Orthopoxvirus (OPXV) is a genus of the family Poxviridae, subfamily Chordopoxvirinae, which includes several viruses infecting humans and animals. Among these are Ectromelia virus (ECTV), which infects only mice, and Cowpox virus (CPXV), which recognises wild rodents as reservoirs, while several species, including humans and cats, are considered incidental hosts. Here, we report a case of lethal infection in a cat that was caused by a putative novel OPXV displaying an intermediate position between CPXV and ECTV at the genomic level.

The cat, a European 6-month-old male, was presented with wide alopecic and erythematous areas on the face and paws. After a few days, the skin lesions spread to other areas of the body and ulcers appeared on the tongue and palate. After the diagnosis of OPXV infection, the cat was treated with antivirals (famciclovir and subsequently acyclovir), but after an initial improvement the animal rapidly worsened and was euthanized.

Bioptic samples, collected intra-vitam from the skin lesions, were processed for histologic and virological investigations. By histopathology, leukocyte infiltration and eosinophilic cytoplasmic inclusion bodies were evident in the skin punches. Different PCR approaches for detection and characterisation of poxviruses showed that the collected samples contained a poxvirus, but they were unable to assign definitively the virus to a species within the genus Orthopoxvirus. The OPXV strain, Italy-09/17, was isolated on African green monkey kidney CV-1 cells and also on embryonated eggs, as demonstrated by the development of the typical pocks in the chorioallantoic membrane. A large amount of typical brick-shaped virions, approximately 320x240 nm in size, morphologically related to the genus Orthopoxvirus, were observed by negative-staining electron microscopy. The nearly full-length genome of the virus was obtained through a next-generation sequencing approach carried out on the isolated virus. By sequence and phylogenetic analysis of selected genomic regions, which are commonly used to classify OPXVs, isolate Italy-09/17 was proven to form a separate cluster from both CPXV and ECTV. Extensive epidemiological surveillance in cats and wild animals, including rodents, will assess whether this feline OPXV circulates in domestic cat populations and whether cats are incidental hosts or represent the main reservoir of the virus.

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