Cause of Encephalitis
Four common forms of mosquito-transmitted viral encephalitis are seen in the United States:
Equine encephalitis affects horses and humans. Eastern equine encephalitis also infects birds that live in freshwater swamps of the eastern U.S. seaboard and along the Gulf Coast. In humans, symptoms are seen 4 to 10 days following transmission and include sudden fever, general flu-like muscle pains, and headache of increasing severity, followed by coma and death in severe cases. About half of infected patients die from the disorder. Fewer than 10 human cases are seen annually in the United States.
Western equine encephalitis is seen in farming areas in the western and central plains states. Symptoms begin 5 to 10 days following infection. Children, particularly those under 12 months of age, are affected more severely than adults and may have permanent neurologic damage. Death occurs in about three percent of cases.
Venezuelan equine encephalitis is very rare in this country. With this disorder, children are at greatest risk of developing severe complications, while adults generally develop flu-like symptoms. Epidemics in South and Central America have killed thousands of people and left others with permanent, severe neurologic damage.
LaCrosse encephalitis occurs most often in the upper midwestern states (Illinois, Wisconsin, Indiana, Ohio, Minnesota, and Iowa) but also has been reported in the southeastern and mid-Atlantic regions of the country. Most cases are seen in children under age 16.