AJTMH Transactions of the Royal Society of Tropical Medicine and Hygiene
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Am. J. Trop. Med. Hyg., 78(1), 2008, pp. 53-62
Copyright © 2008 by The American Society of Tropical Medicine and Hygiene

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Risk Factors Associated with Human Infection during the 2006 West Nile Virus Outbreak in Davis, a Residential Community in Northern California

Carrie F. Nielsen*, M. Veronica Armijos, Sarah Wheeler, Tim E. Carpenter, Walter M. Boyce, Kara Kelley, David Brown, Thomas W. Scott, AND William K. Reisen
Graduate Group in Epidemiology, Center for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, Wildlife Health Center, School of Veterinary Medicine, Department of Entomology, University of California, Davis, California; Sacramento Yolo Mosquito and Vector Control District, Elk Grove, California

We collected a total of 15,329 mosquitoes during weekly sampling in Davis, CA, from April through mid-October 2006 at 21 trap sites uniformly spaced 1.5 km apart over an area of ~26 km2. Of these mosquitoes, 1,355 pools of Culex spp. were tested by multiplex reverse transcriptase-polymerase chain reaction, of which 16 pools (1.2%) were positive for West Nile virus (WNV). A degree-day model with a developmental threshold of 14.3°C accurately predicted episodic WNV transmission after three extrinsic incubation periods after initial detection. Kriging interpolation delineated that Culex tarsalis were most abundant at traps near surrounding agriculture, whereas Cx. pipiens clustered within residential areas and greenbelt systems in the old portion of Davis. Spatial-temporal analyses were performed to test for clustering of locations of WNV-infected dead birds and traps with WNV-positive Cx. tarsalis and Cx. pipiens; human case incidence was mapped by census blocks. Significant multivariate spatial-temporal clustering was detected among WNV-infected dead birds and WNV-positive Cx. tarsalis, and a WNV-positive Cx. pipiens cluster overlapped areas with high incidences of confirmed human cases. Spatial analyses of WNV surveillance data may be an effective method to identify areas with an increased risk for human infection and to target control efforts to reduce the incidence of human disease.

Received July 14, 2007. Accepted for publication October 16, 2007.

Acknowledgments: The authors thank Roy Jones, City of Davis Parks supervisor, and the residents of Davis for allowing us to trap mosquitoes on their property and reporting dead birds, Ryan Carney and the dead bird hotline staff at the California Department of Health Services, the California Animal Health, and Food Safety and the Center for Vectorborne Diseases laboratory staff for necropsying and testing the dead birds, Chris Barker and Bborie Park for providing technical assistance, and Tim Wilson of the Yolo County Health Department for providing information on the human cases.

Financial support: This study was supported by funds from the Sacramento-Yolo MVCD and grants from the University of California Mosquito Research Program, the University of California Wildlife Health Center, and the National Institutes of Allergy and Infectious Diseases, NIH (5R01AI55607).

* Address correspondence to Carrie Nielsen, Department of Medicine and Epidemiology, Center for Vectorborne Diseases, School of Veterinary Medicine, University of California, Davis, CA 95616. E-mail: cfnielsen{at}ucdavis.edu

Authors’ addresses: Carrie Nielsen, M. Veronica Armijos, Sarah Wheeler, and William K. Reisen, Center for Vectorborne Diseases, University of California-Davis, Old Davis Road, Davis, CA 95616. E-mail: cfnielsen{at}ucdavis.edu. Walter M. Boyce, Wildlife Health Center, University of California-Davis, Old Davis Road, Davis, CA 95616. Thomas W. Scott, Department of Entomology, University of California-Davis, One Shields Road, Davis, CA 95616. Tim E. Carpenter, Center for Animal Disease Modeling ad Surveillance, 279 Cousteu Place, Suite 500, Davis, CA 95618. David Brown and Kara Kelley, Sacramento-Yolo Mosquito and Vector Control District, 8631 Bond Road, Elk Grove, CA 95624.






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