An epidemic of St. Louis encephalitis occurred during July and August of 1957 in Cameron County, Texas. A total of 114 clinical cases was reported, of which laboratory evidence of SLE infection was available in 35 instances.
Ecologic conditions were conducive for SLE since this area has a very large wild bird population, and after heavy rains followed by drought Culex quinquefasciatus mosquitoes were extraordinarily abundant. SLE virus was isolated on 8 occasions from these mosquitoes.
The attack rate for the entire community was 155 per 100,000. The urban and rural rates were similar and no spread from foci was noted.
The age distribution of cases was similar to that in previous SLE epidemics east of the Rockies, with greatest numbers occurring in young adults and highest attack rates in older age groups. Cases among males were somewhat more prevalent than those among females. Three deaths occurred, all in people over 60.
The clinical attack rates were higher in a military population within the epidemic area than among the resident population. Inapparent attack rates, however, were higher among civilians (30% to 18%). The ratio of inapparent to apparent infections in civilians was 209:1, while in the military group it was 64:1.
The inapparent infection rate was the same in all age groups, while clinical attack rates were higher in the older age groups, suggesting a greater susceptibility to symptomatic illness among older people.
Present address: Middle America Research Unit, National Institutes of Health, Balboa Heights, Canal Zone.
Present address: Greeley Field Station, Communicable Disease Center, Greeley, Colorado.