August 1996
SHIPPENSBURG, Pa. — Shrews may be a reservoir of the bacterium that causes Lyme disease, according to a study.
If so, then investigators may have to consider shrew populations as well as those of the white-footed mouse — a known reservoir of the bacterium — when assessing the risk for Lyme disease transmission to humans in a particular region.
Researchers led by Penelope J. Padgett, PhD, of Shippensburg University here, obtained tissue samples from 125 shrews and 70 white-footed mice captured in southcentral Pennsylvania, a region that is not endemic for Lyme disease. Most of the shrews were Sorex fontinalis, but investigators also tested Sorex fumeus and Sorex hoyi.
Using polymerase chain reaction (PCR) to identify DNA sequences from the Lyme disease bacterium, Borrelia burgdorferi, Padgett and colleagues found that about 5% of mice and shrews harbored the bacterium. "If we were in an endemic area, the infection rate would have been significantly higher," said Padgett, who is a professor of biology.
Though mouse-like, shrews are classified as insectivores, not rodents. They are among the tiniest mammals. Although common in the northeastern United States, Padgett said, shrews are seen much less often than white-footed mice but they do not usually enter dwellings.
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Identifying shrews as a reservoir for B. burgdorferi "broadens the suspect list" of mammal species that reflect the potential risk for the spread of Lyme disease, Padgett said. If many shrews inhabit an area, small populations of white-footed mice may not necessarily mean a reduced risk for Lyme disease transmission.
Padgett and colleagues are not the first investigators to report that shrews harbor B. burgdoferi. For example, in 1993, Swedish researchers reported in the Journal of Medical Entomology that the shrews Sorex araneus and Sorex minutus can be competent reservoirs for the bacterium. Previous studies also have shown that voles, rabbits and foxes can harbor B. burgdorferi.
Padgett and colleagues reported their findings at the American Society for Microbiology general meeting in New Orleans.
The risk of B. burgdorferi infection depends on multiple ecologic factors, including the population of vector ticks, principally the deer tick Ixodes scapularis. Stephen Eppes, MD, an associate professor of pediatrics at Jefferson Medical College, Philadelphia, has investigated ecologic risk factors for contracting Lyme disease. He said the Padgett team's finding "is potentially important, especially if areas are inhabited by a significant population of shrews." For shrews to increase the risk of Lyme disease, he pointed out, vector ticks must feed on them.
Although I. scapularis probably feeds on shrews, scant research has looked at the interaction between these animals. It is difficult to catch shrews alive, Padgett pointed out, and ticks leave their hosts when they die.
In the United States, health departments reported 11,603 cases of Lyme disease to the Centers for Disease Control and Prevention in 1995. Northeastern states, particularly Connecticut, Rhode Island, New York and New Jersey, had the highest incidence of reported cases in 1995.
For more information, see:
- Padgett PJ, Bamford L, Gallas T. Determination of the infection rate of small mammals using the polymerase chain reaction. Presented at the 96th general meeting of the American Society for Microbiology, New Orleans, May 19-23.
- Talleklint L, Jaenson TG. Maintenance by hares of European Borrelia burgdorferi in ecosystems without rodents. J Med Entomol 1993;30:273-6.
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