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 disease agents from ticks collected from November 2018 to June 2019. 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 pathogens are carried and transmitted by Ixodes pacificus, the Western black-legged tick. 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 2,353 adult Ixodes pacificus ticks were collected from November 2018 to March 2019. County-wide, the MIP for adult ticks of B. burgdorferi s.l. ranged from 0.00% to 1.26% and the MIP of B. miyamotoi ranged from 0.00% to 2.55%. These values are similar to previous years.

From March 2019 to June 2019 District staff collected 546 Ix. pacificus nymphs. On the West Coast, Ixodes pacificus ticks are more likely to have a higher MIP for vector-borne diseases at a nymphal stage than at an adult or larval stage. The county-wide MIP for nymphal ticks of B. burgdorferi ranged from 0.00% to 1.72% and the MIP of B. miyamotoi ranged from 0.00% to 2.82%. These values are also similar to previous years.

The risk of acquiring tick-borne disease in this county remains relatively low, however, 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.

2018-2019 Testing Results for Ixodes pacificus Adult Ticks

Adult Ixodes pacificus ticks were also collected at Frontierland Park (6), Half Moon Bay Coastal Trail (1), Memorial Park (44), Rancho Corral de Tierra (43), Sweeney Ridge (15) and Wavecrest Open Space (9), but sample sizes were too low to calculate Minimum Infection Prevalence.

2018-2019 Testing Results for Ixodes pacificus Nymphal Ticks

Nymphal Ixodes pacificus ticks were also collected at Año Nuevo State Park (3), Big Canyon Park (7), Costanoa Recreational Area (8), and Water Dog Lake Park (36), but sample sizes were too low to calculate Minimum Infection Prevalence.

Additional updates from throughout the 2018-2019 Water Year:

July 2019 update

Laboratory staff continued collecting Ixodes pacificus ticks in the nymph life stage and adult Dermacentor ticks during the month of June. Collections in June occurred at Año Nuevo and the nearby Costanoa campground. A very high number of Dermacentor ticks have been collected from this location because District staff have made multiple collecting trips as part of a study on Tularemia. 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. Surveillance for Ixodes pacificus nymphs will be concluded at the end of June. 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. Ticks of the genus Dermacentor will be tested for tularemia.

June 2019 update

Laboratory staff continued collecting Ixodes pacificus ticks in the nymph life stage and adult Dermacentor ticks during the month of May. Collections in May occurred at Thornewood Open Space Preserve near Woodside, Wunderlich County Park in Woodside, Water Dog Lake Park in Belmont, Big Canyon Park and Eaton Park in San Carlos, Edgewood Park in Redwood City, Costanoa campground near Año Nuevo, and Laurelwood Park in San Mateo. 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. Ticks of the genus Dermacentor will be tested for tularemia.

 May 2019 Update

During April, laboratory staff shifted focus from collecting adult stage to nymphal stage Ixodes pacificus (western black-legged) ticks. Tick nymphs are more difficult to collect than adults because they don’t quest on vegetation along trails. Instead, they are found on tree stumps, downed longs, rocks, and in leaf litter. Nymphal ticks are considered more dangerous than adult ticks because they are harder to see and remove, and in some regions of California they have been found to be more likely than adults to carry Borrelia burgdorferi bacteria, which causes Lyme disease. Areas that were surveyed in April were: Rancho Corral de Tierra National Recreation Area near Half Moon Bay, Edgewood Park in Redwood City and Costanoa, south of Pescadero.  The ticks will be tested for Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma pgagocytophilum. Results are not yet available.

April 2019 Update

During March, laboratory staff continued surveillance for adult Ixodes pacificus (Western black-legged ticks). The continuous on-and-off rainfall continued into March and greatly limited the number of tick-flagging opportunities this winter. The weather reduces the questing activity of the ticks and moisture saturates the tick flag, making it less effective as a collecting tool. Areas that were surveyed in March were: Año Nuevo State Park, south of Pescadero; a neighborhood in Hillsborough; Sweeney Ridge near San Bruno; Costanoa campground south of Pescadero and Rancho Corral de Tierra National Recreation Area near Half Moon Bay. Adult Ixodes pacificus tick surveillance will end in April, and collecting efforts will shift to focus on ticks in the nymphal stage. Ixodes pacificus ticks will be tested for the presence of bacteria Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma phagocytophilum.

March 2019 Update

During February, laboratory staff continued surveillance for adult Ixodes pacificus (Western black-legged ticks). The continuous on-and-off rainfall greatly limited the number of tick flagging opportunities in February, which is typically a peak month for collecting adult Ixodes pacificus. The weather reduces the questing activity of the ticks and moisture saturates the tick flag, making it less effective as a collecting tool. Parks that were surveyed in February were Ano Nuevo State Park, south of Pescadero and Sweeney Ridge in San Bruno. Adult tick surveillance will continue through March, and nymphal tick surveillance is planned for spring. Ixodes pacificus ticks will be tested for the presence of bacteria Borrelia