The present study reports the first case of PCPV virus infection in American bison (Bison bison) that was euthanized for declining condition and sores on the vulva and udder. Gross necropsy and histopathological examination confirmed the presence of pox viral infection. Further, real-time PCR and electron microscopy characterized the causative agent as a PPV. Phylogenetic analysis based on the B2L gene confirmed the causative agent as PCPV. The presence of PCPV and absence of other infectious agents in skin lesions was confirmed by metagenomic sequencing.
PCPV infections are generally reported as mild infections with only a few animals affected in a herd, and commonly seen in milking cows as skin infections of teats, udder and foot. However, an unusual presentation of pseudocowpox associated outbreak with pustular ulcerative vulvovaginits was reported in a Swedish dairy herd of approximately 80 cows. Among the affected cows, 90% of the cows had vesicles, papules and scabs affecting the vulval and vaginal mucosa . An outbreak of PCPV affecting 14 of 17 male cattle was also reported from southern Brazil. Histological examination of affected tissues showed acanthosis with parakeratotic hyperkeratosis, ballooning degeneration of superficial keratinocytes and thickening of corneal layer. Electron microscopic examination of scab samples showed presence of oval shaped (260 nm X 160 nm) enveloped particles. The histological features and electron microscopic appearance of viral particles seen in the present study were consistent with these reported features of unusual presentation of PCPV infection . Further, phylogenetic analysis based on the B2L gene was used to identify and classify the causative agent as a PCPV . Similar to this report, in the present study, phylogenetic analysis based on the B2L gene showed that the bison PPV is closely related to bovine PCPV virus. Phylogenetic analysis showed clear segregation of different viruses into their respective genus groups with all PCPV clustering together. The bison PCPV was found to cluster closely (100%) with other PCPV of cattle origin.
Metagenomic sequence analysis revealed a majority of the reads were related to viruses, of which 87% corresponded to poxviruses and 5% of the viral reads were papillomavirus related. The small percentage of reads matching papillomavirus were found to be the conserved gene sequences that are common to poxviruses and papillomaviruses .
Cases of severe atypical PPV infections were previously reported in sheep from the United States . These atypical cases were present in sheep over 12 months of age, characterized by lesions similar to classic orf but more severe, did not resolve spontaneously, and led to debilitating conditions in all affected sheep . Diagnosis of PPV was made based on histological and electron microscopic examination but lacked molecular characterization. Histopathologically, all PPV infections have similar lesions hence are indistinguishable.
Investigating the PPV outbreaks in Finnish reindeer using phylogenetic analysis based on the B2L gene, showed that the PPV outbreak of 1999–2000 was closely related to bovine PCPV whereas the PPV outbreak of 1992–93 was closely related to sheep orf virus . PPV outbreaks were reported in red deer from New Zealand in 1986  and from Italy in 2008–09 . Phylogenetic analysis based on the B2L gene of the PPV from red deer in Italy showed that the virus was closely related to New Zealand PPV of red deer . Further genetic analysis of the bison PPV is required to determine if it is a bovine PCPV that accidently infected bison or is genetically distant enough to be classified as bison variant of PCPV virus.
Transmission of poxvirus to a new species can have detrimental effects on the new population. When squirrel pox virus was transmitted from grey squirrels to red squirrels in Great Britain, it led to significant losses in the red squirrel population .
Although the bison PPV from the current study is closely related to bovine PCPV, the pathological lesions seen in the bison were very distinct from PCPV lesions seen in cattle. The source of the PPV infection in bison is not known. PCPV virus infection in cattle is generally a mild infection characterized by small “ring” or “horseshoe”- shaped papules on the teats and udder that progress to form vesicles or pustules and heal by forming scabs . In comparison to cattle, the lesions seen in bison were more severe with the papules extending over a larger area of the body, caused ill thrift and significantly affected productivity. Several primary infections could predispose or increase the severity of PPV infection. No histologic lesions suggestive of any immunosuppressive viral or bacterial infections were seen in various other tissues examined.
In conclusion, the present report describes an atypical and severe form of PCPV infection in a new host species. The infection was characterized by multifocal raised nodules, extending over large cutaneous areas in an American bison. Definitive identification of the PCPV was made by gross and histopathological examination, real-time PCR, electron microscopy, metagenomic sequencing and phylogenetic analysis based on the B2L gene. Further investigation to determine the pathogenicity and zoonotic potential of this bison PCPV is warranted.