Equine encephalitis—Eastern, Western and Venezuelan—is another mosquito-borne virus that resides in birds as a vector, similar to how West Nile virus (WNV) WNV is passed. A bird harboring virus is bitten by a mosquito, and then virus is passed to a horse when the mosquito feeds.
Eastern equine encephalitis (EEE) is the predominant encephalitic virus circulating in the USA currently. Western equine encephalitis (WEE) has not been reported in horses in the western United States since 2004, although it is still found in birds and mosquitos. Areas potentially at risk for encroachment of Venezuelan equine encephalitis (VEE) into the USA are along the southern borders of New Mexico, Texas, California, Louisiana, Mississippi, Alabama, and along the west coast of Florida.
Equine encephalitis signs include a sleepy appearance, hence the moniker “sleeping sickness.” The horses develop a fever, and an uncoordinated (ataxic) gait coupled with involuntary muscle twitching. Progressive encephalitic signs develop, such as head pressing, aimless wandering, seizures, hyperexcitability, and coma. A horse that goes down is unable to rise. Mortality is high at a rate of 70-90%.
Effective vaccines are available for mosquito-borne viruses of Eastern and Western encephalitis, and West Nile virus. These immunizations should be boosted annually, and more often in climates with year-round mosquito populations.