Disease-carrying ticks are much more widespread in Marin than scientists once thought. In hotspots like the Bolinas Lagoon, roughly one third of ticks may carry human pathogens.
Ticks are commonly associated with woodlands, and scientists have mainly studied Lyme disease rather than other tick-borne illnesses. But a recent study took a more open-ended approach and found that the arachnids were plentiful in open coastal areas of Marin, and that many carried bacterial diseases other than Lyme.
Dan Salkeld, the study’s lead author, suspected that Marin hikers probably understood the risk before scientists caught up. “I think locals have known this for a while, and I just wasn’t aware of it,” he said.
Mr. Salkeld, a research scientist at Colorado State University, said ticks are still not nearly as abundant, and infection rates are not nearly as high, as on the East Coast. But he said the study opened his eyes to the risk of tick-borne illness out west. If someone had told him they had contracted Lyme disease from a tick bite in California, he said, “Logically, six months ago, I would have been able to say, ‘That’s extremely unlikely.’” Now, he’d quickly accept it.
Some ticks do carry Lyme disease in California, but this study looked for a range of other bacterial infections that can be transmitted to humans from tick bites. “If you look at more than one pathogen, then suddenly this realization of the potential for tick-borne diseases goes up,” Mr. Salkeld said.
His team dragged white flannel blankets along the sides of hiking trails in a variety of habitats, picking off any ticks that attached themselves and storing them in an ethanol solution before lab testing them for bacteria. Besides Borrelia burgdorferi, the bacteria that causes Lyme disease, the study team found Borellia miyamotoi and Anaplasma phagocytophilum. Both of these bacteria cause fevers and chills in humans and, like Lyme, they can have long-term effects if not diagnosed and treated with antibiotics.
Besides the alarming rate of tick-borne infection at the Bolinas Lagoon, Mr. Salkeld’s study found relatively high rates at Olompali State Park, the Marin Headlands, and the Tennessee Valley Trail. But infections were less common off Lucas Valley Road, in Samuel P. Taylor State Park and at McClure’s Beach, Tomales Point and the Five Brooks Trail in the Point Reyes National Seashore.
“Infection rates can really vary mile to mile,” Lia Gaertner, a scientist with the Bay Area Lyme Foundation, said at a webinar on ticks hosted by the Point Reyes National Seashore Association last month. Ms. Gaertner said she knows several people who have been infected from bites at the seashore.
She said Lyme disease is frequently misdiagnosed in California because many doctors still believe it’s an exclusively East Coast problem. Ms. Gaertner herself was misdiagnosed for 10 years.
The seashore’s website warns visitors about the risks of tick-borne illness, and the park co-hosted the webinar partly in response to Mr. Salkeld’s study. Chris Chamberlain, assistant director of Marin County Parks, which administers the Bolinas Lagoon Preserve, said his department just learned of the tick study and hasn’t responded yet. He pointed out that public access to the lagoon is limited, particularly in areas with tall grasses where Mr. Salkeld’s team found the most disease-carrying ticks.
Mr. Salkeld observed that many trails in California do have warnings about ticks but, he suspected, “it doesn’t infiltrate your brain as you’re going for a walk.”
“There’s rattlesnakes, mountain lions and ticks,” he said, “and I think people just take it for granted.”
The Bay Area Lyme Foundation recommends doing tick checks after any outdoor activity. If you have been bitten, remove the tick as quickly as possible by gently pulling it up and away from your skin, preferably with tweezers. Many public health departments test ticks for disease, as do some private companies.