PHILADELPHIA -– A study led by a University of Pennsylvania biologist in the tick-infested woods of the Hudson Valley is challenging the widely held belief that mice are the main animal reservoir for Lyme disease in the U.S.

The paper, published in the Proceedings of the Royal Society B, demonstrates that chipmunks and two shrew species, not just mice, are the four species that account for major outbreaks.

According to the study, white-footed mice account for about a quarter of infected ticks. Short-tailed shrews and masked shrews were responsible for a quarter each and chipmunks for as much as 13 percent. According to the team, vaccination strategies aimed solely at mice are unlikely to bring the disease under control. Efforts to control Lyme disease and prevent its spread, the team said, must include strategies that account for multi-species carriers.

“The majority of zoonotic diseases, those that can be transmitted from wild or domestic animals to humans, are generally assumed to have one natural animal host,” Dustin Brisson, professor of biology in the School of Arts and Science at Penn, said. “For Lyme disease, this host has been the white-footed mouse. Data are beginning to accumulate to suggest that the story is much more complex, mice being one of an assemblage of vertebrate species contributing to feeding ticks and transmitting B. burgdorferi. Deer, a popular culprit of the Lyme disease epidemic, play a rather minor role in transmitting the bacteria to feeding ticks, although they are a major cause of the elevated tick densities that are important for the spread of the disease to humans.”

Borrelia burgdorferi sensu stricto, the bacterium that causes Lyme disease, is transmitted to humans by infected, blacklegged ticks. The ticks, infected as larvae during their first meal — the blood of a vertebrate — are middle men.

Mice were thought to be the primary natural reservoir of the disease because nearly 90 percent of ticks feeding on an infected mouse contract the disease, nearly twice as much as any other species. In addition, mice are common, conspicuous and easy to research in the field and in the lab, promoting their status as primary natural reservoir. Yet other factors, such as population densities and tick burdens of other disease-carrying species, led investigators to rethink mice as the principal reservoir species for the disease.

The team employed genetic and ecological data, including dynamics of an outer surface protein of Lyme disease that provides clues about how the disease was transmitted, to discover that mice feed only 10 percent of all ticks and 25 percent of B. burgdorferi-infected ticks in the northeastern Lyme disease endemic zone. Shrews feed 35 percent of all ticks and 55 percent of infected ticks.

Emerging zoonotic pathogens, the 132 infectious diseases that cross the line between animal and human species, like Lyme disease, are a constant threat to world health. The research team, focused on improving existing strategies to protect the public health, is promoting the notion that targeting a single host species, in this case the white-footed mouse, may have been a faulty assumption.

While public-health strategies to control Lyme disease in North America have focused on interrupting transmission between blacklegged ticks and white-footed mice, Lyme disease infects more than a dozen vertebrate species, any of which can infect feeding ticks and increase human Lyme disease risk.

The research was performed by Brisson of the Department of Biology at Penn, Daniel E. Dykhuizen of Stony Brook University and Richard Ostfeld of the Institute of Ecosystem Studies. It was supported by the U.S. Public Health Service, the National Institutes of Health and the National Science Foundation.

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