ARTICLE Patterns of Avian Seroprevalence to Western Equine Encephalomyelitis and Saint Louis Encephalitis Viruses in California, USA WILLIAM K. REISEN, 1 JAN O. LUNDSTROM, 2 THOMAS W. SCOTT, 3 BRUCE F. ELDRIDGE, 3 ROBERT E. CHILES, ROBERT CUSACK, VINCENT M. MARTINEZ, HUGH D. LOTHROP, DAVID GUTIERREZ, STAN E. WRIGHT, 4 KEN BOYCE, 4 AND BOYD R. HILL 5 Arbovirus Research Unit, Center for Vector-borne Disease Research, School of Veterinary Medicine, University of California, Davis, CA 95616 J. Med. Entomol. 37(4): 507Ð527 (2000) ABSTRACT Temporal and spatial changes in the enzootic activity of western equine encepha- lomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were monitored 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 females, (2) seroconversions in ßocks of 10 sentinel chickens, and (3) seroprevalence in wild birds collected by mist nets and grain baited traps. Overall, 74 WEE and one SLE isolates were obtained from 222,455 Cx. tarsalis females tested in 4,988 pools. In addition, 133 and 40 seroconversions were detected in 28 chicken ßocks, and 143 and 27 of 20,192 sera tested from 149 species of wild birds were positive for antibodies to WEE and SLE, respectively. 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 seroprevalence rates in wild birds. Avian seroprevalence rates did not provide an early warning of pending enzootic activity in chickens, because positive sera from after hatching year birds collected during spring most probably were the result of infections acquired during the previous season. Few seroconversions were detected among banded recaptured birds collected during spring and early summer. Age and resident status, but not sex, were signiÞcant risk factors for wild bird infection, with the highest seroprevalence rates among after hatching year individuals of permanent resident species. Migrants (with the exception of mourning doves) and winter resident species rarely were positive. House Þnches, house sparrows, GambelÕs quail, California quail, common ground doves, and mourning doves were most frequently positive for antibodies. The initial detection of enzootic activity each summer coincided closely with the appearance of hatching year birds of these species in our study areas, perhaps indicating their role in virus ampliÞcation. Bird species most frequently positive roosted or nested in elevated upland vegetation, sites where Cx. tarsalis host-seeking females hunt most frequently. These serosurveys provided important background information for planned host competence and chronic infection studies. KEY WORDS western equine encephalomyelitis virus, St. Louis encephalitis virus, avian serology, surveillance, wetlands, California IN CALIFORNIA, WESTERN equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses are maintained and ampliÞed during summer in a trans- mission cycle involving wild birds and the primary vector, Culex tarsalis Coquillett (Reeves and Hammon 1962, Reeves 1990a). Humans and equines are infected tangentially to the primary cycle and are dead-end hosts for these viruses. Domestic equines currently are protected from WEE by annual vaccination. A state- wide Encephalitis Virus Surveillance program was de- veloped in the 1960s (Longshore 1960, Reeves et al. 1990) and monitors mosquito abundance, mosquito Procedures for the housing and sampling of sentinel chickens were approved in Protocol 8155 approved by the Animal Use and Care Ad- ministrative Advisory Committee of the University of California, Davis. The collection, banding and bleeding of wild birds were conducted under Protocol 8141 approved by the Animal Use and Care Admin- istrative Advisory Committee of the University of California, Davis, California Resident ScientiÞc Collection Permit 801049-02 by the State of California Department of Fish and Game, and Master Station Federal Bird Marking and Salvage Permits Nos. 21615 and 22763 from the U. S. Geological Survey Bird Banding Laboratory. Special exper- imental use permits were obtained for bird collection on the Salton Sea State Park, Kern National Wildlife Refuge and Stone Lakes Na- tional Wildlife Refuge. 1 Arbovirus Field Station, 4705 Allen Road, BakersÞeld, CA 93312. 2 Department of Population Biology, Evolutionary Biology Center, Uppsala University, Norbyvagen 180D, SE-75236 Uppsala, Sweden. 3 Department of Entomology, University of California, Davis, CA 95616. 4 Sacramento and Yolo County Mosquito and Vector Control Dis- trict, Elk Grove, CA 95624. 5 Kern Mosquito and Vector Control District, 4705 Allen Road, BakersÞeld, CA 93312. 0022-2585/00/0507Ð0527$02.00/0  2000 Entomological Society of America Downloaded from https://academic.oup.com/jme/article/37/4/507/839426 by guest on 05 April 2022