Role of peridomestic birds in the transmission of St. Louis encephalitis virus in southern California

In response to the 1984 St. Louis encephalitis (SLE) epidemic in the Los An geles Basin of southern California (USA), an investigative program was init iated to evaluate the interactive components of the SLE virus transmission cycle. From 1987 through 1996 (10 yr), 52,589 birds were bled and their ser a tested for SLE and western equine encephalomyelitis (WEE) virus antibodie s by the hemagglutination inhibition (HAI) test. Eighty-three percent of th e birds tested were house finches (Carpodacus mexicanus) (48.7%) and house sparrows (Passer domesticus) (34.6%); 1.1% of these birds were positive for SLE antibodies. Prevalence of WEE antibodies was negligible. The analysis of 5,481 sera from rock doves (Columbia livia) yielded 3.6% SLE positives a nd 0.4% WEE positives. Collection sites were maintained as study sites when identified as positive bird, mosquito, and SLE virus activity localities; others were abandoned. Serial serum samples from 7,749 banded house sparrow s and 9,428 banded house finches from these selected sites demonstrated yea r-round SLE virus transmission. One location exhibited significant numbers of house finches undergoing annual SLE seroconversion and a number of seroc onversion-reversion-reconversion sequences suggesting either viral reinfect ion from mosquitoes or recrudescence by latent virus. A proportion of both bird species also lived for longer than 1 yr, thus, increasing the possibil ity of virus carry-over from autumn to spring. Assessment of concurrently c ollected mosquitoes indicated no correlative association between mosquito p opulations and SLE seroconversion and reconversion. European house sparrows introduced in the 1800's may have provided a supplemental link to the exis ting SLE virus enzootic cycle involving endemic house finches. Meteorologic al factors are reviewed as possible important correlates of SLE epidemics. The house finch/house sparrow serosurveillance system is also evaluated for use as an "Early Warning" indicator of SLE virus activity.