Subfamily: Chordopoxvirinae

Genus: Parapoxvirus

Distinguishing features

Members of the genus Parapoxvirus generally infect ungulates with disease apparent in livestock species and wildlife. Exceptions are members of the species Grey sealpox virus. Disease is found world-wide wherever livestock species are farmed. It is generally self-limiting and normally manifests as a localised erythematous pustular dermatitis. Infection does not generate sterile immunity and animals can be re-infected with the virus albeit resulting in milder disease. Although individual parapoxvirus species have a very narrow host range, all are considered zoonotic or potentially zoonotic. Parapoxvirus species show extensive DNA cross-hybridization and serological cross-reactivity. They exhibit a narrow host range based on cell culture.

Virion

Virions are ovoid, 220–300 × 140–170 nm in size, with a surface filament that may appear as a regular cross-hatched, spiral coil involving a continuous thread. Infectivity is ether-sensitive.

Genome organization and replication

The dsDNA genome is 128–140 kbp, encoding approximately 119–134 genes; G+C content is generally around 64%, the outlier being members of the species Grey sealpox virus which have a G+C content of only 55%. Eighty-eight of the genes in the central core of the genome are colinear with those of orthopoxviruses and most other members of the subfamily Chordopoxvirnae. Parapoxviruses lack the F15 and D9 genes found in other chordopoxviruses in this region of the genome (Delhon et al., 2004). The variable genomic regions at the ends contain host range genes and those which are presumed to modulate the immune response to infection. Phylogenetic analyses demonstrate a closer relationship between orf virus and pseudocowpox virus than between pseudocowpox virus and bovine papular stomatitis virus despite the latter two being considered bovine viruses. However, pseudocowpox virus is considered to have a broader host range, with infection reported additionally in reindeer and dromedaries (Hautaniemi et al., 2010). All parapoxviruses sequenced to date contain an orthologue of the mammalian vascular endothelial growth factor gene which is thought to facilitate virus growth by stimulating epidermal wound repair. Orf virus lacks the thymidine kinase and ribonucleotide reductase genes commonly encoded by poxviruses. Orf virus encodes a protein that mimics a component of the anaphase-promoting complex and promotes the transition into (or stabilizes) an S-phase state (Mo et al., 2009).

For replication please see discussion under family description.

Species demarcation criteria

Originally, the host species, RFLP and cross-hybridization analyses at the terminal regions of the genome, were used to differentiate taxa. More recently genomic sequence comparisons with respect to gene content, synteny and phylogenetic analyses have been used to infer species demarcation. Orf viruses isolated from goats can be separated in phylogenetic analyses from those isolated from sheep.

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.

* reported in (Airas et al., 2016)