Equine Diseases: EEE, WEE, WNV, and VEE

Eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), West Nile virus (WNV), and Venezuelan equine encephalomyelitis (VEE) have some things in common. All can infect horses; all are spread by mosquitoes, and all can generally be prevented through vaccination.

EEE is of particular concern because it has a fatality rate of about 40% in humans and 90% in horses. Outbreaks most often occur in horses that have never been vaccinated or that have skipped annual revaccination. EEE is on the AAEP’s list of core vaccines recommended for all horses.

No WEE cases have been reported in the United States in almost 20 years, but horses should continue to be vaccinated because the virus is still present in some states. Outbreaks could easily occur if owners allow vaccinations to lapse. There is also a chance that a viral mutation could introduce a strain that is able to produce infection even in horses that receive the standard vaccination.

WNV was first identified in New York in 1999, and a vaccine was licensed in 2001. Its appearance was unexpected so there was no vaccine to protect against it. The virus spread rapidly across the country, infecting birds, horses, humans, and other species. Several species of mosquitoes became vectors. Vaccines are available, and the incidence of cases in equines seems to have declined as horse owners responded to the AAEP’s listing of WNV as another core vaccine that should be given to all horses.

VEE, a threat to humans as well as horses, was identified in the United States in 1971. Unlike WNV, VEE had been expected to increase its range, and steps were taken immediately to curb its spread in this country. Aerial spraying to kill mosquitoes, mandatory immunization of horses, and local quarantines were put in place, and the disease was eradicated in the U.S.

Each of these diseases continues to be a potential threat because the viruses that cause them are still present in the environment, even though cases of infection are being prevented by widespread immunization of horses. Owners need to be aware that they should continue to vaccinate, watch for disease signs, and report illnesses to veterinarians.