Tick Tactic

Why the incidence of Lyme disease in North America has grown enormously and spread geographically during the past decade.

The disease is caused by the bacterium Borrelia burgdorferi, transmitted to humans and various animal species by infected blood-sucking ticks of the genus Ixodes. It has long been assumed that Lyme disease expanded following the recovery of deer populations, major reproductive hosts for adult ticks. But ecologist Taal Levi of the University of California, Santa Cruz, and colleagues think that the cause might instead lie in the decline of a key predator of small rodents: the red fox.

Molecular evidence points to four species of small mammals—the white-footed mouse, the eastern chipmunk, the short-tailed shrew, and the masked shrew—as the main reservoirs of B. burgdorferi. Together they are responsible for infecting 80 to 90 percent of ticks. Levi and his colleagues analyzed the correlations between Lyme disease and the abundance of coyotes, red foxes, and deer. Their conclusion: Lyme disease is more common in areas with fewer small-mammal predators.

With the extirpation of gray wolves from large areas of the United States, coyotes have become the dominant predator across the country, displacing red foxes and causing their range-wide decline (foxes won’t build dens when coyotes are nearby). While coyotes rely more on deer, red foxes feed primarily on small mammals, killing them in large quantities and stashing them away for later feeding. The team found that Lyme disease did not consistently increase with deer abundance, but it did correlate positively with an abundance of coyotes and a scarcity of foxes. In western New York State, where foxes are abundant, Lyme disease is rare even though deer are plentiful. (PNAS)