St Louis encephalitis. The 1975 epidemic in Mississippi.

In 1975, 229 laboratory-documented cases of St Louis encephalitis, 36 of which resulted in death, occurred in Mississippi. A total of 58% of these persons had encephalitis, 15% had aseptic meningitis, 21% had a febrile illness with no known CNS involvement, and 6% had other syndromes. Attack rates and case-fatality ratios increased with age. Although 60% of the cases occurred in a four-county area of northwest Mississippi, cases occurred in 42 of the 82 counties throughout the state. The epidemic erupted almost simultaneously, with variable intensity, in widely separated areas. Mosquito-control, measures were applied, but the results were not quantified.