The San Mateo County Mosquito and Vector Control District conducts annual tick collection and molecular testing via real-time PCR analysis for the pathogens that cause Lyme disease, hard-tick relapsing fever, and human granulocytic anaplasmosis (HGA). All three diseases may be vectored by both nymphal and adult black-legged ticks (Ixodes pacificus).  In the 2020-2021 water year, a total of 4,728 adult and 220 nymphal Ixodes pacificus ticks were collected from nineteen parks and open space areas.

Minimum Infection Prevalence of Pathogens in Ixodes pacificus ticks

County wide, adult ticks had a minimum infection prevalence (MIP) of 1.1% of Borrelia burgdorferi sensu lato, 0.70% for B. miyamotoi, and 0.2% for Anaplasma phagocytophilum. Nymphs had an MIP of 2.7% of B. burgdorferi s. l., 1.8% for B. miyamotoi, and 0.9% for A. phagocytophilum. These findings do not differ significantly from prior years.

The MIPs of the three tick-borne disease pathogens in ticks from each site are presented in the next chart.  Sites with fewer than 150 ticks collected are omitted because their MIPs are unlikely to reflect the true prevalence of these diseases. Complete pathogen testing results for adult and nymphal ticks, including Dermacentor tick species, will be available on the District website when all testing for the season is complete.