Ticks carrying the bacterium responsible for Lyme disease are infesting Northern California’s birds and may be hitching rides on them into suburban settings, according to a new UC Berkeley study.

The tick-borne spirochete scientists know as Borrelia burgdorferi is found in a wide variety of mammals in California including wood rats, gray squirrels and deer.

Although Lyme disease cases are relatively rare in California, DNA sequencing showed B. burgdorferi’s presence in 23 of 53 species of birds tested in Mendocino County. The disease-causing bacteria were detected in blood samples taken from species including the American robin, lark sparrow, dark-eyed junco, lesser goldfinch and oak titmouse.

The golden-crowned sparrow was infected more frequently than any of the other species, according to the study published Wednesday in the journal PLOS ONE.


“Our findings underscore the importance of bird behavior to explain local tick infestation and Borrelia infection in these animals,” the study says, “and suggest the potential for bird-mediated geographic spread of vector ticks and spirochetes in the far-western United States.”

“This is the most extensive study of the role of birds in Lyme disease ecology in California, and the first to consider the diversity of bird species, their behaviors and their habitats in identifying which birds are truly the most important as carriers,” said ecologist Erica Newman, the lead author of the study.

More research is needed to better understand the ability of birds to infect feeding ticks, and how changes in habitat due to warming temperatures and surbanization may be affecting the spread of Lyme disease.

Lyme disease is contracted by an estimated 30,000 people in the United States each year, usually through the bite of infected ticks. The black-legged deer tick transmits B. burgdorferi in the eastern regions of the United States, while the western black-legged tick spreads the spirochete in the West.


The researchers were surprised to discover another species of spirochete closely related to, but distinct from B. burgdorferi, in birds for the first time anywhere in the world. That spirochete, Borrelia bissettii, has been known to cause a Lyme disease-like illness in people in central and southern Europe.

Another surprise: While there are plenty of ticks in chaparral, the researchers found that chaparral harbored the lowest counts of larvae and nymphs on birds.

“This is important because the fire-management strategy in California calls for removal of chaparral,” Newman said. “It may be, however, that getting rid of chaparral invites a replacement community of birds we now know are highly competent carriers of Lyme disease.”

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