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FOREST FRAGMENTATION LIKELY TO INCREASE LYME DISEASE RISK, SUGGESTS NEW STUDY FROM BARD COLLEGE BIOLOGIST Results of Study Published in the February Issue of Conservation Biology; National Institutes of Health Grant Will Support Further Research
ANNANDALE-ON-HUDSON, N.Y.—While many homeowners might covet a patch of woods separating them from their neighbors, a new study coauthored by Felicia Keesing, an assistant professor of biology at Bard College, strongly suggests that the destruction and fragmentation of forests in populated areas of the northeastern United States leads to an increased risk of contracting Lyme disease. The new study, published in the February issue of the journal Conservation Biology, finds that the density of infected nymphal blacklegged ticks—the main culprit in the spread of Lyme disease—is dramatically higher in smaller forest patches.
"These results suggest that, by influencing the community composition of vertebrate hosts for disease-bearing vectors, habitat fragmentation can influence human health," write lead author Keesing and coauthors Richard Ostfeld of the Institute of Ecosystem Studies in Millbrook, N.Y., and Brian Allan of Rutgers University. Their study is based on a survey, conducted in the summer of 2000, of tick densities and infection rates in 14 maple-dominated forest patches in Dutchess County, New York, which in recent years has led the nation in both the number of cases of Lyme and incidence (cases per 100,000 people) of the disease.
The study showed that the density of nymphal blacklegged ticks and the infection rates of those ticks are dramatically higher in smaller forest fragments, especially those totaling less than five acres. The five smallest areas studied supported an average of seven times as many infected nymphs per square meter as did the larger fragments. In some instances, as many as 80 percent of the nymphs were infected—the highest rate the researchers have seen. The risk of contracting Lyme disease is therefore much greater in these smaller forest fragments because people are not only likely to encounter more nymphal ticks, but also more infected ticks. Nymphal ticks are the main risk factor for contracting Lyme disease because their small size makes them hard to detect and because their activity peaks during the summer months—the same time that humans are most likely to encounter them.
The authors deduce that there is an elevated risk of Lyme in smaller forest patches because white-footed mice—the chief reservoir of the Lyme bacterium—thrive in smaller fragments, while their predators and competitors do not. The resulting increase in the relative abundance of mice in these areas means that more tick meals are taken from mice and fewer are taken from species that are less likely to carry or transmit the Lyme bacterium. The inflationary effect of this phenomenon on tick densities and infection rates is compounded by the fact that ticks that feed on white-footed mice are more likely to successfully molt into nymphs than are larvae that feed on non-mouse hosts.
The authors conclude there should be a greater awareness of the potential health risks of forest destruction and fragmentation. "Efforts to reduce the risk of Lyme disease should be directed toward decreasing fragmentation of the deciduous forests of the northeastern United States into small patches, particularly in areas with a high incidence of Lyme disease," the authors say. "The creation of forest fragments of less than five acres should especially be avoided."
The next phase of Keesing's and her colleagues' research will explore whether the results of their New York study hold true in other states in the Northeast. That research is being funded with the support of a $1.6 million grant from the National Institutes of Health. Lyme disease is concentrated in highly populated suburban areas in the United States, especially in the Northeast, and is the fastest growing vector-borne disease in the country, more common than West Nile virus and other mosquito-borne diseases.
Keesing has received the Presidential Early Career Award for Scientists and Engineers (in 2000), as well as grants from the National Geographic Society, National Science Foundation, and the National Institutes of Health. Her work has been published in Ecology, BioScience, and Oecologia, among others.
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