San Mateo County: Ticks and Lyme Disease

Entomology Report November 2008, San Mateo County Mosquito and Vector Control District

In November, district staff began annual surveillance activities for Lyme disease bacteria in ticks. This year, the program will focus on surveying sites of potential risk that have not been thoroughly evaluated in the past. Tick surveys were conducted this month in Portola Valley Ranch and on a previously unsurveyed trail in Laurelwood Park (San Mateo). The western black-legged tick is the primary vector of the Lyme disease spirochete in California. Adults of this tick begin appearing on trails in November, reach peak abundance in January-February and persist into early May. The immature stage (nymph) reaches peak abundance in April-May and is found in dense hardwood forest. Nymphs present a higher risk for transmitting Lyme disease to people because they are harder to see and a high proportion may be infected.

In San Mateo County, Lyme disease spirochetes have been found in 1-3% of both adult and nymphal western black-legged ticks. However, nymphal ticks are more difficult to collect than adult ticks and have only been evaluated from four sites. The lab will be working to expand collection and testing of nymphs this spring in a greater number of locations.