Risk of human exposure to vector ticks (Acari: Ixodidae) in a heavily used recreational area in northern California

The risk of humans encountering vector ticks along hiking trails or in picnic grounds in a Lyme disease-endemic area was evaluated in a multipurpose recreational area (Tilden Regional Park) in the populous San Francisco Bay region of California. Four hillside hiking trails (two high-use, two low-use) were sampled by dragging and walking through low vegetation biweekly for one year; four heavily used picnic areas were sampled concurrently by dragging. Adults of three human-biting ticks were enumerated (n = 1,911) along all trails: Dermacentor occidentalis (63.6% of total), Ixodes pacificus (26.2%), and D. variabilis (0.2%). Subadults (n = 1,669) of D. occidentalis (0.06% of total) and two nonhuman-biters, D. albipictus (70.3%) and Haemaphysalis leporispalustris (29.7%), also were collected. Dragging yielded many more adult ticks than walking year-round for all trails. These methods were significantly correlated during periods of peak tick abundance, but the associations were not sufficiently strong or consistent to allow prediction of captures for either method based on the other. Adult ticks were distributed largely in clusters along the uphill sides of trails. Several adult ticks collected adjacent to trails were found to contain spirochetes identified with polyclonal antibodies as Borrelia burgdorferi (D. occidentalis, 0 of 861; D. variabilis, 2 of 126 [1.6%]; 1. pactficus, 1 of 609 [0.2%]). Picnic areas produced low numbers of adult D. occidentalis and I. pacificus, which prohibited testing them for spirochetes. Two measures for evaluating risk were calculated, the encounter distance (= mean number of meters traveled before encountering a vector tick by either dragging or walking) and the mean number of spirochete-infected ticks encountered by these methods per kilometer of trail. These measures revealed that the risk of exposure to spirochete-infected adult ticks along trails was low year-round irrespective of usage, and risk was even lower in picnic areas. Future studies evaluating human exposure to vector ticks in recreational areas should incorporate, whenever possible, testing for multiple pathogens because most ixodid ticks that commonly bite people in the United States are capable of transmitting two or more microbial disease agents.