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UCSB researchers aim to raise awareness of tick-borne Lyme disease in California

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Photo: SAM SAMBADO
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UCSB
A western blacklegged tick caught during research.

Lyme disease is the most common vector-borne disease in the United States with an estimated 476,000 cases per year according to the Centers for Disease Control (CDC) .

Transmitted by a bite from a tiny infected tick, Lyme disease can cause serious illness if left undetected. Environmental science experts at UC Santa Barbara say understanding the ecology of where those ticks and their animal hosts live can raise public awareness and reduce its spread in humans.

Most cases of Lyme disease occur on the east coast, however, there’s an average of about 100 cases reported in California each year.

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Photo: Andy MacDonald
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UCSB
In the study, dusky-footed woodrats were a key variable to identifying high-risk infection areas. They make their homes in the thick underbrush of California's chaparral, woodlands and forests, much like the ticks that transmit Lyme disease.

“When we go hiking out in the oak woodlands and we pick up a tick that’s infected and it bites us, that’s when transmission occurs and causes disease,” Andy MacDonald said.

MacDonald, from UCSB’s Bren School of Environmental Science, recently published a paper linking the incidence of Lyme disease in California to environmental factors like species distribution and habitat suitability for ticks and their hosts.

“We’re essentially trying to predict where infected ticks are distributed across California,” he said.

He said the species that causes Lyme disease in California is called the western blacklegged tick. Not all ticks are infected with the bacteria that causes the disease, so MacDonald said they looked for variables that might play a role, and they identified a small mammal as the main vector host of the disease-causing bacteria.

“The most important variable by far was the dusky-footed woodrat,” he said.

MacDonald said habitats with this particular woodrat have high rates of tick infection, so identifying high-risk areas where the two co-exist can provide important information for people hiking, camping or doing outdoor activities.

Using field work, surveillance data, and statistical modeling, the research showed a strong association between the presence of dusky-footed woodrats, infected ticks, and Lyme disease.

“We had data on Lyme disease incidence and we used a statistical model to link the infection risk in tick populations to human Lyme disease, and we found that it actually was a pretty strong predictor,” MacDonald said.

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cdph.ca.gov
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The western fence lizard helps to control infection rates in tick populations. When infected ticks feed on lizards, they drop off uninfected.

Raising awareness about where and how Lyme disease is transmitted, he said, can help people avoid high-risk locations.

He also said many people believe tick season is only in the summer, but that’s not the case in California.

“It’s a species that in California is mostly active in the winter and spring and to some extent into the summer as well,” he said.

MacDonald said ongoing research is needed to better understand how environmental factors like weather or habitat change might impact disease transmission.

For instance, more cases of Lyme disease occur in northern California than in southern California or the Central Coast. MacDonald said one explanation might be our drier landscape with abundant lizard population.

“If a tick that is infected feeds on a lizard, it will actually drop off of that lizard uninfected, so they’re serving this role of reducing infection rates in tick populations,” he said.

The article is published online in Environmental Research Letters. You can also find information about preventing and treating tick bites on the CDC website.

Beth Thornton is a freelance reporter for KCBX, and a contributor to Issues & Ideas. She was a 2021 Data Fellow with the USC Annenberg Center for Health Journalism.
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