Severe La Crosse encephalitis with significant neurologic sequelae.

La Crosse encephalitis, a member of the California arbovirus group, is the most common cause of reported mosquito-borne illness in the United States. Approximately 70 cases of La Crosse encephalitis are reported each year. The principal vector is the mosquito Aedes triseriatus. During the summer the virus is amplified horizontally in a cycle among small mammals such as chipmunks and squirrels. Infected female A. triseriatus deposit eggs in the basal holes of hardwood trees, although man-made containers and old tires containing water also supply a suitable breeding site. Some of these eggs infected with La Crosse virus hatch the next spring and give rise to infected adult A. triseriatus, and the host-vector cycle is renewed. Only a minority of children infected with the virus become ill. Clinical disease caused by La Crosse is usually mild, and neurologic sequelae are relatively uncommon. In this report we describe six patients with severe La Crosse meningoencephalitis diagnosed within a 4-week period. All patients required intensive care management, and there was a high rate of neurologic sequelae, suggesting that La Crosse is not necessarily a benign meningoencephalitis.