Stanford researchers find surprising level of tick-borne disease risk on local trails

The risk of acquiring Lyme disease from ticks, such as this western black-legged tick, is higher than had been assumed, according to a new Stanford study.
The San Francisco Bay Area’s broad swaths of trail-lined open space hold higher risks of tick-borne disease than previously thought, according to a new study by Stanford researchers.

western black-legged tick
Western black-legged tick. (Kaldari/Creative Commons)

The scientists collected 622 ticks (typically western black-legged ticks) from 20 sites in recreational areas from Sonoma County in the north to Santa Cruz County in the south by dragging white flannel blankets along the vegetation and leaf clutter. After bringing the ticks to a lab, the researchers extracted and analyzed their DNA.
Among other surprising discoveries, they found that a higher percentage of nymphal (young) ticks were infected with the bacteria Borrelia miyamotoi, a recently discovered human pathogen, than on the East Coast. The corkscrew-shaped bacteria produce Lyme disease-like symptoms.
Nymphal ticks are much smaller than adult ticks and thus are less likely to be discovered when they hitch themselves onto humans walking outdoors.
“Users of recreation areas in the Bay Area need to know the risk of tick-borne diseases such as Lyme is real, and not limited to other parts of the country,” said co-author Eric Lambin, the George and Setsuko Ishiyama Provostial Professor in the School of Earth, Energy & Environmental Sciences and a senior fellow at the Stanford Woods Institute for the Environment.
The research, published in PLOS ONE, found the density of non-infected ticks, infected ticks and disease risk varies widely and unpredictably among different habitats (e.g., coast live oak, redwood, grassland) and geographic areas. However, tick-borne disease risk appears to be higher in redwood forests than previously believed.
Although ticks are found in lower densities among redwoods than some other habitats, they are consistently present and harbor B. miyamotoi and Borrelia burgdorferi, which causes Lyme disease. Also, tick-borne disease exposure appears highest in coast live oak-dominated woodlands. The authors caution, however, that the statistical association is too weak to serve as a basis for targeted preventive public health policies and awareness campaigns.
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