Prevalence of antibodies to mosquito-borne encephalitis viruses in residents of the Coachella Valley, California.

Sera from 19 (2.6%) and 118 (16.4%) of 719 outpatients attending clinics in the southeastern Coachella Valley, California during 1993 and 1994 exhibited IgG antibodies to western equine encephalomyelitis and St. Louis encephalitis (SLE) viruses, respectively, using enzyme immunoassays. However, only seven (1.0%) and 36 (5.0%) outpatients were positive by plaque-reduction neutralization tests (PRNTs), and seven (1.0%) and 84 (11.7%) outpatients were positive by sera hemagglutination inhibition assays, respectively. None were positive for IgM antibodies indicative of recent infection or were diagnosed clinically with central nervous system disease. Prevalence of PRNT antibody to SLE increased as a function of patient age, but did not vary significantly in relation to years of residence, sex, race, postal zip code, occupation, or month of collection.