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The California Mosquito-Borne Virus Surveillance and Response Plan recently was developed to provide a semi-quantitative means for assessing risk for western equine encephalomyelitis (WEE) or St. Louis encephalitis (SLE) viruses and to provide intervention guidelines for mosquito control and public health agencies during periods of heightened risk for human infection. West Nile virus recently has arrived in California, and the response plan also will provide a baseline for assessing the risk for human and equine infection with this virus. In the response plan, overall risk is calculated by averaging risk due to 1) environmental conditions, 2) adult mosquito vector abundance, 3) vector infection rates, 4) sentinel chicken seroconversion rates, 5) equine cases (for WEE), 6) human cases, and 7) the proximity of virus activity to populated areas. Overall risk is categorized into three levels: normal season, emergency planning, or epidemic conditions. We evaluated this response plan using historical data from years with no, enzootic, and epidemic activity of WEE and SLE in several areas of California to determine whether calculated risk levels approximated actual conditions. Multiple methods of risk calculation were considered for both viruses. Assessed risk based on cumulative temperature, rainfall, and runoff levels over the entire season provided more or equally accurate assessments than biweekly assessments based solely on the previous half-month. For WEE, during years with enzootic activity or early-season periods of years with WEE epidemic activity, combining horse and human cases as a single risk factor improved the model’s ability to forecast pending WEE activity, but separating the two factors allowed a better indication of WEE activity during epidemics and periods with no activity. For SLE, assignment of higher risk to drier conditions as measured by rainfall and runoff yielded the most accurate representation of actual virus activity during all recent study periods.
Received September 17, 2002. Accepted for publication February 13, 2003.
Acknowledgments: We thank the management and staff of the Kern MVCD, the Sutter-Yuba MVCD, the Sacramento-Yolo MVCD, the Greater Los Angeles County VCD, and the Coachella Valley MVCD for their invaluable assistance in providing historical surveillance records. In particular, we are grateful for the help of Richard Takahashi, Debbie Lemenager, Kenneth Boyce, Rhonda Laffey, Deborah Dritz, Minoo Madon, Jacqui Spoehel, Branka Lothrop, and Arturo Gutierrez.
Financial support: This study was supported by grant NAO6GP0665 from the Office of Global Programs, National Oceanic and Atmospheric Administration.
Authors’ addresses: Christopher M. Barker, Arbovirus Field Station, 4705 Allen Road, Bakersfield, CA 93312, Telephone: 661-588-6957, E-mail: cmbarker{at}ucdavis.edu. William K. Reisen, Arbovirus Field Station, 4705 Allen Road, Bakersfield, CA 93312, Telephone/Fax: 661-589-0891, E-mail: arbo123{at}pacbell.net. Vicki L. Kramer, California Department of Health Services, Vector-Borne Disease Section, 601 North 7th Street, MS 486, PO Box 942732, Sacramento, CA 94234, Telephone: 916-324-3738, E-mail: vkramer{at}dhs.ca.gov
Reprint requests: William K. Reisen, Arbovirus Field Station, 4705 Allen Road, Bakersfield, CA 93312.
* An abbreviated description of this research was published previously in the Proceedings and Papers of the 70th Annual Conference of the Mosquito and Vector Control Association of California.1
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