Characterization of a novel poxvirus associated with an epidermal keratinocytic dermatopathy in a captive population of the endangered Knysna seahorse Hippocampuscapensis.

The poxviruses are large, epitheliotropic, double-stranded DNA viruses that are pathogens of invertebrates and the mammalian, avian and reptilian vertebrate taxa including the familiar poxviruses of humans and domestic animals such as the variola virus or smallpox virus and the related vaccinia virus used for the live smallpox vaccine, mpox virus (formerly monkeypox virus), cowpox virus of rodents, and orf virus of sheep and goats. Poxviruses have also previously been described from teleost fishes including the common carp Cyprinus carpio, ayu Plecoglossus altivelis, Atlantic salmon Salmo salar, Atlantic cod Gadus morhua, red or Japanese seabream Pagrus major, and black bullhead Ameiurus melas. This report documents a novel poxvirus infection in a captive population of the endangered Knysna seahorse Hippocampus capensis. Histopathological examination revealed an epidermal keratinocytic dermatopathy with hypertrophy and subsequent progressive degeneration and necrosis of the affected cells; karyomegaly with a prominent nucleolus was also a feature of the affected cells. Ultrastructural examination of the affected keratinocytes revealed large and complex, intracytoplasmic virions with elliptical to discoid profiles and an average size of 548.5 nm (SD = 100.58) by 205.65 nm (SD = 21.19) consistent with a typical poxvirus morphology. Furthermore, metagenomic analysis revealed distinct genomic features, whereas phylogenetic analysis of nine core poxvirus genes demonstrated that the seahorse poxvirus forms a unique branch within the teleost poxvirus clade supporting its classification as a novel poxvirus with a proposed species name of Syngnathid cutaneouspox virus. Finally, this study confirms the utility of next-generation sequencing as a methodology for obtaining phylogenetically useful genomic sequences from archival formalin-fixed paraffin-embedded tissues.