Resting on a log or leaning against a tree in California's tick-infested hardwood forests can increase the risk of acquiring ticks harboring the Lyme disease bacterium, UC Berkeley researchers found.

“We sat on logs for only five minutes at a time, and in 30% of the cases, it resulted in exposure to ticks,” said Insect Biology professor Robert Lane. “The next riskiest behavior was gathering wood, followed by sitting against trees, which resulted in tick exposure 23% and 17% of the time.”

The study, published in the March Journal of Medical Entomology, is the first quantitative analysis of human behaviors that may increase the risk of tick exposure in California's hardwood forests. Lane and study co-author Denise Steinlein, a UC Berkeley graduate student in insect biology, conducted the research at the UC Hopland Research and Extension Center in southeastern Mendocino County. UC Berkeley research specialist Jeomhee Mun is also a co-author.

The western black-legged tick, found primarily in the far western United States as well as British Columbia, is the primary carrier of the corkscrew-shaped spirochete Borrelia burgdorferi, which is responsible for Lyme disease ( see California Agriculture 55[6]:13-8 ). The young nymphal ticks that generally spread Lyme disease are notoriously difficult to detect. California's nymphal tick season begins in early spring and continues into summer.

DNA tests revealed that 3% to 4% of the ticks the researchers found on their bodies, as well as by sampling leaf litter with a white flannel cloth, tested positive for B. burgdorferi and another, less prevalent human disease-causing bacterium, Anaplasma phagocytophilum.

The researchers wore white clothing from head to toe, and engaged in a series of typical outdoor activities to attract ticks, such as sitting on logs and leaning against trees. Top, Denise Steinlein carries wood at the tick-infested UC Hopland Research and Extension Center. Bottom, the nymphal stage of the western black-legged tick, which can carry the spirochete that causes Lyme disease, is the size of a poppy seed.