Annadel State Park, the popular 5,000-acre wildland on Santa Rosa’s east flank, is a hotbed of ticks that can transmit Lyme disease, according to a Stanford University study that sampled 20 recreational areas from Sonoma County to Santa Cruz.
Researchers found six immature blacklegged ticks, also known as deer ticks, per 100 meters on trails in Annadel, the second-highest concentration of the tiny arachnids reported in the study, which was financed by the Bay Area Lyme Foundation and published in the scientific journal PLOS ONE.
The only higher concentration was 10 ticks picked up along a trail in the Windy Hill Open Space Preserve in Portola Valley.
Nearly 10 percent of the ticks from Annadel tested positive for the bacteria that causes Lyme disease, a condition that afflicts about 30,000 people a year nationwide and almost eight per year in Sonoma County.
Aside from some fine points overall, the study said nothing new about the county, which has a 10-year average of 1.41 Lyme disease cases per 100,000 people a year, seven times higher than the statewide rate, according to the Sonoma County Department of Health Services.
“I wouldn’t say it’s a big surprise,” said Dan Salkeld, a disease ecologist who was the study’s lead author, regarding the Annadel tick population.
“Sonoma is a beautiful place to go and look for ticks,” he said. “Other people go there for wine; I go there for ticks.”
Neill Fogarty, the supervising ranger at Annadel, said he’s accustomed to finding the immature ticks, known as nymphs, on his body after he has walked through the park, which attracts about 150,000 hikers, runners, mountain bikers and horseback riders annually.
“I usually pick up a few of them every year,” he said.
Of all the environments sampled, the Stanford study found that deer ticks favor live oak-dominated woodlands most of all, and Annadel has plenty of oaks — including coast live oak and black oak — on the west side of the park, around Lake Ilsanjo, Fogarty said.
Salkeld said he expected Sonoma County to have a higher concentration of ticks than Santa Clara and San Mateo counties, where all but four of the study’s tick collection areas were located. Jack London State Historic Park in Sonoma Valley, the only other site in the county, had 0.7 ticks per 100 meters of trail.
Tick concentrations increase significantly in Mendocino County, owing to a damper climate and greater abundance of black oak woodlands, the deer tick’s most favored environment, Salkeld said.
One of the study’s more surprising findings was that redwood forests also harbor deer ticks infected with the Lyme disease bacteria, he said.
Mendocino, which wasn’t included in the study, averages four Lyme disease cases a year per 100,000 people, one of the highest rates in the state.
Salkeld said the study carried no alarming message for the public.
“You should just be aware that ticks are out there and you should check for them when you get home,” he said.
Ticks were collected for the study in May 2012 and 2013, and Salkeld said he had no evidence of how deer ticks have fared during two additional years of drought. The “conventional wisdom” is that tick populations have declined, he said.
Karen Milman, the county’s public health officer, said the study was “a good reminder” that deer ticks inhabit the county and “there’s certainly a risk of Lyme disease.”