Patterns of avian seroprevalence to western equine encephalomyelitis and Saint Louis encephalitis viruses in California, USA

Temporal and spatial changes in the enzootic activity of western equine enc ephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were monitore d at representative wetland study sites in the Coachella, San Joaquin, and Sacramento valleys of California from 1996 to 1998 using three methods: (1) virus isolation from pools of 50 host-seeking Culex tarsalis Coquillett fe males, (2) seroconversions in flocks of 10 sentinel chickens, and (3) serop revalence in wild birds collected by mist nets and, grain baited traps. Ove rall, 74 WEE and one SLE isolates were obtained from 222,455 Cx. tarsalis f emales tested in 4,988 pools. In addition, 133 and 40 seroconversions were detected in 28 chicken flocks, and 143 and 27 of 20,192 sera tested from 14 9 species of wild birds were positive for antibodies to WEE and SLE, respec tively. WEE was active in all three valleys, whereas SLE only was detected in Coachella Valley. Seroconversions in sentinel chickens provided the most sensitive indication of enzootic activity and were correlated with seropre valence rates in wild birds. Avian seroprevalence rates did not provide an early warning of pending enzootic activity. in chickens, because positive s era from after hatching year birds collected during spring most probably we re the result of infections acquired during the previous season. Few seroco nversions were detected among banded recaptured birds collected during spri ng and early summer. Age and resident status, but not sex, were significant risk factors for wild bird infection, with the highest seroprevalence rate s among after hatching year individuals of permanent resident species. Migr ants (with the exception of mourning doves) and winter resident species rar ely were positive. House finches, house sparrows, Gambel's quail, Californi a quail, common ground doves, and mourning doves were most frequently posit ive for antibodies. The initial detection of enzootic activity each summer coincided closely with the appearance of hatching year birds of these speci es in our study areas, perhaps indicating their role in virus amplification . Bird species most frequently positive roosted or nested in elevated uplan d vegetation, sites where Cx. tarsalis host-seeking females hunt most frequ ently. These serosurveys provided important background information for plan ned host competence and chronic infection studies.