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Myxomatosis is primarily a disease of rabbits caused by infection with the myxoma virus. It mainly occurs in domestic and wild rabbits. The virus is harmless to humans. Myxomatosis can result in lumps developing around the ears and face. These lumps are named myxomas and the disease virus was named after this lesion. It was first discovered in South America, California and Mexico in 1896. Sick animals will die within a few days to two weeks after infection, and the fatality rate is close to 100%. Currently, there is no effective treatment.
Pathogen
Myxoma virus is the type species of the Leporipoxviruses, a genus of Chordopoxvirinae, double stranded DNA viruses, whose members infect leporids and squirrels, inducing cutaneous fibromas from which virus is mechanically transmitted by biting arthropods. Pathogenesis studies confirm that the virus initially replicates in dermal cells at the inoculation site, likely dendritic cells. From there, the virus spreads to local macrophages and epidermal cells, and to the draining lymph node. Virus replication in the latter results in lymphoid depletion, with extensive loss of cortical and paracortical lymphocytes. From the lymph node the virus spreads via blood leukocytes to distal tissues including the spleen and other lymphoid tissues, testis, lungs, and skin.
Transmission and Clinical Signs
Currently, myxoma virus is enzootic to the Americas, Europe, Australia and other regions. The principal mode of transmission of the virus is mechanical transport of virus on mouth parts by arthropod vectors such as ticks, mosquitoes, and fleas through bites. It can also transmit the virus to other rabbits via direct contact.
The general incubation period is 3-7 days, up to 14 days. The first sign of disease is conjunctivitis that rapidly becomes more severe and is accompanied by a milky discharge from the eye. The rabbit has no energy and no appetite, with a fever that may reach 42°C. In severe outbreaks, some rabbits die within 48 hours after signs appear. Those that survive become progressively weaker and develop a rough coat. The eyelids, nose, lips, and ears become puffy, which gives a swollen appearance to the head. The ears may droop. In females, the vulva becomes inflamed and swollen with fluid; in males, the scrotum swells. Other signs include discharge of pus from the nose, difficulty breathing, and coma. Death usually occurs within 1 to 2 weeks after signs appear, and the fatality rate is close to 100%.
Diagnosis
Gross lesions: The most prominent gross lesions in in rabbits with myxomatosis are the skin tumors and the pronounced cutaneous and subcutaneous edema, particularly in the area of the face and around body orifices. Hemorrhages of the skin, heart, and subserosa of the gastrointestinal tract may be observed.
Microscopic lesions: Lesions in the skin involve epithelial cells, fibroblasts, and endothelial cells and range from proliferative to degenerative, depending on the strain of virus. The skin tumors result from proliferation of undifferentiated mesenchymal cells, which become large stellate (myxoma) cells surrounded by a homogeneous matrix of mucinous material interspersed with capillaries and inflammatory cells
Lab tests include serology and molecular diagnosis.
Serology: immunofluorescent assay (IFA), ELISA, complement fixation test (CFT)
Molecular diagnosis: PCR
Treatment and Vaccination
Unfortunately, there is no specific treatment for myxomatosis. Vet can only offer supportive care, including fluids, antibiotics to prevent secondary infections, and pain medication. The myxomatosis vaccine is available in some countries.
Reference
- Bertagnoli S, Marchandeau S. Myxomatosis. Rev Sci Tech. 2015 Aug;34(2):549-56.
- Espinosa J, Ferreras MC, Benavides J, et al. Causes of Mortality and Disease in Rabbits and Hares: A Retrospective Study. Animals (Basel). 2020 Jan 17;10(1):158.
