Myxomatosis
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Description
Myxomatosis is a highly contagious viral condition of rabbits caused by the myxoma virus and carried prinicpally by the rabbit flea,Spilopsyllus cuniculi. It is also transmitted by direct or indirect contact with ocular or skin discharges or by mechanical vectors. Myxomatosis is enzootic in rabbits of the genus Sylvilagus in both South and North America and in wild rabbits of the genus Oryctolagus in South America, Europe, and Australia.
Pathogenesis
The myxoma virus infects several cell types including mucosal cells, lymphocytes, and fibroblasts. In addition to primary and secondary tumour development, there is severe immunosuppression leading to overwhelming infections by opportunistic gram-negative bacteria particularly affecting the conjunctiva and nasal passages.
Virus multiplication and tumour-like lesions occur initially at the site of intradermal inoculation. This is followed by spread to regional lymph nodes and cell-associated viraemia, with generalization to the skin and internal organs. Gelatinous proliferative nodules develop all over the body, especially at orifices such as the eyes, anus, nose. The rabbit usually dies within 12 days, if not killed by predators.
Clinical signs
The clinical disease varies with the virus strain and host species. Sylvilagus species are relatively resistant, and are probably the natural host of the virus. In this species, infection usually results in the development of skin tumours at the site of innoculation. The tumours appear 4-8 days after exposure and persist for up to 40 days.
In the European rabbit (Oryctolagus cuniculus), infection with a virulent virus (i.e. the South American or California strains) results in severe disease with up to a 99% case fatality rate. Initial signs include oedema of the eyelids accompanied by inflammation and oedema around the anal, genital, oral, and nasal orifices. Oedema of the head and ears, drooping ears, and bacterial infections resulting in mucopurulent conjunctivitis and pneumonia are seen. Death (8-15 days post infection) is usually preceded by labored breathing and convulsions.Very young rabbits may succumb to generalized disease. Lepus species (hares) however are highly resistant; occasional individuals develop mild to severe generalized myxomatosis.
Pathology
The most prominent gross lesions in European rabbits with myxomatosis are the skin tumours and the pronounced cutaneous and subcutaneous oedema, particularly in the area of the face and around body orifices. Skin hemorrhages and subserosal petechiae and ecchymoses may be observed in the stomach and intestines. Subepicardial and subendocardial hemorrhages may also occur.
Adult rabbits of the genus Sylvilagus usually develop localized skin tumors resembling fibromas. Hares or young Sylvilagus rabbits may develop fibromatous to myxomatous nodules, however, lesions are usually mild and localized.