Subfamily: Chordopoxvirinae

Genus: Salmonpoxvirus

Distinguishing features

The Salmonpoxvirus genus includes a single virus species, Salmon gillpox virus, members of which cause a proliferative disease in gill tissue in Atlantic salmon and can lead to significant mortality on fish farms (Gjessing et al., 2015). On its own the poxvirus has been associated with up to 20% mortality, but when combined with other causes of gill disease, such as amoebic gill disease, losses of up to 80% have been reported. Although salmon in freshwater can be affected, evidence suggests fish returning from sea pose the greatest risk of introducing the virus to freshwater and to salmon farms, whether they be in freshwater or seawater. It is not known if salmon are the natural host of the virus, but other species such as brown trout, seatrout and arctic char that share the same water courses as infected salmon do not generally appear to be infected by the virus. Other poxvirus-like virions and poxvirus-like DNA sequences have been detected in carp and ayo, but how these are related to viruses in the genus Salmonpoxvirus is currently unknown. There is no evidence that members of the genus Salmonpoxvirus are zoonotic.

Virion

Virion morphogenenesis of salmon gillpox virus is described as more reminiscent of that of the entomopoxviruses than the chordopoxviruses, and the virion itself is described as “boatshaped” and approximately 300 nm long. The description may have arisen from it possessing a single lateral body, unlike the two normally found in the other chordopoxviruses. The majority of the conserved chordopoxvirus genes involved in virus membrane biogenesis are missing from the salmon gillpox virus genome.

Genome organization and replication

The dsDNA genome is approximately 242 kbp, containing a predicted 206 unique genes; the G + C content is approximately 37.5%. Of the 206 predicted proteins encoded by the virus, only 59 are recognised as being homologues of other chordopoxvirus proteins; functions for others have been suggested through conservation of motifs but the majority (118) remain completely uncharacterised due to lack of homology to any sequences in public databases. Conserved genes are mainly associated with viral DNA replication and gene expression, together with the structure and morphogenesis of the virus core and capsid. Although blocks of conserved genes are found in the central region of the salmon gillpox virus genome there is a lack of synteny normally found in this region when compared with the majority of other chordopoxviruses. Two proteins encoded by salmon gillpox virus are also found in crocodylidpoxviruses, reflecting potential retention of ancestral genes from the nucleocytoplasmic large DNA viruses, but which have been lost from other chordopoxviruses, presumably after divergence from their common chordopoxvirus ancestor. Phylogenetic analyses of 13 proteins conserved across all poxviruses indicates that salmon gillpox virus has the lowest branchpoint within the phylogeny of chordopoxviruses discovered to date.

For replication please see discussion under family description.

Species demarcation criteria

There is only one species in the genus and therefore criteria have not been defined.