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Myxomatosis is a disease of rabbits caused by Myxoma virus, a poxvirus in the genus Leporipoxvirus. The natural hosts are brush rabbits (Sylvilagus bachmani) in North America and tapeti (S. brasiliensis) in South and Central America, in which the myxoma virus causes only a mild disease, involving skin nodules. In European rabbits (Oryctolagus cuniculus), it causes a severe, often fatal, disease. Symptoms include fever, swelling of the eyelids and anogenital area, a mucopurulent ocular and nasal discharge, respiratory distress and hypothermia. Death generally occurs 10–12 days after infection. Myxoma virus is transmitted passively (without replication) by arthropod vectors, usually via the bites of mosquitoes and fleas, and also mites, flies and lice. It can also be transmitted by direct contact, and is shed in the ocular and nasal discharge and from eroded skin.
Myxoma virus was intentionally introduced in Australia, France and Chile in the 1950s to control wild European rabbit populations. This resulted in short-term 10–100-fold reductions in the rabbit population, followed by its recovery with the emergence of myxomatosis-resistant animals and attenuated virus variants. The introduction of myxomatosis is regarded as a classical example of host–pathogen coevolution following cross-species transmission of a pathogen to a naive host.