BERKELEY -- Ticks harboring the Lyme disease bacterium can be cleansed of the infection when they feed on the blood of the common western fence lizard, UC Berkeley researchers have discovered. The new finding may explain why Lyme disease is less common in California but epidemic in some northeastern states, where lizards are rare.

"Lizards are doing humanity a great service here," said Robert Lane, professor of insect biology in the College of Natural Resources at UC Berkeley and principal investigator on the tick findings, published this week in California Agriculture and before that in the Journal of Parasitology. "The lizard's blood contains a substance - probably a heat-sensitive protein - that kills the Lyme disease spirochete, a kind of bacterium."

Even better news, the newly discovered protein apparently leaches into the mid-gut of infected nymphal ticks as the tick feeds and destroys spirochetes stored there, permanently cleansing the ticks before they mature to adult size.

The western fence lizard is an even more important host of immature nymphal ticks that transmit Lyme disease in Northern California than most rodents, said Lane.

But unlike wood rats and some other wild rodents, western fence lizards don't contract the Lyme disease bacterium when infected ticks attach, said Lane. The newly discovered "spirochete-killing factor" in their blood, not yet identified, seems to prevent infection.

In California, the western black-legged tick is the primary carrier of Borrelia burgdorferi, the bacterium causing Lyme disease. Cleansing of the tick gut by the lizard protein occurs in the nymphal stage of tick development, which is an immature stage usually smaller than 1/20th of an inch in length.

Tiny though they are, these ticks can do big damage, causing most cases of Lyme disease in California, where the disease occurs sporadically in people who frequent tick-infested areas during the spring and summer.

In California, some populations of western black-legged ticks are three to four times more likely to carry the dangerous Lyme disease spirochete as nymphs than as adults. This is contrary to logic, since "you would expect that the older the tick is, the more likely it is to be infected," said Lane. "To reach the adult stage, a tick must have fed twice before, whereas to reach the nymphal stage, it must have fed only once," leaving less opportunity for exposure to infection.

Lane's recent study of Tilden Park in the East San Francisco Bay Area showed that in one area 1.3 percent of adult ticks carry the Lyme-disease bacterium, compared to 5.7 percent of nymphal ticks. These rates are much lower than in the northeastern U.S., where, for instance, fifty percent of adult ticks and 25 percent of nymphal ticks carry the disease in parts of Rhode Island, Massachusetts, Connecticut and New York.

Lane points out that along most of the trails surveyed at Tilden, the infection rate in adult ticks was even lower, as low as zero percent in some areas. As for picnic areas, several yielded few ticks year around, showing the risk there is lower than along trails. All in all, the risk of being infected with Lyme disease following a tick-bite in Tilden is very low.

Nymphal ticks are most active from April through July, Lane said. They live in shady, moist wooded areas carpeted with dead leaves and organic matter. People are most likely to contract Lyme disease from nymphal ticks while gardening, picnicking, resting or otherwise enjoying the outdoors in such areas.

"Because of their small size, nymphal ticks are hard to detect on human skin," said Lane. "You could easily have them and not know it. Probably only 20 to 30 percent of people who acquire Lyme disease as a result of a nymphal bite are aware that they've been bitten."

Despite the new lizard finding, "people should not go out into the woods and collect lizards and put them in their backyards to protect themselves from Lyme disease," said Lane. Not only would this be ineffective because Lyme disease in California primarily is contracted in rural or semi-rural areas occupied by various kinds of wildlife including lizards, but "there are problems with moving lizards from one locality to another and it's an illegal activity," said Lane.

Lane collaborated with UC Berkeley researcher Gary Quistad on the recent work, which was funded by the National Institutes of Health.