Ticks stay loyal to lizard hosts

UC BERKELEY (US) — Take a tick’s favorite host—the Western fence lizard—out of the equation and you would think the tick population and the threat of Lyme disease would rise. A new study finds the opposite is true.

Lyme disease—characterized by fever, headache, fatigue, and a bullseye rash—is spread through the bite of ticks infected with spirochete bacteria called Borrelia burgdorferi. In the Western region of the United States, the Western black legged tick (Ixodes pacificus) is the primary vector for Lyme disease bacteria.

A protein in the lizard’s blood kills Borrelia bacteria, and as a result, Lyme-infected ticks that feed on the lizard’s blood are cleansed of the disease-causing pathogen.

Ninety percent of the juvenile ticks in this species feed on the Western fence lizard, prevalent throughout California and neighboring states—so the lizard has been credited for the relatively low incidence of Lyme disease in the Western United States.

“Our expectation going into this study was that removing the lizards would increase the risk of Lyme disease, so we were surprised by these findings,” says Andrea Swei, who conducted the study while she was a Ph.D.
student in integrative biology at University of California, Berkeley.

“Our experiment found that the net result of lizard removal was a decrease in the density of infected ticks, and therefore decreased Lyme disease risk to humans.”

The research is reported in the journal Proceedings of The Royal Society B.

“When you have an animal like the Western fence lizard that supports such a huge population of ticks, you can’t assume that all those juvenile ticks will go to another host if the lizard population drops,” says Robert Lane, professor of entomology and co-author of the study.

For the field test, researchers selected 14 plots, each measuring 10,000 square meters and spread out over two sites in Marin County, Calif.

Half the plots were located at China Camp State Park, and the other half were at the Marin Municipal Water District Sky Oaks headquarters. The researchers had already been extensively surveying tick density in those plots over the course of two years, so they had detailed data on tick and vertebrate populations before this experimental field trial.

From March to April 2008, before tick season went into full swing, 447 lizards were captured and removed from six plots, three from each site. The remaining plots were left unaltered as controls. The lizards that had been captured were marked before being relocated so the researchers could determine whether any wandered back into their old haunts.

After the lizards were removed, the researchers spent the following month trapping other mammals known to harbor ticks, particularly woodrats (Neotoma fuscipes) and deer mice (Peromyscus maniculatus), to determine whether they bore an uptick in ticks as a result of the lizards’ absence.

Lizards accommodate ticks with a specialized fold in their neck called a “mite pocket.” (Credit: Anand Varma photo)

The researchers also checked for differences between control and experimental plots in the abundance of host-seeking ticks by systematically dragging a large white flannel cloth over the ground.

The researchers found that in plots where the lizards had been removed, ticks turned to the female woodrat as their next favorite host. On average, each female woodrat got an extra five ticks for company when the lizards disappeared.

However, the researchers found that 95 percent of the ticks that no longer had lizard blood to feast on failed to latch on to another host.

“One of the goals of our study is to tease apart the role these lizards play in Lyme disease ecology,” says Swei, who is now a post-doctoral associate at the Cary Institute of Ecosystem Studies in New York.

“It was assumed that these lizards played an important role in reducing Lyme disease risk. Our study shows that it’s more complicated than that.”

Notwithstanding the results in this new study, Lane pointed out that the Western fence lizard are key to keeping infection rates down among adult ticks.

“This study focused only on the risk from juvenile ticks, specifically those in the nymphal stage,” he says. “The earlier finding that adult ticks have lower infection rates because they feed predominantly on the Western fence lizard at the nymphal stage still holds.”

Researchers from University of California, Santa Barbara contributed to the study.

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