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Eastern equine encephalitis, also known as sleeping sickness, is a viral disease that affects horses, some other animals, and humans. EEE occurs in the eastern half of the United States, most commonly on the eastern seaboard and the Gulf coast. It is also found in Central and South America and the Caribbean. A similar disease, western equine encephalitis (WEE) is present in the western United States.

How is EEE spread?

Mosquitoes are the vectors of this disease, with Culiseta melanura being the principal carrier. The mosquito picks up the virus from an infected bird and can then spread it to horses. There is no record of transmission from infected horses to other horses or to humans.

How widespread is this disease?

EEE seems to follow a pattern of low annual incidence with heavier outbreaks occurring about every five years in horses, and every ten years in humans, in particular areas. Wisconsin, Minnesota, and Florida have had a significantly increased incidence in the last few years.

What are the signs of EEE infection in horses?

Signs are usually noticed one to three weeks after the horse is bitten. The virus causes inflammation of the brain and spinal cord. Affected horses may show depression, fever, droopy eyes and lips, elevated heart rate, decreased appetite, seizures, and erratic behavior. Blood tests may reveal abnormalities in the white cell count. EEE is almost always fatal in equines.

How common is the disease in humans?

EEE infection is infrequent in humans, with only about 200 confirmed cases in the U.S. in the last forty years. The low incidence is attributed partly to the fact that humans are unlikely to encounter infected mosquitoes. The virus is limited to mosquito species that live in swamps and coastal areas rarely visited by people. In addition, the type of mosquito that most commonly carries the disease, and is exclusively responsible for spreading it to horses, does not bite humans, and incidence of infection is very low in species that are attracted to humans.

Although human infection is uncommon, EEE is fatal in 35% to 50% of human cases, and many who survive have severe brain damage and require long-term institutional care.

How is EEE treated?

There is no specific medical treatment for EEE. Care of infected horses and humans is centered around relieving fever and other symptoms. Antibiotics are not effective against the virus that causes EEE.

Can EEE be prevented?

A vaccine is available to protect horses against EEE. The recommended use is one or two doses per year, although three doses per year are sometimes given in areas where heavy outbreaks have been reported. Horses that have been properly vaccinated have good protection, but some owners choose not to vaccinate their horses as incidence of the disease is normally low. Horse owners should check with local veterinarians for appropriate vaccination protocol in their regions.

An experimental human vaccine has been developed for laboratory use, but at this time preventive vaccination is not available to the public. Because EEE is spread by mosquitoes, measures to control populations of this insect can limit the likelihood of exposure among humans and unvaccinated horses. Other protective measures for humans include using a mosquito repellent, staying indoors at dusk when mosquitoes are most active, avoiding wetlands, and wearing clothing that covers the arms and legs.

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