Download Chipmunks And Shrews, Not Just Mice, Harbor Lyme Disease

Chipmunks And Shrews, Not Just Mice, Harbor Lyme Disease
ScienceDaily (Dec. 6, 2007) —
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 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. Shorttailed 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.
This research was published in the Proceedings of the Royal Society B.