The arboviruses reside within the alphavirus family. Their are three major alphaviruses which are arthropod borne. The virus was first isolated in the U.S. in 1933 and is maintained in an enzootic cycle involving wild birds and mosquitoes in fresh water swamps. The vector involved is Culiseta melanura which rarely bites mammals, explaining the rare transmission to humans and equines. The infection can be systemic or encephalitic, with most systemic cases being inapparent.
Children seem to show a higher susceptibility and a more abrupt onset of symptoms than adults.
A vaccine is available for horses and also for laboratory workers who culture the virus. In MS the first confirmed case of EEE occurred in 1989, their were no cases in 1990 and 1 case in 1991. Was first isolated in the U.S. in 1930. In 1941 their was a U.S. epidemic involving 300,000 horses and 3,336 humans. It is most prevalent in the western plain states where human cases may follow outbreaks in horses.