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Lyme Disease in California: Ecology and Epidemiology

Abstract

Lyme disease (LD), caused by the spirochete Borrelia burgdorferi (Bb), is the most commonly reported vector-borne disease in the United States. We have been studying the ecology, epidemiology, and prevention of LD since 1982. The primary objectives of this research are intended to clarify the transmission cycles of Bb and closely related spirochetes in diverse habitats; to determine what behaviors and environmental factors elevate the risk of human exposure to Bb-infected ticks; and to evaluate preventive methods. Several genospecies of LD spirochetes are maintained in transmission cycles involving western gray squirrels, dusky-footed wood rats, California kangaroo rats or deer mice, and three species of Ixodes ticks. The western gray squirrel has been implicated as a primary reservoir host of Bb in dense woodlands. The western black-legged tick, Ixodes pacificus, especially the nymphal stage, is the primary bridging vector from wildlife reservoirs to people. In Mendocino County, the prevalence of spirochete-infection in the nymphs typically averages between 5 and 10% (range, <1% to 41% among individual sites) in various subtypes of dense woodlands. Some activities that were found to contribute significantly to risk of exposure to LD or host-seeking I. pacificus ticks included cutting wood, and contact with either low vegetation bordering the uphill margins of hillside-hiking trails or leaf-litter areas and wood in dense woodlands. To determine how people might become infested with I. pacificus nymphs, six behaviors were evaluated as potential risk factors for encountering the nymphs in a hardwood forest. Activities entailing considerable contact with wood (e.g., sitting on logs or against tree trunks) resulted in greater infestation by nymphs than behaviors involving exposure solely to leaf litter. The ultimate goal of this long-term research is to use the basic knowledge gleaned to develop and implement strategies for reducing human exposure to LD and other tick-borne agents.

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