Eastern Equine Encephalitis

Eastern equine encephalitis (EEE), also known as sleeping sickness, is a viral disease that affects horses, some other animals, and humans. EEE occurs in the Eastern half of North America, 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 Western and Central North America and is most often seen in Canada in Saskatchewan and Manitoba.

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.

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, Michigan, and Florida have had a significantly increased incidence in the last few years. It is a relatively rare disease in Canada, though has been identified in horses in recent years in Southwestern Ontario and Nova Scotia.

Blood tests may reveal abnormalities in the white cell count. EEE is almost always fatal in equines.

EEE infection is infrequent in humans, with only about 200 confirmed cases in the US in the last 40 years and no confirmed cases in Canada.

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.

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 horses and humans include using mosquito repellent, staying indoors at dusk when mosquitoes are most active, avoiding wetlands, and wearing a fly sheet or clothing that covers the arms and legs.