Subfamily: Chordopoxvirinae

Genus: Leporipoxvirus

Distinguishing features

The genus includes viruses of lagomorphs and squirrels that have a broad cell culture host range. It is likely that the viruses co-evolved with their natural hosts in the Americas, as on these Continents disease manifests normally as localized cutaneous fibromas, that persist for several weeks before spontaneously regressing. In the European rabbit, however, members of the species Myxoma virus, causes myxomatosis, a generalized disease characterized by cutaneous lesions all over the body, blepharoconjunctivitis with ocular and nasal discharge, involvement of lymphoid tissue, lungs and testes, and is normally lethal. Indeed, in the 1950s, myxoma virus was used as a biological control for the European rabbit which had been introduced to Australia in the mid-19^th century. Upwards of 98% of the European rabbit population are estimated to have been killed by the virus in Australia, and also in France and the UK where the virus had also been introduced (Kerr et al., 2015). Since then, however, co-evolution of the virus with the European rabbit host has resulted in less virulent viruses and more resilient rabbits such that rabbit populations have gradually recovered. In 2019, a recombinant poxvirus derived from myxoma virus and a second unknown poxvirus (predicted to be from cervids), called myxoma virus-Toledo, crossed from rabbits into hares in the Iberian peninsula where it caused a new myxomatosis-like disease in the hare population. Viruses are mechanically transmitted by biting arthropods, especially mosquitoes and fleas; but they are also transmitted by direct contact and fomites. Serological cross-reaction and cross-protection have been demonstrated between members of different virus species. There is no evidence that leporipoxviruses are zoonotic, but myxoma virus exhibits tropism for tumor cells from outside the rabbit species.

Virion

The virion is brick-shaped with dimensions of approximately 286 × 230 × 275 nm and generally indistinguishable from those of orthopoxviruses (Compare Figure 1A.Poxviridae with Figure 1D.Poxviridae). Infectivity is ether-sensitive.

Genome organization and replication

A complete genomic sequence is available for two viruses, representative of two species within the genus, myxoma virus (MYXV) and rabbit (Shope) fibroma virus (SFV). The dsDNA genomes are approximately 160 kbp and 162 kbp in length and encode approximately 165–171 genes, at least 12 of which are present within the inverted terminal repeat sequences (ITRs) which are approximately 12 kbp in length: G + C content is approximately 39.5–43.5 % (Cameron et al., 1999, Willer et al., 1999). The internal core of the genome, from the equivalent of vaccinia virus gene F9L to vaccinia virus gene A37R, is generally colinear with the genome of the majority of the mammalian chordopoxviruses with only a few gene deletions. Leporipoxviruses have in addition up to 10 unique genes not found so far in members of other genera. The genomes of MYXV and SFV are highly similar, essentially encoding identical gene complements. However, a number of the genes have been disrupted in the SFV genome, with at least one completely missing. Whether or not these changes in the SFV genome are sufficient to dictate the different disease presentation in rabbits between the two viruses is currently unknown. A MYXV-SFV hybrid virus (malignant rabbit virus) has been isolated.

For replication, please see discussion under family description.

Species demarcation criteria

The general chordopoxvirus classification criteria of <98% nucleotide identity across the central core of the genome is used to delineate species, whereas >98% identity indicates separate strains of the same species.

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.

* reported in (Simon and Bullard 1973)