Farm and Home Biosecurity
Livestock/Row Crop Producer
Eastern Equine Encephalomyeltis
There are 3 equine encephalomyelitis strains caused by viruses
which are transmitted by mosquitoes and occasionally other blood-sucking
insects. Insects transmit the disease to horses from wild birds and rodents. Western
Equine Encephalomyelitis (WEE) is prevalent throughout the United States,
whereas the distribution of Eastern Equine Encephalomyelitis (EEE) is
restricted to the eastern and southeastern states. Venezuelan Equine
Encephalomyelitis (VEE) occurs in South and Central America but has not been
diagnosed in the U.S.A. for more than 20 years.
Humans are susceptible to the disease if bitten by infected
mosquitoes. The disease is not directly contagious from horse to horse, and
indirect horse-to-human transmission by mosquitoes is highly unlikely because
the amount of virus in the blood of horses affected with EEE or WEE is very
small.
Following transmission of the virus by infected mosquitoes,
the horse develops various progressive clinical signs related to degeneration of
the brain. These include: a high fever, mild depression, loss of appetite, and
increased sensitivity to sound. In stage II, affected horses show depression,
drowsiness, and a weak wobbly gait. In severe cases, blindness, head pressing,
circling, and convulsions may occur. Approximately 50 percent of horses affected
with WEE die, whereas the mortality rate for EEE and VEE is 70 to 90 percent.
Vaccination with one of the many encephalomyelitis vaccines
containing EEE, WEE, and VEE initiates effective control of these diseases and
is highly recommended for all horses. The bivalent encephalomyelitis vaccines
containing EEE and WEE are the most commonly used. VEE vaccination is not
routinely recommended in most areas of the U.S.A. except the border areas of
Texas, New Mexico, Arizona, and California.
Primary immunization of unvaccinated horses involves
intramuscular administration of two doses of a EEE and WEE vaccine 3 to 4 weeks
apart. Annual revaccination is best completed in the spring, prior to peak
insect vector season. Booster vaccination of pregnant mares 4 to 6 weeks before
foaling provides passive colostral protection to their foals. Foal vaccinations
should begin at 3 to 4 months of age.
Annual encephalomyelitis immunization can be conveniently
combined with tetanus and, where appropriate, influenza, rhinopneumonitis,and
Potomoc Horse Fever using combination vaccines.
EEE in horses is usually fatal and is diagnosed by submitting
the horse or head (brain) for tests to the Livestock and Poultry Commission
Laboratory in Little Rock. Emu birds are also very susceptible to
encephalomyelitis viruses. The potential for EEE exists every year in Arkansas
so horses should be vaccinated yearly to prevent infection.
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