San Mateo County: Tularemia Detected in Coastal Ticks

Entomology Report September 2006, San Mateo County Mosquito Abatement District

Tularemia has been detected in ticks collected south of Pescadero this summer. The ticks were collected in July. Results of testing (conducted at the Centers for Disease Control) were received this month. This survey was conducted along with staff from the California Dept. of Health Services in response to a human case of tularemia. The patient had exposure to ticks in coastal San Mateo County, as well as locations in Marin and Alameda Counties. Tularemia was detected by PCR in 1 of 6 pools of Dermacentor occidentalis (totaling 43 ticks) and 1 of 17 pools of D. variabilis (211 ticks). The minimum infection rate for these ticks was 1 out of 43 or 2.8% for D. occidentalis, 1 out of 211 or 0.5% for D. variabilis and 2 out of 247 or 0.8% overall. This is somewhat lower than the prevalence of Lyme disease spirochetes in western black-legged ticks (1-3% countywide). The MIR calculated for D. occidentalis is probably not an accurate estimate for that species because the number tested was small.

Dermacentor ticks reach their peak numbers in the summer (June-July). They are found in drier areas and later in the year than the western black-legged tick which transmits Lyme disease.