CORVALLIS - This summer the private and commercial rabbit growers of western Oregon may be facing one of the periodic epidemics of myxomatosis, a disease with an extremely high mortality rate that shows up somewhat unpredictably in the European rabbits most commonly reared in Oregon.

A sudden outbreak of the disease in Linn and Benton counties in early July prompted the closure of rabbit shows at the county fairs in those two areas. But whatever combination of viral, population or climatic conditions caused these cases may also lead to more widespread outbreaks, according to experts in the College of Veterinary Medicine at Oregon State University.

Myxomatosis, which last caused major problems in the state more than a decade ago, is extremely infectious, is transmitted naturally by mosquitoes or other insects, can be spread from rabbit to rabbit by human handlers and has no cure. In Oregon there is no diagnostic test for live animals and no readily available vaccine. "The last time we had a major outbreak in the 1980s this caused a horrible problem in the Willamette Valley and southern Oregon," said Dr. Donald Mattson, an associate professor of veterinary medicine at OSU. "It can have a mortality rate higher than 90 percent in European rabbits."

There are a few primary precautions that rabbit owners can take to protect their animals, Mattson said. The most important would be mosquito netting, which may help protect against mosquito and other insect transmission. But animal handlers should be careful about use of such netting in very hot weather, since it may impair air flow in rabbit hutches. Beyond that, the best prevention is avoiding groups of other rabbits which may be infected, such as at rabbit shows or county fairs.

"For rabbits, this is a very deadly disease," said Dr. Beth Valentine, an associate professor of veterinary medicine at OSU. "From the point of view of a rabbit owner, the best place for these animals is behind mosquito netting, at home, until the worst of the mosquito season passes later this fall. People who own rabbits in western Oregon should be very cautious until this problem has passed."

Myxomatosis is caused by a poxvirus that has a natural reservoir, perhaps among brush rabbits, the OSU experts say. It is far less deadly to wild rabbits, although they, too, can be affected. Exactly what triggers the periodic outbreaks among domesticated rabbits is less clear. It may be some combination of immunity levels in wild populations, heat stress, other weather conditions, mosquito populations and other factors.

The disease is not easily diagnosed, and might be misdiagnosed by veterinary doctors who see it infrequently. Symptoms can include high fever, loss of appetite, swelling of mucus membranes or sluggishness. The underlying cause of mortality is a profound suppression of the animals' immune system, making them vulnerable to a host of other health problems. Skin nodules called "myxomas" may appear in some cases. But at times an animal has appeared fairly healthy, and then died the next day.

There is no treatment other than supportive care for secondary infections, veterinary doctors say, and no vaccine is readily available. Experts are now getting information on vaccines that have been used with some success outside the U.S., but in any case it would probably be too late for a vaccine to provide any protection this summer, OSU doctors said.

Some of the facts known about myxomatosis include:

  • Myxomatosis first was described in Uruguay in 1898, and spread northward from there into Mexico and California, where the "California" strain of the virus evolved, which often has a high mortality but fewer overt symptoms than some other strains.
  • All domesticated rabbits in the U.S. are highly susceptible to the virus, but humans are not.
  • Mosquitoes, fleas, mites and biting flies may all help transmit the virus, and it may also be carried in the air for unknown distances.
  • In rabbit colonies, mechanical transmission of the virus is observed, often by human caretakers or even a judge at a rabbit show.
  • If a rabbit is exposed to an infected rabbit, it should be quarantined for 14 days and assumed to be infected during that period.

It's unknown exactly how severe this year's epidemic may become and how far it will spread, the OSU experts say, although history suggests it may not move much beyond western Oregon.

In the interests of monitoring the spread of the disease, anyone who owns a rabbit that dies from an unknown cause should consider contacting their local veterinarian or arranging for a necropsy by the Veterinary Diagnostic Laboratory at OSU, at 541-737-3261. There will be a fee for the necropsy. It's probable that the disease has already caused many more deaths than have so far been confirmed because it cannot be specifically diagnosed in live animals and few people arrange for necropsies.

More detailed information on myxomatosis will also soon be posted on the website of the OSU College of Veterinary Medicine, at Dr. Brad LeaMaster, the state veterinarian, is collaborating with the OSU veterinary experts on the monitoring and management of this disease.

"We'll try to monitor the spread of this disease as carefully as we can and keep fair officials and the public advised of any developments," LeaMaster said. "At this point we're allowing officials in local areas to make decisions about closing down such things as rabbit shows at county fairs.


Dr. Donald Mattson, 541-737-6877

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