West Nile Virus Emergence and Persistence in Los Angeles, California, 2003–2008

Jennifer L. Kwan Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Davis, California; Greater Los Angeles County Vector Control District, Santa Fe Springs, California

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Susanne Kluh Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Davis, California; Greater Los Angeles County Vector Control District, Santa Fe Springs, California

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Minoo B. Madon Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Davis, California; Greater Los Angeles County Vector Control District, Santa Fe Springs, California

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William K. Reisen Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Davis, California; Greater Los Angeles County Vector Control District, Santa Fe Springs, California

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West Nile virus (WNV) invaded Los Angeles in September 2003, and during the subsequent five-year period followed a pattern of amplification, subsidence, and resurgence. Enzootic transmission was tracked by abundance and infection incidence in Culex pipiens quinquefasciatus and Cx. tarsalis and by seroprevalence in peridomestic passerine birds, infection in dead birds, and seroconversions in sentinel chickens. Culex p. quinquefasciatus served as the primary vector of WNV, with gravid traps serving as the best sampling method and the most consistent indicator of viral activity. Spatial scan statistics applied to mosquito infection and positive dead bird data delimited three major clusters of WNV transmission, with introduction occurring in the Los Angeles Basin, and amplification and dispersal events carrying transmission to the San Fernando and Santa Clarita valleys. Los Angeles experienced major epidemics in 2004 and 2008, providing a unique opportunity to investigate specific patterns of enzootic amplification preceding epidemics.

Author Notes

*Address correspondence to William K. Reisen, Center for Vectorborne Diseases, University of California, Old Davis Road, Davis, CA 95616. E-mail: wkreisen@ucdavis.edu

Financial support: This study supported, in part, by grant R01-AI055607 from the National Institutes of Allergy and Infectious Diseases, National Institutes of Health, and grant RM08-6044 from the National Aeronautics and Space Administration Applied Sciences Program Decision Support through Earth Science Research Results. Additional funding and resources were provided by the Greater Los Angeles County Vector Control District and by the Research and Policy for Infectious Disease Dynamics Program of the Science and Technology Directorate, the U.S. Department of Homeland Security, and the Fogarty International Center, National Institutes of Health.

Authors' addresses: Jennifer L. Kwan and William K. Reisen, Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Davis, CA, E-mails: jnwilson@ucdavis.edu and wkreisen@ucdavis.edu. Susanne Kluh and Minoo B. Madon, Greater Los Angeles County Vector Control District, Santa Fe Springs, CA, E-mails: skluh@glacvcd.org and minoovecterminator@yahoo.com.

Reprint requests: William K. Reisen, Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Old Davis Road, Davis, CA 95616, E-mail: wkreisen@ucdavis.edu.

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