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.
This year, in addition to testing for Lyme disease and hard-tick relapsing fever, the District conducted testing for Anaplasma phagocytophilum which causes the disease human granulocytic anaplasmosis (HGA). HGA is a tick-borne disease that causes reoccurring bouts of mild to moderate fever, aches, nausea and vomiting. It is carried by adults and nymphs of the western black-legged tick (Ixodes pacificus). The District conducted real-time PCR testing of adult ticks collected from parks in San Mateo County for the 2017-2018 water year (October through September).
Every year the District collects ticks from recreational areas in the county to assess the risk of tick-borne disease. One way to measure this is by determining the minimum infection prevalence (MIP), which estimates what percentage of these ticks are expected to be carrying a given disease agent in a population. The laboratory has completed testing for two vector-borne diseases from ticks collected from November 2017 to May 2018.
Laboratory staff continued collecting ticks in the nymph life stage during the month of May. Collections in May occurred at Laurelwood Park in San Mateo, Wunderlich County Park in Woodside, Thornewood Open Space Preserve near Woodside, Coal Creek Open Space Preserve near Portola Valley and Water Dog Lake Park in Belmont. Ixodes pacificus nymphs are active in the spring, and more difficult to collect in abundant numbers than adult ticks because they are typically on fallen logs or in leaf litter, whereas adult ticks are easily found on the edges of vegetation along trails.
The nymph stage is the second of three stages in a tick’s life cycle. It is thought that nymphs are the most dangerous stage of ticks because they are more likely to be carrying bacteria that causes a tick-borne disease such as Lyme, and because they are significantly smaller and harder to see than adult ticks. Tick nymphs are most active in the spring months in California and can be found on tree stumps, downed logs, rocks, and in leaf litter.
During March, laboratory staff continued winter surveillance for adult Ixodes pacificus (Western black-legged ticks). Ticks are collected by dragging a 1 meter square piece of white flannel over the vegetation alongside trails. Laboratory staff have begun testing collected Ixodes pacificus ticks for the presence of bacteria Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma phagocytophilum. Parks that were surveyed in March were Año Nuevo and Memorial Park near Pescadero, and Big Canyon Park and Eaton Park in San Carlos.
During February, laboratory staff continued winter surveillance for adult Ixodes pacificus (Western black-legged ticks). Ticks are collected by dragging a 1 meter square piece of white flannel over the vegetation alongside trails. Ixodes pacificus ticks will be tested for the presence of bacteria Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma phagocytophilum.
During January, laboratory staff continued winter surveillance for adult Ixodes pacificus (Western black-legged ticks). Ticks are collected by dragging a 1 meter square piece of white flannel over the vegetation alongside trails. Ixodes pacificus ticks will be tested for the presence of bacteria Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma phagocytophilum. Parks that were surveyed in January were Thornewood Open Space Preserve near Portola Valley and Memorial County Park near Pescadero.
From January to April of 2017, laboratory staff from the San Mateo County Mosquito and Vector Control District collected ticks from 15 different parks and open spaces around the county. Ticks were tested for three different disease-causing bacteria using the districts quantitative PCR machine. Results displayed on the graph are Minimum Infection Rates (MIR). MIR is an estimated percentage of infected ticks that used when ticks are pooled together before testing to save time and cost. The following pathogens were tested for in the 2017 tick-born disease surveill
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.
Please see the attached spreadsheet for the latest tick-borne disease surveillance data for 2014-2015.
The MIR with Borellia bacteria for Ixodes pacificus (western black legged tick) in parks in San Mateo County was 1.49%. A MIR of 2-3% is considered normal for our county, and does not indicate an elevated level of risk.
