Surveillance for California encephalitis group virus illness in Wisconsin and Minnesota, 1978

During the summer and fall of 1978, active surveillance for encephalitis in southwestern Wisconsin and southeastern Minnesota resulted in the detection of the largest number of cases of California encephalitis group (CEG) virus infections ever documented in that region, where CEG virus is known to be endemic. A total of 75 laboratory-confirmed and eight presumptive cases of CEG virus infections were identified as a result of serologic testing completed in the Wisconsin and Minnesota State Laboratories. Cases included 46 residents of Wisconsin, 25 of Minnesota, 10 residents of Iowa and two from Illinois. The outbreak peaked during the last half of August and the first half of September. Eighty percent of patients were less than 10 years of age; only one case was documented in an adult (greater than or equal to 18 years). The clinical picture in patients ranged from mild aseptic meningitis to encephalitis with coma; convulsions occurred in 31% (22/72) of patients. One patient, a 3-year-old girl, died. The high incidence of infection detected in this investigation may be representative of the endemic incidence of CEG illness in the region and suggests that CEG infections in children residing in endemic areas in Wisconsin and Minnesota may be a greater public health problem than previously recognized.