Post

Tick Testing Results for 2017-2018
Water Year 2017-2018

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.

Collection and testing results for Ixodes pacificus adults collected from parks in San Mateo County 2017-2018

Park

Nearest City

# Collected

# Positive Pools Borrelia burgdorferi s.l.

MIP

# Positive Pools Borrelia miyamotoi

MIP

Ano Nuevo State Park

Pescadero

211

6

2.84%

0

0.00%

Big Canyon Park

San Carlos

200

0

0.00%

0

0.00%

Coal Creek OSP

Redwood City

229

0

0.00%

0

0.00%

Crystal Springs Regional Trail

Millbrae

245

1

0.41%

2

0.82%

Eaton Park

San Carlos

393

0

0.00%

2

0.51%

Laurelwood Park

San Mateo

304

2

0.66%

1

0.33%

Los Trancos OSP

Portola Valley

225

2

0.89%

2

0.89%

Memorial Park

Pescadero

145

2

1.38%

0

0.00%

Montara Mountain

Montara

148

0

0.00%

1

0.68%

Purissima Creek Redwoods OSP

Half Moon Bay

234

3

1.28%

2

0.85%

Skyline Ridge OSP

Portola Valley

476

4

0.84%

3

0.63%

Thornewood OSP

Woodside

211

4

1.90%

3

1.42%

Water Dog Lake Park

Belmont

293

1

0.34%

1

0.34%

Wunderlich County Park

Woodside

241

0

0.00%

2

0.83%

County-wide Totals

 

3555

25

0.70%

19

0.53%

Pool size = 5

           

Collection and testing results for Ixodes pacificus nymphs collected from parks in San Mateo County 2017-2018

 

nearest City

# Collected

# Positive Pools Borrelia burgdorferi s.l.

MIP

# Positive Pools Borrelia miyamotoi

MIP

Wunderlich County Park

Woodside

69

0

0.00%

0

0.00%

Crystal Springs Regional Trail

Millbrae

72

1

1.39%

0

0.00%

Skyline Ridge OSP

Portola Valley

74

0

0.00%

2

2.70%

Coal Creek OSP

Redwood City

81

1

1.23%

1

1.23%

Thornewood OSP

Woodside

83

0

0.00%

1

1.20%

Los Trancos OSP

Portola Valley

66

3

4.55%

0

0.00%

Water Dog Lake Park

Belmont

27

0

0.00%

0

0.00%

Laurelwood Park

San Mateo

62

2

3.23%

1

1.61%

County-wide Totals

 

534

7

1.31%

5

0.94%

Pool size = 2

           

 

 

 

 

 

Collection and testing results for Ixodes pacificus larvae collected from parks in San Mateo County 2017-2018

Park

nearest City

# Collected

# Tested

# Positive Pools Borrelia burgdorferi s.l.

MIP

# Positive Pools Borrelia miyamotoi

MIP

Wunderlich County Park

Woodside

63

63

0

0.00%

0

0.00%

Crystal Springs Regional Trail

Millbrae

23

23

0

0.00%

0

0.00%

Skyline Ridge OSP

Portola Valley

23

23

0

0.00%

0

0.00%

Coal Creek OSP

Redwood City

200

200

0

0.00%

1

0.50%

Los Trancos OSP

Portola Valley

17

17

0

0.00%

0

0.00%

Laurelwood Park

San Mateo

116

116

0

0.00%

0

0.00%

County-wide Totals

 

442

442

0

0.00%

1

0.23%

Pool size = 5

             

 

Commands