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SPATIAL PATTERNS OF LYME DISEASE RISK IN CALIFORNIA BASED ON DISEASE INCIDENCE DATA AND MODELING OF VECTOR-TICK EXPOSURE

Ixodes pacificus, particularly the nymphal life stage, is the primary vector to humans of the Lyme disease agent Borrelia burgdorferi in California. During 2004, we collected I. pacificus nymphs from 78 woodland sites in ecologically diverse Mendocino County, which has a moderately high incidence of Lyme disease. Within this county, nymphal density was elevated in forested areas with a growing degree day range of 2,600–3,000 (10°C base). Using a geographic information systems approach, we identified all areas in California sharing these environmental characteristics and thus projected to pose high acarologic risk of exposure to host-seeking nymphal ticks. Such areas were most commonly detected in the northwestern part of the state and along the Sierra Nevada foothills in the northeast, but the analysis also identified isolated areas with high acarologic risk in southern California. This mirrors the spatial distribution of endemic Lyme disease during 1993–2005; most cases occurred in counties to the northwest (58%) or northeast (26%), whereas fewer cases were reported from southern California (16%). Southern zip-codes from which Lyme disease cases had been reported were commonly located in close proximity to areas with high projected acarologic risk. Overall, Lyme disease incidence in zip code areas containing habitat with high projected acarologic risk was 10-fold higher than in zip code areas lacking such habitat and 27 times higher than for zip code areas without this habitat type within 50 km. A comparison of spatial Lyme disease incidence patterns based on county versus zip code units showed that calculating and displaying disease incidence at the zip code scale is a useful method to detect small, isolated areas with elevated disease risk that otherwise may go undetected.

Received March 24, 2006. Accepted for publication May 20, 2006.

Acknowledgments: We thank the involved private landowners, Brooktrails Township Community Services District, Mendocino County General Services, the University of California Angelo Coast Range Reserve and Hopland Research and Extension Center, the California Department of Parks and Recreation, the California Department of Forestry and Fire Protection, the U.S. Department of the Interior Bureau of Land Management, the U.S. Department of Agriculture Forest Service, and the U.S. Army Corps of Engineers for permission to collect ticks. The University of California Hopland Research and Extension Center and A. M. Merenlender kindly provided logistical support. C. N. Brooks and E. Heaton assisted in the development of remote sensing/GIS models. Field and laboratory assistance was provided by J. E. Kleinjan and P. Rogers. J. F. Piesman provided valuable comments on an early draft.

Financial support: This research was supported in part by a grant from the National Institutes of Health (AI22501) to Robert S. Lane.

^* Address correspondence to Lars Eisen, Department of Microbiology, Immunology, and Pathology, 1690 Campus Delivery, Colorado State University, Fort Collins, CO 80523. E-mail: lars.eisen{at}colostate.edu

Authors’ addresses: Rebecca J. Eisen, Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720 (Current address: Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Fort Collins, CO 80522), Robert S. Lane, Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720. Curtis L. Fritz, Division of Communicable Diseases, California Department of Health Services, Sacramento, CA 95814. Lars Eisen, Department of Microbiology, Immunology, and Pathology, Colorado State University, Fort Collins, CO 80523.

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