Tick Testing Results for August 2019

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): . You may also purchase and apply a spray containing Permethrin to your clothing or purchase clothing already infused with permethrin.