Encephalitis, St. Louis: A disease caused by a virus (first observed in Illinois in 1932) transmitted from birds to the common Culex mosquito to people. St. Louis encephalitis occurs typically in the summer and early fall. In the U.S., the virus is rarely found outside the Southeast.

Signs and symptoms include the sudden onset of drowsiness, headache and nausea (due to the encephalitis), pain in the abdomen, a rash, and swollen glands (lymphadenopathy). These features are usually but not always mild. The fatal cases tend to involve infants and small children (under age 5), the aged, and people with an impaired immune system.

The virus is closely related to other flaviviruses including those responsible for West Nile encephalitis, Japanese encephalitis and Murray Valley encephalitis.

See also: Flavivirus encephalitis.