St. Louis Encephalitis: The Chicago Experience

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  • Enteric and Neurotropic Pathogens Branch, Viral Diseases Division, Center for Disease Control, Cook County Department of Public Health, Chicago City Board of Health, Illinois Department of Public Health, Atlanta, Georgia 30333

The largest laboratory-documented outbreak of St. Louis encephalitis (SLE) in the United States occurred in the Chicago metropolitan area in the summer and early fall of 1975. Of 1,456 illnesses investigated, 326 cases of confirmed or probable SLE and 420 cases of suspected SLE were found in the six-county area. The outbreak peaked in early September, but cases continued to occur through October. Cases clustered geographically in the south-western suburbs. There was a direct correlation between age and attack rate, severity of illness, and mortality rate—all of which increased with successive age groups. Thirty-six persons died. Males and females were equally affected. This epidemic marked the first time that St. Louis encephalitis has been documented in the Chicago area.

Author Notes

Present address: Blue Cross/Blue Shield, Chicago, Illinois 60601.

Present address: Deputy Assistant Secretary for Health Programs, Department of Health, Education, and Welfare, Washington, D.C. 20201.

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