Subfamily: Chordopoxvirinae

Genus: Avipoxvirus

Distinguishing features

Pox-like diseases have been described worldwide in over 278 species of birds, spread across 23 orders, but it is likely there are many more affected than currently reported. Two forms of disease are commonly found, cutaneous and respiratory/enteric, dependent on the site of infection. The cutaneous form tends to be less serious than the respiratory/enteric form with mortality ranging from the low percentages up to 80–100% in some affected flocks (Giotis and Skinner 2019). Although viruses have not been isolated from all these 278 bird species, the similarity in clinical signs suggests a common etiological link. In commercial poultry flocks, poxvirus infection can result in egg drop, impaired fertility and slower growth rates, whereas in wild birds increased susceptibility to secondary bacterial infections, impaired vision and ability to feed at will results in greater predation. Viruses are primarily transmitted mechanically by arthropods, by direct contact with fomites or through aerosols. Avipoxviruses grow productively in avian cell cultures, but abortively in mammals and mammalian cell lines. There is no evidence that avipoxviruses are zoonotic.

Virion

Virions are brick-shaped, about 330×280×200 nm. Infectivity is usually ether-resistant. Viruses exhibit extensive serological cross-reactivity, but the degree of cross-protection between species normally seen within a genus is not so evident within the avipoxviruses. Avipoxviruses produce A-type inclusion bodies with considerable amounts of lipid.

Genome organization and replication

There are currently 6 fully sequenced Avipoxvirus viruses, each representative of a species. Their dsDNA genomes range from 189–360 kbp, encoding a predicted 171–328 proteins; G + C content ranges from 28–31%. Comparison of all sequenced viruses to date indicates 130 potential genes are conserved across all avipoxvirus species, but only 83 of the 90 genes historically reported to be conserved across all chordopoxviruses are present. There is also some disruption to the collinearity of the central core region of the genome (where these 90 genes are normally found within the chordopoxviruses) with at least three major insertions of novel genes within this area as well as inversions of some of the conserved blocks of genes. Between the various avipoxviruses there is significant variation in the size and gene content of these insertions. Most avipoxviruses contain a large number of genes encoding members of the ankyrin repeat family of proteins although their functions remain obscure. The Avipoxvirus genus includes 12 species that can be further sub-divided phylogenetically, using a subset of genes, into at least 3 deep clades. The distances between these clades are as great if not greater than the distances between individual genera of mammalian poxviruses, suggesting that the Avipoxvirus genus may require reclassifying into several genera based on fowlpox-like, canarypox-like and psittacinepox-like viruses.

For replication please see discussion under family description.

Species demarcation criteria

Provisional species demarcation criteria include disease characteristics, nature of the host and ecological niche, growth characteristics on the chicken chorioallantoic membrane, host range in cell culture and cross-neutralization. Restriction enzyme fragment length polymorphisms (RFLP) analysis and cross-hybridization have been used but whole genome sequencing is now more common. The significant divergence found between individual species has rendered it extremely difficult to identify pan-genus oligonucleotide probes for PCR amplification and sequencing, and consequently existing phylogenetic information across the entire genus, when not based on full genome sequence, is limited to a small number of gene loci.

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.