PREVALENCE OF ANTIBODIES TO WESTERN EQUINE ENCEPHALOMYELITIS AND ST.-LOUIS ENCEPHALITIS VIRUSES IN RESIDENTS OF CALIFORNIA EXPOSED TO SPORADIC AND CONSISTENT ENZOOTIC TRANSMISSION

Sera from outpatients attending county health department clinics in ar eas of California with consistent (Imperial Valley) and sporadic (Sacr amento Valley) enzootic transmission of western equine encephalomyelit is (WEE) and St. Louis encephalitis (SLE) viruses exhibited neutralizi ng antibody prevalence rates of 1.3% (n = 690) and 0.5% (n = 1,066) fo r WEE and 11.0% and 0.8% for SLE, respectively. Seroprevalence for SLE virus in Imperial County increased as a function of both age and year s of residence, indicating that this virus was endemic with a low rate of annual infection. Of 26 sera that tested positive for SLE virus an tibody by an enzyme immunoassay, but were negative by plaque reduction neutralization test, 14 (53%) had neutralizing antibody that reacted with greater than or equal to one type of dengue (DEN) virus. The DEN virus infections presumably were acquired elsewhere because neither th e vectors nor DEN virus transmission occurs in California. The low pre valence of neutralizing antibody for WEE and SLE in the California hum an population indicated that despite recent increases in enzootic tran smission, contact between humans and infectious mosquitoes have remain ed low.