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. Complete results for Borrelia burgdorferi sensu lato (the agent of Lyme disease) and Borrelia miyamotoi (the agent of hard tick relapsing fever) are shown in the table below. Results for Anaplasma phagocytophilum (the agent of anaplasmosis) are pending. All three of these diseases are carried and transmitted by the Western black-legged tick (Ixodes pacificus). This small dark-colored tick is most commonly found questing on tall grasses by the side of trails during the winter and early spring. A total of 3555 adult black-legged ticks were collected from November 2017 to March 2018. County-wide, the MIP of B. burgdorferi s.l. was 0.7% and the MIP of B. miyamotoi was 0.53%. These values are similar to previous years.
This year, the District also focused on collecting the immature stages of black-legged ticks including the nymphs and larvae. Nymphs and larvae are most commonly found on downed logs or piles of wood and rocks, or in leaf litter throughout the spring from March till May. Both nymphal and larval ticks prefer to feed on birds, small mammals and reptiles but nymphal ticks will willingly attach to a human. It is rare for larvae to attach to humans. On the West Coast, Western black-legged ticks are more likely to have a higher MIP for vector borne diseases at a nymphal stage than at an adult or larval stage stage. This season, the District collected 534 nymphal ticks and 442 larval ticks in April and May of 2018. The county-wide MIP of B. burgdorferi was 1.31% and of B. miyamotoi was 0.94%. For larval ticks, the MIP for B. burgdorferi was 0% and 0.23% for B. miyamotoi. Research has shown that B. miyamotoi can be passed from the adult tick to her offspring (called transovarial transmission) but B. burgdorferi s.l. cannot, which is consistent with our results.
The tick-borne disease risk in this county remains relatively low. Low risk is not the same as zero risk so we caution all residents to be careful when walking on trails or in regions with tall grasses. Check yourself for ticks after hiking or walking through areas with high grass or bushes that may extend into the trail area. Tuck your pants into your socks and your shirt into your pants and apply insect repellents on your skin that are labeled for ticks and registered with the Environmental Protection Agency. This link to the EPA insect repellent web page has a useful search tool to ensure the repellent you use is effective for ticks (and mosquitoes): https://www.epa.gov/insect-repellents/find-repellent-right-you You may also purchase and apply a spray containing Permethrin to your clothing or purchase clothing already infused with Permethrin.
Adult Ixodes pacificus collection and testing results, San Mateo County, 2017-2018 Water Year
|Park||Nearest City||# Collected||# Positive Pools for Bbsl||MIP Bbsl||# Positive Pools for Bm||MIP Bm|
|Año Nuevo State Park||Pescadero||211||6||2.8%||0||0%|
|Big Canyon Park||San Carlos||200||0||0%||0||0%|
|Coal Creek Open Space Preserve||Redwood City||229||0||0%||0||0%|
|Crystal Springs Regional Trail||Millbrae||245||1||0.4%||2||0.8%|
|Eaton Park||San Carlos||393||0||0%||2||0.5%|
|Laurelwood Park||San Mateo||304||2||0.7%||1||0.3%|
|Los Trancos Open Space Preserve||Portola Valley||225||2||0.9%||2||0.9%|
|Purissima Creek Redwoods Open Space Preserve||Half Moon Bay||234||3||1.3%||2||0.9%|
|Skyline Ridge Open Space Preserve||Portola Valley||476||4||0.8%||3||0.6%|
|Thornewood Open Space Preserve||Woodside||211||4||1.9%||3||1.4%|
|Water Dog Lake Park||Belmont||293||1||0.3%||1||0.3%|
|Wunderlich County Park||Woodside||241||0||0%||2||0.8%|
*Pool size = 5 adult ticks
Nymphal Ixodes pacificus collection and testing results, San Mateo County, 2017-2018 Water Year
|Park/Location||Nearest City||# Collected||# Positive Pools for Bbsl||MIP Bbsl||# Positive Pools for Bm||MIP Bm|
|Coal Creek Open Space Preserve||Redwood City||81||1||1.2%||1||1.2%|
|Crystal Springs Regional Trail||Millbrae||72||1||1.4%||0||0%|
|Laurelwood Park||San Mateo||62||2||3.2%||1||1.6%|
|Los Trancos Open Space Preserve||Portola Valley||66||3||4.6%||0||0%|
|Skyline Ridge Open Space Preserve||Portola Valley||74||0||0%||2||2.7%|
|Thornewood Open Space Preserve||Woodside||83||0||0%||1||1.2%|
|Water Dog Lake Park||Belmont||27||0||0%||0||0%|
|Wunderlich County Park||Woodside||69||0||0%||0||0%|
*Pool size = 2 nymphal ticks
Larval Ixodes pacificus collection and testing results, San Mateo County, 2017-2018 Water Year
|Park/Location||Nearest City||# Collected||# Positive Pools for Bbsl||MIP BBsl||# Positive Pools for Bm||MIP for Bm|
|Wunderlich County Park||Woodside||63||0||0%||0||0%|
|Crystal Springs Regional Trail||Millbrae||23||0||0%||0||0%|
|Skyline Ridge Open Space Preserve||Portola Valley||23||0||0%||0||0%|
|Coal Creek Open Space Preserve||Redwood City||200||0||0%||1||0.5%|
|Los Trancos Open Space Preserve||Portola Valley||17||0||0%||0||0%|
|Laurelwood Park||San Mateo||116||0||0%||0||0%|
*Pool size = 5 larval ticks
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). County-wide, the minimum infection prevalence (MIP) for adult ticks was 0.42%. This value is low compared to many areas of the state, but is considered normal for San Mateo County. Los Trancos Open Space Preserve and Coal Creek Open Space Preserve had the highest MIPs for adult ticks at 1.78% and 1.75% respectively.
Adult Ixodes pacificus collection and Anaplasma phagocytophilum testing results, San Mateo County, 2017-2018 Water Year
|Park/Location||Nearest City||# Tested||# Positive Pools Ap||MIP Ap|
|Año Nuevo State Park||Pescadero||211||0||0%|
|Big Canyon Park||San Carlos||200||2||1.0%|
|Coal Creek Open Space Preserve||Portola Valley||229||4||1.8%|
|Crystal Springs Regional Trail||Millbrae||245||0||0%|
|Eaton Park||San Carlos||393||1||0.3%|
|Laurelwood Park||San Mateo||304||1||0.3%|
|Los Trancos Open Space Preserve||Portola Valley||225||4||1.8%|
|Memorial County Park||Loma Mar||145||1||0.7%|
|Purisima Creek Redwoods Open Space Preserve||Half Moon Bay||234||2||0.9%|
|Skyline Ridge Open Space Preserve||Portola Valley||476||0||0%|
|Thornewood Open Space Preserve||Woodside||211||0||0%|
|Water Dog Lake Park||Belmont||293||0||0%|
|Wunderlich County Park||Woodside||241||0||0%|
*Pool size = 5 adult ticks
June 2018 update
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. Ticks of the genus Ixodes will be tested for the Lyme disease causing bacteria Borrelia burgdorferi and other disease causing bacteria Borrelia miyamotoi and Anaplasma phagocytophilum.
May 2018 update
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.
Lab staff are making an extra effort this year to collect nymphs from a select number of parks and open spaces in San Mateo County that have abundant nymphal habitat. So far, nymph collections have been made at Los Trancos Open Space Preserve, Skyline Ridge Open Space Preserve, and Crystal Springs Regional Trail.
Lab staff will be testing all collected nymphs for the presence of disease causing bacteria and comparing results to adult ticks collected earlier in the year. Estimating disease prevalence in nymphal ticks may present a more complete risk assessment because of their role in disease transmission.
April 2018 update
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. Adult tick collections are completed for the 2017-2018 winter season. Nymphal tick surveillance is planned for spring.
March 2018 update
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. Parks that were surveyed in February were Water Dog Lake Park in Belmont, Purisima Creek Redwoods OSP near Half Moon Bay, Coal Creek OSP near Portola Valley, Big Canyon Park in San Carlos, Eaton Park in San Carlos, Montara Mountain in Montara, Ano Nuevo State Park near Pescadero, Laurelwood Park in San Mateo and Skyline Ridge OSP near Portola Valley. Surveillance for adult ticks will continue through March, and surveillance for nymphs is planned for spring.
On February 15, District laboratory staff collected ticks in Laurelwood Park accompanied by three employees of San Mateo County Environmental Health department. The Environmental Health workers participated as part of a program for San Mateo County employees to familiarize them with the public services provided by other County departments and agencies. This cross-training gives them a fuller picture of the role of County and local government, knowledge to make decisions about their future career path and better prepares them to assist in other departments or agencies if needed. District staff showed them techniques for collecting ticks and educated them about ticks, tick-borne disease, and the tick surveillance that the District conducts. Meanwhile, with their assistance flagging along the trail, abundant numbers of ticks were quickly collected at Laurelwood Park.
February 2018 update
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. Adult tick surveillance will continue through February and March, and nymphal tick surveillance is planned for spring.