Family: Poxviridae

Subfamily: Entomopoxvirinae

Distinguishing features

Entomopoxviruses have been identified in over 40 species of insects. The four genera in the subfamily each comprise viruses that infect a separate order of insects, namely Coleoptera, Lepidoptera, Orthoptera and Diptera (Becker and Moyer 2007). However, further entompoxviruses have been reported in other orders of insects such as Hymenoptera. Although some entomopoxviruses have a broad tissue tropism, the main site of infection tends to be hemocytes or adipose tissue cells. The course of infection is generally slow, but usually lethal for the host (Williams et al., 2017). An exception is the entomopoxvirus, Diachasmimorpha entomopoxvirus, found in a braconid parasitoid wasp that exists as a heritable symbiont, whilst also remaining pathogenic in the parasitized fruit-fly host (Lawrence 2005).

The virions of entomopoxviruses can be brick-shaped or ovoid. They range from 250–400 nm in length and 150–250 nm in width and are similar in composition to the virions of chordopoxviruses. Virions generally have globular surface units that give a mulberry-like appearance. Three morphotypes of virion are recognised, based on the shape of the core.

As with other members of the family Poxviridae, the genome comprises a linear double-stranded DNA molecule with covalently-closed termini. Genomes are approximately 229–308 kbp, encoding a predicted 247–334 genes: G + C content is approximately 20%. A common co-linear signature of core genes, different from those of members of the subfamily Chordopoxvirinae, is beginning to become apparent, and is characteristic of the subfamily. The direction of gene transcription at the termini and within the core does not mirror that of the chordopoxviruses. No serologic relationships have been demonstrated between entomopoxviruses and chordopoxviruses. Mature virions are usually occluded in spheroids, comprised of a major crystalline occlusion body protein (termed “spheroidin”), and thought to protect infectivity in the environment over prolonged periods. Transmission is normally via the per os route whereby, after ingestion, the virus particles infiltrate the internal tissues through the gut. Vertical transmission has also been reported.

The process of viral replication is thought to be similar to that described for the chordopoxviruses.

Genus demarcation criteria

• · Natural host range. The subfamily Entomopoxvirinae includes four genera; Alphaentemopoxvirus, Betaentemopoxvirus, Deltaentomopoxvirus and Gammaentemopoxvirus. Generally, each of the entompoxvirus genera infect a different order of insect, respectively Coleoptera, Lepidoptera, Orthoptera and Diptera.

• · Virion morphology. Entomopoxviruses infecting Coleoptera have a kidney shaped core and single lateral body located in the concave side, those infecting Lepidoptera and Orthoptera have a cylindrical core flanked by two lateral bodies and finally, those infecting Diptera have a dumbbell-shaped core flanked by two lateral bodies.

• · Gene content and phylogeny. Within a genus, both gene content and synteny in the central region of the genome are relatively well conserved between members of different species. The numbers of conserved genes differs between genera. Phylogenetic analyses, both with the 49 genes conserved among members of the family Poxviridae and of the highly conserved spheroidin gene supports the current genus classification.

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.