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.
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.
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.
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.
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.
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.
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.
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.
