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Where Ticks Come From

Medical Authors: Barbara K. Hecht, PhD, Frederick Hecht, MD, FAAP, FACMG
Medical Editor: William C. Shiel Jr., MD, FACP, FACR

Ticks carry the bacteria that cause Lyme disease. People contract Lyme disease by being bitten by these ticks.

In order to investigate whether specific human behavior increases the risk of tick exposure, researchers from the University of California at Berkeley took to the woods. They found that sitting on a log carried with it the greatest risk of picking up a tick. If you sit on a log (at least in Northern California) for only five minutes, you have a 30% chance of getting a tick on you! Gathering wood was also cited as a risky activity as well as leaning up against a tree.

Comment: Aside from issuing a press release (below) about this study, it was also published in the current issue of the Journal of Medical Entomology. In case you aren't familiar with this journal, it is chock-full of everything you might want to know (or not know) about the dangers that mosquitoes, cat fleas, ticks, flies, scorpions, mange mites and other insects pose to you. We don't recommend it for bedtime reading.

Researchers find no safe place to sit in California tick-infested forest

BERKELEY - After a long hike through some of California's forests, it may be tempting to rest on a log or lean against a tree. Wrong move, say researchers at the University of California, Berkeley, who found that such activities may increase the risk of acquiring ticks harboring the Lyme disease bacterium.

UC Berkeley researcher Denise Steinlein demonstrates the three actions found to be riskiest for acquiring the western black-legged tick: leaning against a tree, carrying wood and sitting on a log. (Photos by Robert Lane)

"We sat on logs for only five minutes at a time, and in 30 percent of the cases, it resulted in exposure to ticks," said Robert Lane, professor in the Division of Insect Biology at UC Berkeley's College of Natural Resources and lead investigator of the study. "It didn't matter if we sat on moss or the bare surface; the ticks were all over the log surface. The next riskiest behavior was gathering wood, followed by sitting against trees, which resulted in tick exposure 23 and 17% of the time, respectively."

The study, published in the current issue of the 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. The paper has come just weeks before the start of northwestern California's nymphal tick season, which begins in early spring and continues into summer.

Once on a person, the nymph of the western black-legged tick (shown enlarged in inset) is readily overlooked because of its minute size.

Print-quality image available for download

The western black-legged tick, found primarily in the far western United States as well as in British Columbia, is the primary carrier of the corkscrew-shaped spirochete Borrelia burgdorferi, a bacterium named after its discoverer, Dr. Willy Burgdorfer. B. burgdorferi is responsible for Lyme disease, which can lead to debilitating symptoms in humans. Most human cases of Lyme disease in northwestern California appears to be transmitted by young nymphal ticks, which are notoriously difficult to detect because they are as small as poppy seeds.

Lane and study co-author Denise Steinlein, a UC Berkeley graduate student in insect biology, trekked through a hardwood forest at the UC Hopland Research and Extension Center in southeastern Mendocino County to conduct the field trials. The area, dominated by California black oak, is endemic for Lyme disease.

Jeomhee Mun, a UC Berkeley research specialist in insect biology, is another co-author of the paper.

Lane and Steinlein conducted the experiments on two back-to-back days in three consecutive weeks in 2002 between late May and mid-June. Decked out in white clothing from top to bottom, with pant legs tucked into white socks and seams sealed with duct tape, the researchers set out to learn how people might acquire nymphal ticks.

"If we're going to develop effective strategies and educational programs for the prevention of Lyme disease, it is critical that we understand how people are exposed to the ticks that transmit the bacteria in the first place," said Lane. "We intentionally looked at behaviors that people would typically engage in while spending time in the woods."

The researchers sat on logs, sat against trees, gathered wood, walked through leaves, sat still on leaf litter and sat and stirred up leaf litter for set amounts of time. Lane noted that turkey hunters can easily spend up to an hour or longer sitting with their backs against trees while trying to call in toms during the spring hunting season.


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