Eastern equine encephalitis (EEE), western equine encephalitis (WEE) and Venezuelan equine encephalitis (VEE) are caused by Alphaviruses. Birds and rodents are the primary reservoirs, but unvaccinated horses are particularly susceptible and often serve as sentinels. These diseases are zoonotic, infecting humans, and are a public health concern. Eastern equine encephalitis is the most common of the three and is found widely in several regions of the United States (especially in areas of high mosquito populations). Western equine encephalitis occurs infrequently, but circulates in wildlife. Venezuelan equine encephalitis is a Foreign Animal Disease and therefore reportable at both the Federal and State level. It occurs in cycles in Central and South America; and there have been incursions into the United States decades ago.
EEE/WEE/VEE are transmitted only by the bite of infected mosquitoes; direct transmission between horses and people does not occur. However, should an incursion of VEE occur, equine infections do present some risk to people.
Infection causes non-specific clinical signs and often affects the central nervous system.
Treatment consists of supportive care. Survival rates for horses infected with EEE or VEE is low; however, WEE survival can be as high as 70-80%.
EEE/WEE/VEE vaccination is very effective at preventing clinical disease. Vaccination needs to be repeated at minimum on an annual basis. Consult with your veterinarian regarding boosters if you live or are travelling to an area with year round mosquito populations or an area that has previously reported cases of EEE/WEE. As VEE has not occurred in the U.S. for over 40 years vaccination is not recommended unless the animal will be travelling to endemic areas.Reduce your horse’s exposure to mosquitoes: eliminate any standing water (mosquito habitat) and place fans inside where the horses rest, limit outdoor activities at dawn and dusk, and use equine approved mosquito repellent.