For centuries the death and disability of hundreds of thousands of horses attributable to Eastern, Western, and Venezuelan Equine Encephalomyelitis (EEE, WEE, and VEE respectively) has had a tremendous impact on both the North and South American continent. The exact incidence of the disease in horses is unknown but EEE, a reportable disease, is known to effect hundreds of horses in the U.S. every year. But these numbers are small compared to the time before a vaccine was available. Humans also contract these viruses though horses do not figure into the infection of humans.

Beside these American continent forms there are other related viruses all over the world with similar properties and identified by their place of origin like the Japanese Equine Encephalitis virus. There are other similar viruses that are not related to this group like West Nile Viral Encephalitis (WNE). Though not closely related, WNV modes of transmission and symptoms greatly resemble the arbovirus encephalitides, ...more.

This article discusses the incidence, distribution, transmission, clinical signs, diagnosis, treatment, and prevention of EEE, WEE, VEE viruses.

These viruses belong to the genus Alphavirus, family Togaviridae. The exact incidence is unknown because many horses with neurological disease are not examined. But it is certain, since the advent of wide spread vaccination, the incidence in horses has fallen. Western equine encephalitis, despite its name, occurs throughout North and South America. Infections east of the Mississippi River are probably a less pathogenic strain of the virus known as the Highland Virus. Major outbreaks of WEE have occurred in the western and Midwestern United States and have occurred in west-central Canada, Mexico, and South America.

EEE is a bit less widespread than WEE and there is overlap of where they occur. It ranges from Quebec in the north to the Caribbean and on into Central America to South America. In the United States, the virus has been isolated in all states in the eastern 1/3 of the country and along the Gulf Coast but also occasionally in some western states. EEE occurs in these regions mainly during warm months along coastal areas and anywhere there is large amounts of standing water.

VEE has a number of subtypes each with its own small geographic distributions. It is believed that pathogenic versions arise periodically from these subtypes giving rise to extremely severe and widespread outbreaks. These have been reported in Venezuela, Colombia, Ecuador, and Peru at 6 to I0 year intervals. This disease occasionally flirts with the US as it did in 1969 when such an outbreak spread to Costa Rica and finally the southwest corner of Texas in 1971. About 1500 horses died of VEE in Texas before further spread of the disease in the United States was halted by immunization of horses with an attenuated vaccine, resulting in an immune belt across the southern United States. Also there is a small focus of type II VEE in the Everglades region of Florida that has not yet been associated with disease.

For recent reports of the incidence of EEE and WEE see the APHIS link under More Info below.