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The western fence lizard doesn't peddle insurance like its relative, the gecko, but it provides a potentially more valuable service to North Coast residents by shielding them from Lyme disease in an unusual way, scientists say.

Researchers working primarily in the Ukiah and Hopland areas of Mendocino County discovered the role of the common lizard, also known as a blue-belly, in cleaning Lyme disease bacteria out of ticks that transmit it to humans.

"It's an incredibly simple system," said Robert Lane, a UC Berkeley professor emeritus of medical entomology whose investigation of Lyme disease transmission in nature began three decades ago.

Simply put, the lizard's blood contains a protein that kills the Lyme disease bacteria in the gut of an immature western black-legged tick, which then molts into a disease-free adult tick.

Under a lab microscope, Lane watched the spiral-shaped bacteria, known as spirochetes, die within an hour of being immersed in lizard blood.

"You could see it with your own eye," he said.

Sonoma County is a ground zero for Lyme disease in California, with 73 confirmed cases — more than any other county — in the past decade.

Mendocino and Humboldt counties have fewer cases, but rank second and third statewide in the rate of cases per 100,000 people.

California's overall Lyme disease infection rate of 0.2 cases per 100,000 pales compared with rates of 50 to 70 cases in New England, but without the lizard's presence it would likely be higher, Lane said.

The little lizard, which kids and cats are fond of catching, "serves a protective function," he said. "It's reducing the likelihood that you or I would be bitten by an infected adult tick."

It's range is the far west, and primarily California.

Lane said his discovery, published in 1998, stemmed from a "conundrum."

A study of about 150 people in a small, rural community near Ukiah in the late 1980s found that 24 percent tested positive for Lyme disease, a level comparable to "hyper-endemic" areas of the northeast United States.

How could that be, the researchers wondered, when only 1.3 percent of adult ticks in the area carried the bacteria?

The answer came a few years later when scientists figured out how to collect tiny nymph ticks — the size of a poppy seed — from leaf litter on the floor of Mendocino County forests.

The infection rate among those nymphs was 12 to 13 percent, also comparable to the northeast.

Now, Lane says, the Lyme disease infection rate of 5 to 15 percent for western black-legged tick nymphs drops to 1 to 2 percent among adults after they have drawn blood from a lizard, which is the nymphs' "preferred host" for their one and only blood meal before becoming adults.

Ticks in their first stage of life, as larvae, pick up the bacteria from their one and only blood meal, typically on a wood rat or tree squirrel, which are the natural "reservoirs" of Lyme disease infection, Lane said.

The low rate of infection in adult ticks is likely due to the lizard's "cleaning" effect on nymph ticks, said Anne Kjemtrup, a researcher at the California Department of Public Health's Center for Infectious Diseases.

Consequently, Californians are at the greatest risk of contracting Lyme disease from the bite of a nymph tick, which is more difficult to detect and remove due its size (less than 1/20th of an inch), Kjemtrup said.

Based on a sample of 200 nymph ticks in Annadel State Park about 10 years ago, Lane said 15 percent were infected with the bacteria.

The infection rate varies from year to year, he said, adding that park users are generally safe from ticks if they stay on established trails.

Lyme disease patient advocates, however, say the lizard's role should not be overplayed, since the North Coast's woodlands — where people work and play — are home to an abundance of nymph ticks.

"There's so much exposure here," said Phyllis Mervine of Ukiah, founder and president of LymeDisease.org, an advocacy group for Lyme patients.

Thora Graves of Santa Rosa, who got Lyme disease from a tick bite 26 years ago, said she appreciates the lizard's function but also thinks the incidence of Lyme disease is vastly underreported.

"It's terrible here on the West Coast and we're not getting the recognition," she said.

Lane said his Lyme disease investigation continues at the 5,300-acre UC Hopland Research & Extension Center east of Hopland.

The first isolation of Lyme disease bacteria in western North American came from a tick collected at the center in 1985, Lane said.

The lab work was done by Willy Burgdorfer, who discovered the bacteria — named Borrelia burgdorferi — in 1982.

(You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.)