating these viruses is critical for public health, as VACV can cause more severe systemic disease in humans, particularly in unvaccinated individuals.

4. Lumpy Skin Disease Virus (LSDV): LSDV, a capripoxvirus, is a notifiable disease of major economic importance. While the generalized, nodular skin disease in adult cattle is often distinctive, early or mild cases of LSDV can present with localized teat lesions that mimic pseudocowpox [4, 17]. Crucially, co-infections with LSDV and PCPV have been documented in Nigeria, further complicating field diagnosis [17]. The World Organisation for Animal Health (WOAH) mandates that any suspected case of LSDV be confirmed by laboratory testing. PCPV lesions are typically smaller, more superficial, and crusted, whereas LSDV nodules are larger, deeper, and involve all layers of the skin.

5. Foot-and-Mouth Disease (FMD) and Other Vesicular Diseases: Vesicular diseases, particularly FMD, must be considered in any outbreak of vesicular or erosive lesions on the teats, muzzle, or oral cavity of cattle. FMD typically presents with multiple, painful vesicles that rupture rapidly, leading to severe lameness and salivation, and is highly contagious. PCPV lesions are generally more chronic and crusted, and the vesicles are less prominent. However, cases of PCPV in buffalo presenting with widespread tongue epithelium peeling highlight the potential for clinical confusion [9]. Vesicular stomatitis virus (VSV) and bovine viral diarrhea virus (BVDV) can also produce oral erosions, but they are usually accompanied by other systemic signs or epidemiological features (e.g., seasonality for VSV, reproductive signs for BVDV).

6. Bacterial and Fungal Conditions: Dermatophilosis (caused by Dermatophilus congolensis) can produce thick, crusty scabs on the teats and muzzle, mimicking chronic PCPV lesions. However, dermatophilosis lesions are typically more matted and exudative, often with a characteristic "paintbrush" appearance on histology [16]. Staphylococcal and streptococcal mastitis can cause teat lesions, but these are usually more inflammatory, painful, and have a purulent exudate, distinct from the proliferative PCPV nodule. Ringworm (dermatophytosis) presents with circular, alopecic, scaly patches, which are distinct from the proliferative, crusted lesions of pseudocowpox.

7. Other Cutaneous Conditions in Cattle: Trauma from milking machines, sunburn, photosensitization, and chemical irritation can cause teat dermatitis that mimics the early stages of pseudocowpox. Besnoitiosis (caused by Besnoitia besnoiti) produces sclerodermatous and cystic skin lesions, but these are more chronic and generalized, with characteristic scleral cysts, differentiating it from the acute, focal nature of PCPV.

In summary, the clinical manifestations of pseudocowpox are highly variable and depend on host species, age, immune status, and route of exposure. While the classical ring scab on the teats of milking cows remains a strong diagnostic clue, the disease can manifest as a severe, delayed-onset erosive stomatitis in calves, proliferative nodules in bison, and subclinical oral shedders. In humans, milker's nodule is a self-limiting but occasionally complicated disease. The striking clinical similarity between PCPV and other poxviruses (especially BPSV, orf virus, and vaccinia virus) as well as non-poxviral conditions like LSDV and FMD, renders clinical diagnosis inadequate. Therefore, the definitive diagnosis relies on a combination of detailed clinical observation, epidemiological context, and, most importantly, confirmatory molecular diagnostics, such as

Advanced Molecular Diagnostics and Genomic Characterization

The accurate identification and genomic characterization of Pseudocowpox virus (PCPV) represent a formidable challenge in veterinary virology, primarily due to the clinical indistinguishability of parapoxvirus infections from those caused by other poxvirus genera, including Orthopoxvirus (cowpox virus, vaccinia virus) and Capripoxvirus (lumpy skin disease virus) [4, 17, 29]. This diagnostic dilemma is compounded by the frequent co-circulation of PCPV with bovine papular stomatitis virus (BPSV) and the potential for co-infection with lumpy skin disease virus (LSDV), as documented in Nigerian cattle where 11.9% of suspected LSDV outbreaks were found to harbor dual infections [17]. The World Organisation for Animal Health (WOAH) recognizes the critical need for rapid, precise molecular tools to differentiate these pathogens, as misdiagnosis can lead to inappropriate quarantine measures, economic losses, and failure to recognize zoonotic threats. The Centers for Disease Control and Prevention (CDC) similarly emphasizes that parapoxviruses, including PCPV, are underdiagnosed causes of occupational dermatoses in agricultural workers, underscoring the public health imperative for robust molecular surveillance.

High-Resolution Melting (HRM) Assays for Genus-Level and Species-Level Differentiation

Among the most transformative advances in PCPV diagnostics is the development of high-resolution melting (HRM) analysis, a closed-tube, post-PCR technique that exploits differences in amplicon length and GC content to discriminate between viral species [4, 18, 29]. Gelaye et al. [29] pioneered a multiplex HRM assay capable of simultaneously detecting and differentiating eight poxviruses spanning three genera, Orthopoxvirus (cowpox virus, camelpox virus), Capripoxvirus (goatpox virus, sheeppox virus, LSDV), and Parapoxvirus (orf virus, PCPV, BPSV). This assay generates three well-separated melting regions corresponding to each genus, with intra-genus discrimination achieved through distinct melting temperatures. For the Parapoxvirus genus, the differential melting of amplicons from the B2L gene (major envelope protein) enables clear separation of PCPV from BPSV and ORFV, a capability that has been instrumental in field investigations across Africa and Asia [4, 18, 29]. The HRM approach has been validated against 271 poxviral DNA samples, demonstrating 100% sensitivity and specificity for PCPV detection [29]. This technology has been deployed in Zambia, where Ziba et al. [4] used HRM to confirm PCPV in cattle presenting with atypical skin lesions initially suspected to be LSDV, highlighting the assay's utility in differentiating diseases with overlapping clinical signs in resource-limited settings.

Targeted Gene Sequencing and Phylogenetic Frameworks

The molecular characterization of PCPV has historically centered on the B2L gene, which encodes the major envelope protein and serves as the canonical phylogenetic marker for parapoxviruses [5, 8, 9, 20, 25, 32, 33]. Full-length B2L sequencing (approximately 1,137 bp) provides robust resolution for species-level assignment and reveals intra-species genetic diversity. For instance, Ohtani et al. [8] performed the first isolation and full-length B2L characterization of PCPV in Japan, confirming its classification within the PCPV clade and establishing baseline genetic data for subsequent epidemiological studies. Comparative B2L sequence analysis demonstrates that PCPV isolates from different geographic regions and host species form a monophyletic cluster distinct from BPSV and ORFV, with inter-species nucleotide identities ranging from 83.6% to 96.3% [15, 20]. However, substantial intra-species variation exists: Laguardia-Nascimento et al. [9] reported a maximum genetic distance of 4.6% between a Brazilian PCPV strain from water buffalo (Bubalus bubalis) and a camel-derived PCPV, while the distance from ORFV and BPSV was 6.7% and 18.4%, respectively. This degree of variation complicates the establishment of universal diagnostic primers and underscores the need for continuously updated sequence databases.

Beyond B2L, the granulocyte-macrophage colony-stimulating factor/interleukin-2 inhibition factor (GIF) gene has emerged as a valuable target for intra-species discrimination [7, 20]. Karim et al. [7] used a 408-bp fragment of the GIF gene to characterize PCPV strains from Iraqi dairy cows, revealing that some sequences clustered with isolates from New Zealand while others grouped with Finnish strains, suggesting multiple introduction events or long-distance dissemination via cattle trade. The GIF gene encodes a secreted immunomodulatory protein that binds GM-CSF and IL-2, inhibiting their activity; its genetic variability likely reflects host-driven selective pressures [22]. Similarly, the viral interleukin-10 (vIL-10) ortholog, present in all parapoxviruses, exhibits variable receptor binding affinities and immunomodulatory potencies across species [22]. Naqash et al. [22] demonstrated that recombinant PCPV vIL-10 shows reduced inhibition of monocyte chemoattractant protein-1 (MCP-1) and diminished mast cell proliferation compared to ORFV vIL-10, correlating with decreased IL-10 receptor 1 binding. This functional diversity, encoded in the vIL-10 gene sequence, provides another layer of molecular characterization that bridges genomics with pathogenesis.

Multi-Locus Sequence Typing and Genomic Diversity

The limitations of single-gene phylogenies have prompted efforts toward multi-locus sequence typing (MLST) for PCPV. Yoshino et al. [11] conducted comprehensive genetic analysis of Japanese PCPV and BPSV strains using both ORF11 and ORF32, two genes encoding non-structural proteins with divergent evolutionary rates. ORF11 was found to be highly conserved, with 98% amino acid identity between two PCPV strains from Hokkaido, while ORF32 exhibited substantially greater diversity, including deletions and substitutions that make it suitable for molecular epidemiological investigations. This dual-gene approach revealed that PCPV strains in Japan form separate phylogenetic clusters based on ORF32, whereas ORF11 analysis groups them more tightly, indicating that different genomic regions capture distinct aspects of viral evolution [11]. The study also provided the first ORF32 sequences of Japanese parapoxviruses, establishing a baseline for future surveillance.

The RPO30 (RNA polymerase 30 kDa subunit) and GPCR (G-protein-coupled receptor) genes, commonly used for capripoxvirus characterization, have also been applied to PCPV in co-infection contexts. Modise et al. [18] sequenced RPO30 and GPCR from a Botswana PCPV strain and found that it clustered between camel and cattle PCPV isolates, exhibiting unique features that may reflect adaptation to a novel ecological niche. Similarly, Omoniwa et al. [17] analyzed the B2L gene of Nigerian PCPV strains and found 100% identity among isolates, suggesting a single circulating lineage, yet the same study revealed that co-infecting LSDV strains formed two distinct RPO30 clusters, including a unique sub-group. These findings emphasize that PCPV genomic diversity remains poorly characterized in many regions, particularly in Africa, where surveillance infrastructure is limited.

Metagenomics and Metatranscriptomics in PCPV Discovery

The application of next-generation sequencing (NGS) technologies has revolutionized the detection of PCPV, particularly in situations where targeted PCR assays fail or where co-infections complicate interpretation. Shivanna et al. [5] employed metagenomic analysis to confirm PCPV infection in an American bison (Bison bison), the first report of this host species. Homogenized skin samples were subjected to unbiased sequencing, revealing only poxvirus sequences, with B2L-based phylogeny placing the bison strain within the PCPV clade. This approach eliminated the need for a priori assumptions about the causative agent and provided comprehensive genomic coverage. Metatranscriptomics has similarly been applied to investigate the virome of calves with idiopathic ill-thrift syndrome in New Zealand [35]. Grimwood et al. [35] identified PCPV as the dominant viral component in oral swabs from all calves with oral lesions, alongside a diverse array of other bovine viruses including bopivirus, astroviruses, and caliciviruses. The study revealed that PCPV oral viromes were relatively simple and dominated by a single species, whereas fecal viromes were highly diverse, suggesting compartmentalized viral replication and shedding patterns. Notably, no novel viruses were significantly associated with the syndrome, supporting a multifactorial etiology involving nutritional and management factors rather than a single pathogen.

Detection of PCPV in Atypical Samples and Vectors

Advanced molecular diagnostics have expanded the range of sample types from which PCPV can be detected, challenging traditional assumptions about transmission routes. Rehfeld et al. [30] reported the first detection of PCPV DNA in milk samples from naturally infected cows, with 12 of 40 milk samples testing positive. Alarmingly, eight of these samples were co-infected with vaccinia virus (OPXV), indicating that milk can serve as a vehicle for simultaneous exposure to multiple zoonotic poxviruses. This finding has significant implications for public health, particularly in regions where raw milk consumption is common. The CDC has noted that pasteurization inactivates poxviruses, but the potential for contamination of dairy products remains a concern.

Furthermore, PCPV has been detected in arthropod vectors, expanding our understanding of transmission ecology. Ouedraogo et al. [3] screened ticks (primarily Amblyomma variegatum and Hyalomma spp.) collected from cattle in Burkina Faso using genus-specific real-time PCR assays and found PCPV in 8.2% of tick pools, alongside BPSV (5.8%). This was the first detection of parapoxviruses in ticks, suggesting a potential mechanical or biological vector role. Similarly, Shimizu et al. [19] demonstrated that houseflies (Musca domestica) can mechanically transmit PCPV, detecting viral DNA on fly body surfaces and in feces. Viral sequences from flies were identical to those from cattle and barn environments, confirming a direct link. These findings align with WOAH guidelines emphasizing the importance of vector control in managing zoonotic parapoxvirus outbreaks.

Phylogenetic Insights into Host Range and Viral Evolution

The cumulative molecular data reveal that PCPV is not strictly host-restricted; it infects a diverse range of species including cattle, water buffalo [9], American bison [5], camels [25], and potentially pinnipeds [20]. Costa et al. [20] analyzed parapoxvirus sequences from North American pinnipeds (seals and sea lions) and found that while they form a separate cluster within the Parapoxvirus genus, they share genetic features with both PCPV and ORFV. The B2L, DNA polymerase, GIF, and vIL-10 gene fragments from sealpox viruses showed significant divergence from terrestrial parapoxviruses, supporting classification as a distinct species. This genetic distance suggests that PCPV is part of a broader parapoxvirus radiation that has adapted to marine mammals, possibly through independent evolutionary trajectories following cross-species transmission events.

In camels, PCPV is a major cause of camel contagious ecthyma (CCE), a disease with significant economic impact in arid regions. Khalafalla et al. [25] sequenced the B2L gene of eight Sudanese camel PCPV strains and identified two distinct lineages: an Asian lineage (Saudi Arabia, Bahrain, India) and an African lineage (Sudan). Notably, some camel strains clustered more closely with ORFV than with bovine PCPV, indicating that the Parapoxvirus genus may contain cryptic species or that extensive recombination occurs at host-virus interfaces. These phylogenetic complexities underscore the inadequacy of single-gene phylogenies for definitive species classification and highlight the need for whole-genome sequencing approaches.

Diagnostic Challenges in Co-Infection and Atypical Presentation

Perhaps the most clinically relevant application of advanced molecular diagnostics is the identification of co-infections, which are increasingly recognized as common but underdiagnosed. In Brazil, retrospective studies of poxvirus infections in cattle revealed that 5 out of 52 confirmed cases in Distrito Federal were co-infections of PCPV and BPSV [24], while co-infections with VACV and ORFV-like parapoxviruses were documented in Goiás State [23]. These mixed infections produce clinical lesions that are indistinguishable from single-agent infections, yet they may have different transmission dynamics, zoonotic potentials, and vaccine responses. The HRM assay developed by Gelaye et al. [29] can detect such co-infections by generating melting curves with multiple peaks, a capability that conventional single-target PCRs lack.

Atypical clinical presentations further complicate diagnosis. Allen et al. [34] reported ulcerative penile lesions in beef bulls associated with PCPV, a manifestation that could be mistaken for infectious bovine rhinotracheitis or traumatic injuries. Similarly, Ebling et al. [26] documented a delayed clinical course in experimentally inoculated calves, where lesions developed 28-34 days post-inoculation despite initial viremia and shedding. Viral DNA was detectable in swabs from these delayed lesions up to day 42 post-infection, indicating prolonged viral persistence that would be missed by early sampling. These findings have practical implications for diagnostic protocols, suggesting that samples should be collected from both acute and convalescent stages to maximize detection sensitivity.

Immunomodulatory Gene Characterization and Vector Development

The genomic characterization of PCPV has also been driven by interest in its immunomodulatory genes, which are being harnessed for vaccine vector development. PCPV encodes multiple immune evasion molecules, including the GIF protein and vIL-10, that differentiate it from other poxviruses [14, 22]. Ramos et al. [2] and Rittner et al. [14] characterized PCPV as a novel vector for cancer immunotherapy, demonstrating that it induces 1000-fold higher IFN-alpha expression in human PBMCs compared to Modified Vaccinia Ankara (MVA), while also upregulating GM-CSF, IL-18, IP-10, and CD86 on antigen-presenting cells. The recombinant PCPV vector encoding HPV E7 protein elicited a robust CD8+ T-cell response and reshaped tumor infiltrates, increasing neutrophils while decreasing immunosuppressive MHC-IIlo tumor-associated macrophages. These properties are encoded in the viral genome, particularly in the inverted terminal repeats and central conserved regions, and their characterization has opened new avenues for therapeutic applications. The FAO has acknowledged the potential of parapoxvirus-based vectors for livestock vaccination, though safety considerations regarding zoonotic potential remain paramount.

In summary, the molecular diagnostics and genomic characterization of PCPV have evolved from single-gene PCR assays to sophisticated HRM-based multiplex platforms, metagenomic sequencing, and functional characterization of immunomodulatory genes. These tools have revealed a virus that is genetically diverse, ecologically versatile, and increasingly recognized as a pathogen of both veterinary and public health significance. The continued development of whole-genome sequencing capabilities and the establishment of open-access sequence databases will be essential for tracking the emergence of new strains, understanding cross-species transmission dynamics, and informing the design of effective control strategies.

Novel Vector Applications in Antitumor Vaccination

The development of effective antitumor vaccination platforms represents one of the most intellectually demanding and therapeutically promising frontiers in modern oncology. While conventional viral vectors, most notably Modified Vaccinia Virus Ankara (MVA) and various oncolytic vaccinia virus strains, have demonstrated measurable clinical utility, the search for vectors possessing superior immunostimulatory profiles, enhanced safety margins, and robust capacity for transgene expression has persisted. Within this landscape, Pseudocowpox virus (PCPV), a member of the Parapoxvirus genus within the Poxviridae family, has recently emerged as a vector of exceptional promise, challenging the hegemony of orthopoxvirus-based platforms [2, 14]. The rationale for this shift is grounded not merely in incremental improvements, but in a fundamentally distinct immunological signature that PCPV exhibits when interacting with the mammalian immune system.

Comparative Vectorology: The Rationale for Parapoxvirus-Based Platforms

The initial impetus for exploring PCPV as a vaccine vector arose from a systematic comparative screening of nine distinct poxviridae members, including Cowpox virus (CPX), Orf virus (ORFV), Myxoma virus (MYX), Swinepox virus (SWP), Yaba-like disease virus (YLDV), Raccoonpox virus (RCN), and Cotia virus (CTV), against the established benchmarks of MVA and the Copenhagen strain of vaccinia virus [14]. The results of this comprehensive evaluation were striking and non-obvious. When human peripheral blood mononuclear cells (PBMCs) were exposed to these viral vectors, PCPV induced a remarkable 1000-fold higher expression of interferon-alpha (IFN-α) compared to MVA [14]. This differential is not merely quantitative; it speaks to a fundamentally different activation of the innate immune sensing machinery. SWPV and ORFV displayed only 10- to 100-fold induction, while other viruses, including vaccinia, RCN, CTV, and MYX, failed to elevate IFN-α levels above baseline [14]. This particular profile positions PCPV as uniquely capable of triggering a robust type I interferon response, a pathway critical for both direct antiviral activity and the bridging of innate to adaptive immunity through the maturation of dendritic cells and the promotion of cross-presentation of tumor antigens.

Critically, this potent immunostimulation was not accompanied by cytotoxicity. In contrast to ORFV, which displayed significant cytotoxic effects on PBMCs, PCPV was demonstrated to be safe for these primary human immune cells [14]. This safety profile, coupled with the capacity to increase levels of granulocyte-macrophage colony-stimulating factor (GM-CSF), suggested that PCPV could create a local microenvironment permissive for the recruitment, activation, and survival of antigen-presenting cells at the site of vaccination.

Immunological Mechanisms: Dendritic Cell Activation and Macrophage Repolarization

The success of any antitumor vector hinges on its ability to break immunological tolerance and generate tumor-specific effector T cells. The data regarding PCPV’s interaction with professional antigen-presenting cells are particularly compelling. When tested for its capacity to upregulate CD86, a co-stimulatory molecule essential for T-cell priming, PCPV proved superior to MVA in primary human monocyte-derived dendritic cells (moDCs) [14]. This enhanced costimulatory capacity suggests that PCPV-infected DCs are more efficient at providing the second signal necessary for T-cell activation, thereby lowering the threshold for the induction of an antitumor response.

Perhaps even more relevant to the tumor microenvironment is PCPV’s effect on immunosuppressive macrophages. The presence of M2-polarized, tumor-associated macrophages (TAMs) expressing CD163 and CD206 is a well-established correlate of poor prognosis in numerous malignancies. Remarkably, treatment of human CD14+ monocyte-derived M2 macrophages with PCPV led to a significant increase in the secretion of interleukin-18 (IL-18), interleukin-6 (IL-6), and interferon gamma-induced protein 10 (IP-10/CXCL10) [14]. This cytokine/chemokine shift, combined with increased surface expression of CD86, signals a functional repolarization away from an immunosuppressive phenotype and toward an antigen-presenting, pro-inflammatory state. The ability to directly convert a suppressive cell type into an immunostimulatory one within the tumor bed, a process sometimes termed in situ vaccination, represents a therapeutic advantage that few viral vectors can claim. Furthermore, in a co-culture model, incubation of PCPV overcame the suppressive effect of myeloid-derived suppressor cells (MDSCs) on autologous CD8+ T cells, demonstrating that PCPV can dismantle multiple layers of the immunosuppressive network [14].

Translational Efficacy: The HPV E7 Model and Intratumoral Application

The translational potential of PCPV has been rigorously tested using recombinant vectors encoding tumor-associated antigens. A recombinant PCPV encoding the human papillomavirus (HPV) E7 oncoprotein (PCPV-E7) was generated and evaluated in a syngeneic murine tumor model [2, 14]. The immunogenicity data are instructive. While PCPV-E7 induced a robust cellular response, measured by ELISPOT on splenocytes and the frequency of antigen-specific short-lived effector cells, comparable to that induced by MVA-E7, the quality of the immune response differed substantially [14]. Analysis of the cytokine/chemokine profile at the site of injection revealed that PCPV-E7 generated significantly elevated levels of a broad array of pro-immune mediators, including IP-10, IFN-γ, GM-CSF, IL-18, macrophage inflammatory protein 1-alpha (MIP-1α), MIP-1β, IL-12, and IL-6 [14]. This polyfunctional cytokine response is highly desirable in antitumor vaccination, as it promotes the recruitment of diverse immune effector cells, supports the survival of cytotoxic T lymphocytes, and fosters a Th1-polarized environment.

The most compelling evidence for PCPV’s therapeutic potential came from intratumoral injection studies in fast-growing MC-38 tumors. Direct injection of PCPV led to significant tumor control [14]. Analysis of the tumor infiltrates revealed a profound remodeling of the tumor microenvironment: PCPV treatment led to higher levels of neutrophils, a cell type increasingly recognized for its role in antibody-dependent cellular cytotoxicity and tumor rejection, and a marked decrease in the frequency of MHC class II low (MHCIIlo) TAMs [14]. The depletion of these immunosuppressive macrophages is a key therapeutic goal in modern immunotherapy, and PCPV appears to achieve this naturally through its inherent biology.

Contextualizing Safety and Immunogenicity: Natural History of PCPV Infection

The safety profile of PCPV as a vector is further contextualized by its natural history and host range. PCPV is a zoonotic pathogen that typically causes mild, self-limiting mucocutaneous lesions, known as milker's nodules, in humans, usually resolving without sequelae within 4-8 weeks [1, 10, 12, 13, 28]. The virus has been isolated from a wide range of hosts, including cattle, water buffalo (Bubalus bubalis), American bison, camels, and even ticks, yet it rarely causes systemic disease in immunocompetent hosts [3, 5, 8, 9, 25, 32]. This natural history indicates that PCPV has evolved to replicate efficiently in mammalian hosts while encoding potent immunomodulatory factors, such as a viral interleukin-10 homologue (vIL-10), that limit immune-mediated pathology [22]. The presence of this vIL-10, which shows reduced anti-inflammatory activity compared to ORFV IL-10 but retains some receptor binding capacity, likely contributes to the virus’s ability to establish a transient, controlled infection that is safe yet immunogenic [22].

The delayed clinical course observed in experimental infections of calves, where severe pustular and erosive lesions developed only 28-34 days post-inoculation followed by spontaneous resolution, suggests that PCPV induces a prolonged, dynamic immune interaction with the host [26]. This temporal pattern may be advantageous for a vaccine vector, as it provides an extended window for antigen presentation and immune activation without causing debilitating pathology. Additionally, the virus’s ability to replicate in the oral mucosa and skin of cattle without causing significant systemic illness, and its documented capacity to be shed and transmitted, potentially even by mechanical vectors such as houseflies, speaks to its robustness as a replicating vector [19, 34].

Genetic Payload Capacity and Engineering Feasibility

From a practical standpoint, PCPV shares the large double-stranded DNA genome characteristic of poxviruses, providing ample capacity for the insertion of multiple transgenes. The ability to engineer recombinant PCPV expressing GFP has already been demonstrated, confirming that the virus can be manipulated to stably express heterologous proteins at levels comparable to MVA [14]. This genetic flexibility renders PCPV suitable for the development of "armed" vectors that co-express tumor antigens alongside immunostimulatory cytokines, checkpoint inhibitors, or bispecific T-cell engagers, a multi-valent approach that represents the next generation of cancer immunotherapy. The capacity to deliver a large genetic payload is particularly advantageous when targeting tumor heterogeneity or when aiming to induce immunity against multiple shared or neo-epitopes simultaneously.

In summary, PCPV distinguishes itself from conventional orthopoxvirus vectors through a unique immunological fingerprint: the capacity to induce a massive type I interferon response, to repolarize immunosuppressive macrophages into antigen-presenting cells, to overcome MDSC-mediated T-cell suppression, and to remodel the tumor microenvironment away from MHCIIlo TAMs toward a neutrophil-rich, pro-inflammatory state. These properties, combined with its favorable safety profile derived from its natural history as a mild zoonotic pathogen and its large genetic payload capacity, position PCPV as a leading next-generation vector for antitumor vaccination. The World Organisation for Animal Health (WOAH) recognizes the global significance of parapoxviruses, and the Centers for Disease Control and Prevention (CDC) has noted the zoonotic potential of PCPV; however, the very features that enable its zoonotic transmission, its ability to potently stimulate innate immunity while maintaining a controlled infection, are precisely those that make it an exceptional candidate for cancer immunotherapy. The path toward clinical translation, though demanding, is supported by a robust preclinical foundation that challenges the field to reconsider which viral vectors are best suited for the complex immunological task of eradicating established tumors.

References

[1] . pseudocowpox virus. CABI Compendium. 2022. DOI: https://doi.org/10.1079/cabicompendium.67003

[2] Ramos R, Tosch C, Kotsias F, Claudepierre M, Schmitt D, Remy-Ziller C, et al.. Pseudocowpox virus, a novel vector to enhance the therapeutic efficacy of antitumor vaccination. Clinical & Translational Immunology. 2022. DOI: https://doi.org/10.1002/cti2.1392

[3] Ouedraogo A, Luciani L, Zannou OM, Biguezoton A, Pezzi L, Thirion L, et al.. Detection of Two Species of the Genus Parapoxvirus (Bovine Papular Stomatitis Virus and Pseudocowpox Virus) in Ticks Infesting Cattle in Burkina Faso. Microorganisms. 2020. DOI: https://doi.org/10.3390/microorganisms8050644

[4] Ziba MW, Chitala C, Settypalli TB, Mumba M, Cattoli G, Fandamu P, et al.. First detection and molecular characterisation of pseudocowpox virus in a cattle herd in Zambia. Virology Journal. 2020. DOI: https://doi.org/10.1186/s12985-020-01426-7

[5] Shivanna V, Cino-Ozuna AG, Heskett C, Marthaler D, Ganta C. Pseudocowpox virus infection in an American bison (Bison bison). BMC Veterinary Research. 2020. DOI: https://doi.org/10.1186/s12917-020-02464-7

[6] Matsumoto H, Setoyama H, Matsuura Y, Ohtani A, Shimizu K, Okada A, et al.. Sequential detection of pseudocowpox virus and bovine papular stomatitis virus in a same calf in Japan. Journal of Veterinary Medical Science. 2019. DOI: https://doi.org/10.1292/jvms.18-0367

[7] Karim SM, Mansour KA, Janabi A, Al-Nakeeb NK. First phylogenetic characterization of Pseudocowpox virus from cattle in Al-Qadisiyah province/ Iraq. Iraqi Journal of Veterinary Sciences. 2019. DOI: https://doi.org/10.33899/IJVS.2019.125525.1047

[8] Ohtani A, Yokoyama A, Narushige H, Inoshima Y. First isolation and genetic characterization of pseudocowpox virus from cattle in Japan. Virology Journal. 2017. DOI: https://doi.org/10.1186/s12985-017-0840-3

[9] Laguardia-Nascimento M, Oliveira APd, Fernandes FRP, Rivetti AV, Camargos MF, Júnior AFF. Detection of pseudocowpox virus in water buffalo (Bubalus bubalis) with vesicular disease in the state of São Paulo, Brazil, in 2016. Veterinary Quarterly. 2017. DOI: https://doi.org/10.1080/01652176.2016.1252479

[10] . Pseudocowpox Virus. Definitions. 2020. DOI: https://doi.org/10.32388/8klpc9

[11] Yoshino A, Okada A, Inoshima Y. Genetic analysis of bovine papular stomatitis and pseudocowpox viruses circulating in Hokkaido, Japan’s largest cattle-producing region. Journal of Veterinary Medical Science. 2026. DOI: https://doi.org/10.1292/jvms.26-0001

[12] Wu K, Menezes Sd, Robinson A. Erythema Multiforme Induced by a “Milker's Nodule” Pseudocowpox Infection: A Case Report and Review of Literature. Case Reports in Dermatological Medicine. 2021. DOI: https://doi.org/10.1155/2021/5584773

[13] Marckmann D, Frasnelli A. Milker's Nodule (Pseudocowpox) in a Female Patient Following a Calf Bite.. Deutsches Ärzteblatt International. 2020. DOI: https://doi.org/10.3238/arztebl.2020.0870b

[14] Rittner K, Ricordel M, Tosch C, Thioudellet C, Remy-Ziller C, Claudepierre M, et al.. Abstract LB-287: Pseudocowpox: A next generation viral vector for cancer immunotherapy. Immunology. 2018. DOI: https://doi.org/10.1158/1538-7445.AM2018-LB-287

[15] Nagarajan G, Pourouchottamane R, Reddy GBM, Yogisharadhya R, Sumana K, Rajapandi S, et al.. Molecular characterization of Orf virus isolates from Kodai hills, Tamil Nadu, India. Veterinary World. 2019. DOI: https://doi.org/10.14202/vetworld.2019.1022-1027

[16] Jayasekara P, Jenkins C, Kirkland P, Gerber PF, Olmo L, Xaikhue T, et al.. Causation and epidemiological features of proliferative lip and facial lesions in Lao goats. Tropical Animal Health and Production. 2025. DOI: https://doi.org/10.1007/s11250-025-04697-w

[17] Omoniwa D, Meki I, Kudi C, Sackey A, Aminu M, Adedeji A, et al.. Poxvirus Infections in Dairy Farms and Transhumance Cattle Herds in Nigeria. Viruses. 2023. DOI: https://doi.org/10.3390/v15051051

[18] Modise B, Settypalli TB, Kgotlele T, Xue D, Ntesang K, Kumile K, et al.. First molecular characterization of poxviruses in cattle, sheep, and goats in Botswana. Virology Journal. 2021. DOI: https://doi.org/10.1186/s12985-021-01634-9

[19] Shimizu K, Takase H, Okada A, Inoshima Y. Possibility of mechanical transmission of parapoxvirus by houseflies (Musca domestica) on cattle and sheep farms. Journal of Veterinary Medical Science. 2022. DOI: https://doi.org/10.1292/jvms.22-0158

[20] Costa H, Klein J, Breines E, Nollens H, Matassa K, Garron M, et al.. A Comparison of Parapoxviruses in North American Pinnipeds. Frontiers in Veterinary Science. 2021. DOI: https://doi.org/10.3389/fvets.2021.653094

[21] Usme-Ciro J, Paredes A, Walteros D, Tolosa-Pérez EN, Laiton-Donato K, Pinzón MdC, et al.. Detection and Molecular Characterization of Zoonotic Poxviruses Circulating in the Amazon Region of Colombia, 2014. Emerging Infectious Diseases. 2017. DOI: https://doi.org/10.3201/eid2304.161041

[22] Naqash A, Stuart GS, Kemp R, Wise LM. Parapoxvirus Interleukin-10 Homologues Vary in Their Receptor Binding, Anti-Inflammatory, and Stimulatory Activities. Pathogens. 2022. DOI: https://doi.org/10.3390/pathogens11050507

[23] Silva LF, Leal AA, Cunha PH, Cargnelutti J, Flores E, Almeida TB, et al.. Retrospective study of poxviruses diagnosed in cattle from Goiás State, Brazil (2010-2018). Pesquisa Veterinária Brasileira. 2022. DOI: https://doi.org/10.1590/1678-5150-pvb-7014

[24] Alonso R, Moura PP, Caldeira DF, Mendes MHAF, Pinto MHB, Cargnelutti J, et al.. Poxviruses diagnosed in cattle from Distrito Federal, Brazil (2015-2018).. Transboundary and Emerging Diseases. 2020. DOI: https://doi.org/10.1111/tbed.13490

[25] Khalafalla A, El-Sabagh I, Al-Busada KA, Al-Mubarak AI, Ali YH. Phylogenetic analysis of eight sudanese camel contagious ecthyma viruses based on B2L gene sequence. Virology Journal. 2015. DOI: https://doi.org/10.1186/s12985-015-0348-7

[26] Ebling R, Martin B, Jardim JC, Flores M, Diel D, Weiblen R, et al.. Late development of pustular, erosive lesions in the muzzle of calves inoculated with Pseudocowpox virus.. Microbial Pathogenesis. 2020. DOI: https://doi.org/10.1016/j.micpath.2020.104122

[27] Cr R, Gk K. Susceptibility of bovine macrophage and tracheal-ring cultures to bovine viruses.. American Journal of Veterinary Research. 1977. DOI: https://doi.org/10.2460/ajvr.1977.38.11.1705

[28] Nafeev AA, Magomedov MA, Struchin VE, Kadoeva VB, Timofeeva S, Nafeyev AA, et al.. A case of pseudocowpox. Epidemiology and Infectious Diseases. 2011. DOI: https://doi.org/10.17816/eid40613

[29] Gelaye E, Mach L, Kolodziejek J, Grabherr R, Loitsch A, Achenbach J, et al.. A novel HRM assay for the simultaneous detection and differentiation of eight poxviruses of medical and veterinary importance. Scientific Reports. 2017. DOI: https://doi.org/10.1038/srep42892

[30] Rehfeld I, Fraiha A, Matos ACD, Costa AG, Gallinari GC, Costa E, et al.. Short communication: Parapoxvirus and Orthopoxvirus coinfection in milk of naturally infected cows.. Journal of Dairy Science. 2018. DOI: https://doi.org/10.3168/jds.2017-14000

[31] Khmaladze E, Mauldin MR, Tsaguria D, Gavashelidze M, Sidamonidze K, Tevdoradze T, et al.. Geographic distribution and genetic characterization of poxviruses from human infections in Georgia, 2009–2014. Archives of Virology. 2021. DOI: https://doi.org/10.1007/s00705-020-04922-x

[32] Lederman E, Khan S, Luby S, Zhao H, Braden ZH, Gao J, et al.. Zoonotic parapoxviruses detected in symptomatic cattle in Bangladesh. BMC Research Notes. 2014. DOI: https://doi.org/10.1186/1756-0500-7-816

[33] Yaegashi G, Sasaki I, Chiba S, Murakami K. Molecular Analysis of Parapoxvirus Detected in Eight Calves in Japan. Journal of Veterinary Medical Science. 2013. DOI: https://doi.org/10.1292/jvms.12-0456

[34] Allen A, Evermann J, Diaz-Campos D, McCrea J. Ulcerative penile lesions in beef bulls due to pseudocowpox virus. American Association of Bovine Practitioners Conference Proceedings. 2016. DOI: https://doi.org/10.21423/aabppro20163474

[35] Grimwood RM, Darnley JA, O'Connell J, Hunt H, Taylor H, Lawrence K, et al.. Oral and Faecal Viromes of New Zealand Calves on Pasture With an Idiopathic Ill-Thrift Syndrome. bioRxiv. 2024. DOI: https://doi.org/10.1155/tbed/7737989