In this study, we have detected PCPV DNA in samples collected on LSD suspicion in a herd of cattle in Zambia using an HRM assay for differential diagnosis of poxvirus infections. Sequencing and the subsequent phylogenetic analyses confirmed that the DNA in these samples belong to PCPV. In the phylogenetic reconstruction, all Zambian PCPVs clustered with previously published sequences of cattle PCPVs.
Although PCPV in cattle is reportedly present worldwide [17–19], there was neither a record of the disease in Zambia nor in most African countries. Therefore, this marks the first occasion that we have identified PCPV in the country. The absence of a proper means for differential diagnosis of pox-like diseases in cattle and the lack of confirmed cases for PCPV contributed to the initial misdiagnosis. As LSD is endemic in Zambia, the current cases were mistakenly reported as LSD-suspected, based on the sole clinical diagnosis. Similar cases of initial LSD diagnosis have been reported [20]. This underlines the importance of determining the etiology of infections in pox-like skin lesions on cattle.
Our findings highlight the relevance of molecular methods for differential diagnosis and the management of pox diseases in ruminants. This robust HRM assay has enabled differentiation between LSDV and PCPV in a single test and identified PCPV, which otherwise would have gone unnoticed. The correct identification of the disease agent avoids confusion with other critical infectious agents present in Zambia and ensures the implementation of proper interventions.
In the phylogenetic analyses, PCPVs from samples collected in December 2017 clustered independently from those in samples taken on the same farm in April 2018, showing an evolution of the virus during the persistence of the infection on the farm. A close inspection of the sequence alignments showed up to 9 amino acid changes in the partial B2L sequence of isolates collected in April 2108 as compared to those collected in December 2017. Interestingly, the analysis of the PCPV B2L gene in samples collected from the same animal at these two-time points showed that two amino acids mutation occurred.
Such rapid genomic variations in the PCPV suggest a swift ability of this virus to mutate during chronic infections. A previous report revealed a similar evolution in the ATPase gene of the ORF virus during the persistence of the virus in a sheep herd in Ethiopia [15]. Such alterations in the genome could potentiate the adaptation of the virus to new tissues and promote shedding, thus enhancing its potential spread.
Our field investigations suggested that the disease started in this herd in 2014, but only drew the attention of farmers when animals’ health conditions worsened. The affected cattle had lesions that were not only confined to the teats and udder. The lesions were present on other parts of the animal, including the limbs, mouthparts, ventrum, and skin folds, suggesting a severe form of the disease. PCPV has previously been isolated from atypical sites apart from the teats and udder [18, 19, 21]. As PCPV is a zoonotic disease, humans coming into direct contact with infected animals are at risk of contracting the infection [22]. We could not find an active case of transmission to humans.
Nevertheless, it came to our attention that the farmworkers previously had sores mainly on their hands and forearms, and sometimes the face. Those lesions cleared within a few weeks without treatment. This observation is consistent with previous reports showing that PCPV lesions in humans usually resolve quickly [23]. It is advisable to handle infected animals with caution to reduce the risks.