| Myxomatosis is a fatal disease of all breeds of domesticated rabbits caused by myxoma virus, a member of the poxvirus group. Myxomatosis is called “big head” and is characterized by mucinous skin lesions or myxomas. Wild rabbits like the cottontail (
Sylvilagus
) and jackrabbits (
Lepus
) are quite resistant. Myxoma virus-infected
Sylvilagus
develop fibroma-like lesions similar to those caused by rabbit fibroma virus. All other mammals are refractory to the virus. Myxomatosis has a worldwide distribution. In the USA, myxomatosis is restricted largely to the coastal area of California and Oregon, where epidemics occur infrequently but sporadic cases are common. These areas correspond to the geographic distribution of the California brush rabbit (
S
bachmani
), the reservoir of the infection. Losses in rabbitries may be 25-90%. Transmission occurs via mosquitos, fleas, biting flies, and direct contact. |
| The initial sign is conjunctivitis that rapidly becomes more marked and is accompanied by a milky ocular discharge. The rabbit is listless and anorectic, with a fever that frequently reaches 108°F (42°C). In acute outbreaks, some rabbits may die within 48 hr after signs appear. Those that survive become progressively depressed and develop a rough coat. The eyelids, nose, lips, and ears become edematous, which gives a swollen appearance to the head. In females, the vulva becomes
inflamed and edematous; in males, the scrotum swells. A characteristic sign at this stage is drooping of the edematous ears. A purulent nasal discharge invariably appears, breathing becomes labored, and the rabbit goes into a coma just before death, which usually occurs within 1-2 wk after clinical signs appear. Occasionally, a rabbit survives for several weeks; in these cases, fibrotic nodules appear on the nose, ears, and forefeet. Rabbits inoculated experimentally with laboratory
strains of the virus invariably develop small nodules at the point of injection after several days; similar nodules develop later on other parts of the body, particularly the ears. |
|
Few characteristic gross lesions are found at necropsy in the acute form of the disease. The spleen is occasionally enlarged and is almost always devoid of lymphocytes when examined histologically. In rabbits that survive longer, subcutaneous edema and nodular skin tumors are seen. The seasonal incidence of the disease, clinical signs (especially the swollen genitalia), and high mortality are all of diagnostic significance. Large, eosinophilic,
cytoplasmic inclusion bodies in the conjunctival epithelial cells are also helpful in diagnosis. |
| An attenuated vaccine prepared from a myxomatosis virus has protected rabbits infected under both field and laboratory conditions. This vaccine is not available in the USA, and because there is no effective treatment, euthanasia and burying or burning of affected rabbits is indicated. Preventive measures include protecting rabbits from exposure to arthropod vectors. |
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