St. Louis encephalomyelitis in Hermosillo, Sonora, Mexico

This report, which describes an epidemic of St. Louis encephalitis in northwestern Mexico, was prompted by the apparent infrequency of previous Mexican outbreaks of this disease. The site of the epidemic was the Municipality of Hermosillo in the State of Sonora. This municipality, with a population of about 273,000, is situated in a hot flat area which is naturally dry but which is also well-irrigated. Here, over an eithy-week period, 51 persons became ill and were hospitalized with symptoms including fever, vomiting, stiff neck, lethargy, and increased cell/protein levels in their cerebrospinal fluid. Sera from 12 of these patients were tested for SLE antibodies by hemagglutination inhibition, complement fixation, and neutralization. The results of these tests were positive and showed a fairly high degree of correlation. In addition, paired serum samples showed a significant rise in SLE antibody titers. Over half of the cases occurred in the City of Hermosillo, the municipal seat; the remainder occurred in 13 outlying areas, most of them quite nearby. Virtually all of the city dwellers affected were residents of low-income districts. The rate of hospital cases was roughly 19 per 100,000 inhabitants, and the case-fatality rate among these cases was approximately 20 per cent. The ratio of male to female patients was 1.3 to 1; sixty-nine per cent of the patients were under 16 years of age.