The District currently tests for several tick-borne diseases., including Borrelia burgdorferi, which causes Lyme disease, and Borrelia miyamotoi, which causes a similar, less-common tick-borne disease.
The results of these tests are reported as the minimum infection rate, or MIR. This is a way of describing the proportion of ticks tested that are infected with a particular pathogen. The table shows the MIR for Borrelia burgdorferi, the MIR for Borrelia miyamotoi, and the total MIR for Borrelia bacteria. These are reported for each park surveyed (with the exception of parks where there were not enough ticks collected to complete testing), and for the parks as a whole.
A MIR of 2-3% is considered normal for our county, and does not indicate an elevated level of risk.
The District laboratory is taking advantage of breaks in rainy weather to collect ticks from parks and open space areas in San Mateo County. Ticks are collected by dragging a tick flag – a large white piece of flannel attached to a wooden rod – over the vegetation alongside trails. The main target species of tick is Ixodes pacificus, the western black-legged tick, which vectors Lyme disease, Borrelia miyamotoi infection, and anaplasmosis. The ticks collected will be tested for the presence of bacteria that cause these diseases. The Ixodes pacificus ticks are in
Tick testing is completed for the 2015-2016 season. Staff flagged for ticks at fifteen parks this season, although only 10 parks had sufficient numbers for estimating infection prevalence. Ticks were tested for the presence of two disease-causing bacteria, Borrelia burgdorferi which causes Lyme Disease and Borrelia miyamotoi which causes a relapsing fever type illness. As seen in the table below, infection prevalence of either of the two Borrelia ranged from 2.6% to 0%, and was highest at Thornewood Open Space Preserve near Woodside.