Ticks, those disease-spreading little bloodsuckers, are more prevalent in Bay Area open space and redwood forests than previously thought.

A new Stanford study shows the tiny arachnids are found in more diverse habitats than suspected, and they appear to carry more strains of Lyme and Lyme-like diseases than earlier suspected. Not only that, but a single tick can be infected with more than one strain of these potentially harmful bacteria, researchers found.

Health advocates say the research helps scientists better understand why Lyme disease and other mysterious tick-borne infections are so difficult to diagnose and treat, particularly in the Bay Area.

“It’s a tricky, tricky bacteria that hides in your system. But because there are so many different strains it makes it difficult to diagnose, and each strain has different symptoms,” said Linda Giampa, executive director of the Bay Area Lyme Foundation. “If you can’t diagnose it, it really makes it difficult to treat and to cure it.”

The Bay Area Lyme Foundation funded the $150,000 study, which appeared in the online scientific journal Plos One.

Lyme is the most common tick-borne illness, and the oldest known and most frequently diagnosed strain of the disease is Borrelia burgdorferi. But scientists have recently discovered additional strains that may be causing human illnesses with similar or different symptoms.

That could complicate things for doctors.

The disease has already been difficult to detect in patients because the definitive symbol of the traditional strain — the bull’s-eye-shaped target around the bite — may or may not be present, and early symptoms, including fever, headache and fatigue, are often mistaken for the flu. Lyme can be treated with antibiotics, but if left untreated, the disease can progress to joint swelling, numbness, muscle weakness, cognitive difficulties and heart problems. Some patients end up suffering from chronic symptoms even with treatment.

More common in the East

Lyme is found in greater numbers in the Eastern part of the country due to the moister climates preferred by ticks. The U.S. Centers for Disease Control and Prevention estimate that at least 300,000 cases of Lyme are diagnosed each year, about two-thirds of them in residents east of the Mississippi.

But the new research, which involves scientists from Stanford and Northern Arizona universities, shows that strains associated with the East Coast appear to be moving west and that the disease is probably more common here than assumed.

One of the newer strains of Lyme, called Borrelia miyamotoi, may be more common on the West Coast than the East Coast. Less is known about the symptoms associated with it. Human infections have rarely been associated with the bulls-eye rash and include fever, chills, headache, fatigue, and joint and body pain.

In another surprising finding, the researchers found a higher-than-expected percentage of young ticks, called nymphs, infected with pathogens, particularly this newer strain. The Western blacklegged tick, the West Coast culprit most likely to carry disease, is small, but the nymphs can be barely the size of a poppy seed.

“You’re less likely to discover them, so they’re more likely to spread disease because they can feed on you for three or four days,” said the study’s lead author, Dan Salkeld, a disease ecologist at Colorado State University who started the work research while at the Stanford Woods Research Institute for the Environment.

Lyme-like diseases

To add to the confusing nature of tick-borne diseases, Salkeld said his research has also found Bay Area ticks infected with bacterial strains that test negative for Lyme, yet still cause some Lyme-like symptoms. He said more research is needed on these diseases, which are lumped into a catchall category known as “tick-borne relapsing fever.”

To conduct the study, scientists dragged flannel blankets in about 20 sites in recreational areas from Sonoma County in the north to Santa Cruz County in the south to gather ticks.

Ticks are most commonly associated with grasslands and oak-dominated woodlands, but the researchers also conducted the tests in redwood areas, more as an attempt to rule those regions out. They were surprised by what they found.

“Ticks are pretty much everywhere,” Salkeld said, adding that they still tend to be found more in wetter parts of the Bay Area. “Everyone just assumed there weren’t many ticks in the redwoods. There still aren’t many, but they’re there.”

The Bay Area Lyme Foundation’s Giampa said the research is not meant to frighten Bay Area residents away from the region’s trails, redwoods and open spaces, but to make them more conscious about checking for ticks and symptoms. She suggested showering after hikes to remove the nymphs, which may be hard to spot with the naked eye.

Showering (briefly, of course) brings to mind the one bright spot of the tick study: California’s historic drought.

“Because of the drought, when we swept for ticks, we found fewer ticks,” Giampa said. “Ticks don’t like dry areas.”

Victoria Colliver is a San Francisco Chronicle staff writer. E-mail: vcolliver@sfchronicle.com Twitter: @vcolliver