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CORVALLIS - The private and commercial rabbit growers of Western Oregon may
be facing this summer 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. There is no diagnostic test in live animals and no 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 a poxvirus that has a natural reservoir in nature, 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 also not easily diagnosed, and often it's 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 animal's
immune system, making them vulnerable to a host of other health problems.
Nodules called "myxomas" may appear in some cases. But at times an animal
has appeared fairly healthy and been dead 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 1989, and spread northward
from there into Mexico and California, where it has evolved into the
"California" strain of virus, which often has 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 often observed, often by human caretakers or
even the 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 an autopsy by the Veterinary Diagnostic
Laboratory at OSU, by calling (541) 737-3261. There will be a fee for the
autopsy. 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 autopsies.
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," Lemaster
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."
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