Virus is spread by the mosquito Culex tarsalis which breeds in ditches. Since the virus can multiply at cool temperatures, epidemics normally occur early in summer and may also be seen farther north into Canada. Eastern Equine encephalitis is rare yet deadly. The virus was first isolated in the U.S in 1933 and is transmitted by Culiseta melanura. The mosquito resides in fresh water swamps and rarely bites mammals, hence the rare appearance of this disease in humans.
Children appear to be more susceptible to infection and show more abrupt onset of symptoms.
A vaccine is currently available for lab workers and for horses. Venezuelan Equine encephalitis virus is maintained in the forests and marshes of tropical America. For the most part the virus circulates around rodents via the mosquito Culex melanoconion. VEE does not pose a significant problem in the U.S. as most humans who become infected usually only suffer from fever an myalgia and encephalitis is not normally seen. There is a vaccine available for horses.