Division of Epidemiology and Public Health Biology, School of Public Health, University of California, Arbovirus Research Unit, Center for Vector-Borne Disease Research, School of Veterinary Medicine, University of California, Berkeley, California
Sera from outpatients attending county health department clinics in areas of California with consistent (Imperial Valley) and sporadic (Sacramento Valley) enzootic transmission of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses exhibited neutralizing antibody prevalence rates of 1.3% (n = 690) and 0.5% (n = 1,066) for 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 years of residence, indicating that this virus was endemic with a low rate of annual infection. Of 26 sera that tested positive for SLE virus antibody by an enzyme immunoassay, but were negative by plaque reduction neutralization test, 14 (53%) had neutralizing antibody that reacted with ≥ one type of dengue (DEN) virus. The DEN virus infections presumably were acquired elsewhere because neither the vectors nor DEN virus transmission occurs in California. The low prevalence of neutralizing antibody for WEE and SLE in the California human population indicated that despite recent increases in enzootic transmission, contact between humans and infectious mosquitoes have remained low.