Occurrence of La Crosse (California serogroup) encephalitis viral infections in Illinois.

La Crosse (LAC) virus (California encephalitis serogroup) was identified as the etiologic agent of disease in 61 cases of California encephalitis (CE) that occurred in Illinois between 1976 and 1980. This group represented 54% of all 114 laboratory-diagnosed cases of CE between 1966 and 1980. Key epidemiologic observations included: 88% of the 61 cases had onset in July, August, or September; the 5- to 9-year-old age group represented 48% of the cases; the male:female sex ratio was 2:1; 97% of the cases occurred in the northern half of the state; and 61% of the cases occurred in Peoria County. Four instances of probable common source of exposure were reported. Average length of hospitalization was 8.3 days (range 3-31 days). Hospitalization costs were estimated at $3,967-$5,750 per case. An "average" year in Illinois yields 12 cases of LAC encephalitis with direct hospitalization costs of $47,604-$69,000.