When a Bear sits in the woods . . .
. . . a tick may hitch a ride, Berkeley research team cautions
| 15 April 2004
After a long hike through the forest, it may be tempting to rest on a log or lean against a tree. Wrong move, say Berkeley researchers who found that such activities may increase the risk of acquiring ticks harboring the Lyme disease bacterium.
“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 the College of Natural Resources and lead investigator of a new study analyzing human behaviors that may increase the risk of tick exposure in California’s hardwood forests. “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 percent of the time, respectively.”
The study, published in the current issue of the Journal of Medical Entomology, comes just weeks before the start of northwestern California’s nymphal-tick season, which begins in early spring and continues into summer.
The western black-legged tick, found primarily in the far western United States as well as in British Columbia, is the primary carrier of Borrelia burgdorferi, the corkscrew-shaped spirochete responsible for Lyme disease, which can lead to debilitating symptoms in humans. Most human cases of Lyme disease in northwestern California appear to be transmitted by young nymphal ticks, which are notoriously difficult to detect because they are as small as poppy seeds.
To conduct the field trials, Lane and study co-author Denise Steinlein, a graduate student in insect biology, trekked through a hardwood forest at the UC Hopland Research and Extension Center in southeastern Mendocino County. The area, dominated by California black oak, is endemic for Lyme disease. The researchers dressed in white clothing from top to bottom, with pant legs tucked into white socks and seams sealed with duct tape, then set out on two back-to-back days in three consecutive weeks between late May and mid-June 2002.
“If we’re going to develop effective strategies and educational programs for the prevention of Lyme disease,” said Lane, “it is critical that we understand how people are exposed to the ticks that transmit the bacteria in the first place. We intentionally looked at behaviors that people would typically engage in while spending time in the woods.”
Accordingly, 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. After each activity, they meticulously picked off and counted the ticks on their clothing and bodies. They also used an adhesive lint roller to pick up ticks that might otherwise have escaped their attention. All told, they found a total of 86 nymphal ticks on their bodies during the field trials.
“Activities that were riskiest involved considerable contact with wood,” said Steinlein. “Of the six behaviors we analyzed, sitting still on leaf litter was the least-risky, resulting in tick exposure only eight percent of the time.”
Why the difference between wood and leaf litter? The clue may be an important animal host for the larvae and nymphs of the western black-legged tick.
“The western fence lizard is an important host for the ticks, and the lizards often use logs in sunlit areas as basking sites,” said Lane. “Nymphal ticks that are seeking hosts to feed upon may be going to the place where they’ll have the greatest chance of finding a lizard. Humans or pets that happen to come along for a picnic lunch or a short rest on a log may [thus] be putting themselves in harm’s way.”
DNA tests revealed that 3 to 4 percent of the ticks the researchers found tested positive for B. burgdorferi and another, less prevalent human-disease-causing bacterium, Anaplasma phagocytophilum. Estimates from prior studies of ticks infected with B. burgdorferi in Mendocino County are higher, ranging from 5 to 10 percent on average, with some spots occasionally yielding rates of 15 percent or higher.
Lane, while cautioning that the findings in this study are not intended to be applicable to forested areas in other regions of the country, recommended precautions against tick exposure for people frequenting areas of California where Lyme disease is endemic.
“I would avoid prolonged contact with wood as well as with leaf-litter areas, and I would strongly suggest that people inspect themselves carefully after spending time in tick-infested areas,” he said. “Moreover, I would advise people to continue checking their skin for two to three days after potential exposure. Nymphal ticks are so hard to see in the beginning — probably less than one in three people bitten by nymphs ever discovers the tick that bit them. But they become easier to detect once they start swelling up a bit after they’ve had a blood meal.
“It usually takes longer than one day after the tick becomes attached for the bacteria to be transmitted to the host, so the sooner the tick is found and removed, the better.”