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Exposure to Lyme Disease-Transmitting Ticks Depends on Activity

By Judy McBride
September 5, 2001

Outdoor enthusiasts would likely pick up fewer ticks that transmit Lyme disease from a brisk walk in the woods than from sitting on fallen logs or kneeling in or handling leaf litter. Blacklegged deer tick nymphs lurk in these venues, waiting for animals or people to latch onto.

The nymphs--tick youths--are the main vectors of Lyme disease. But at less than one-sixteenth of an inch long, their tiny, black bodies are barely visible and seldom felt. So Agricultural Research Service entomologist John F. Carroll wanted to assess the relative risk of picking up blacklegged deer tick nymphs from different activities. He chose moderately infested woods a few miles from his laboratory at ARS’ Henry A. Wallace Beltsville (Md.) Agricultural Research Center.

Carroll usually wears boots, tucks his pantlegs into his socks and covers the exposed sock with tape because ticks can get a better hold on material. Although the nymphs got on his footwear, many were brushed off as he walked along. About the same number of ticks were on his boots after a five-minute walk as after a 30-second one. Wearing sneakers with the sock tops exposed afforded somewhat less protection than the boots and tape, he and ARS statistician Matt Kramer recently reported in the Journal of Medical Entomology.

But the type of footwear appears to be less important than the type of activity. The researchers found tick nymphs on 87 percent of fallen logs they sampled. They specifically tested large logs that would make good seats for tired hikers or climbing kids. Fallen logs make a good hiding place for mice and other rodents that serve as a reservoir for the Lyme disease bacterium, according to Carroll. Stone walls might pose a similar risk.

Kneeling or placing hands in leaf litter--activities that campers, kids at play and homeowners clearing underbrush and leaves are likely to do--also increases risk. The researchers picked up many more nymphs from slowly crawling on their hands and knees for 30 seconds than from walking half a minute. The ticks clung to their pant legs and hands, with the majority on their pants.

ARS is the chief agency for scientific research in the U.S. Department of Agriculture. Carroll works in the Beltsville center's Parasite Biology, Epidemiology, and Systematics Laboratory.