A new study makes the controversial proposal that biodiversity provides a barrier to certain diseases. In particular, it suggests that species diversity reduces the risk of Lyme disease to humans by diluting the effect of mice, the disease's most infectious host. The authors say the pattern could apply to other diseases spread from animals to humans by ticks and insects.
Lyme disease is the most common vector-borne disease in North America, and it causes fever and aching that can occasionally be debilitating. Juvenile ticks pass the disease-causing spirochete bacterium from rodent or bird hosts to humans. Ecologists have assumed that some hosts are better able to transmit the spirochete to ticks, and therefore that the makeup of the animal community affects disease risk for humans.
To find out which host species are best at passing on the disease, Kathleen LoGiudice and colleagues at the Institute of Ecosystem Studies in Millbrook, New York, trapped more than 200 animals from 13 host species. They tested any attached ticks for Lyme disease and then, using local host population sizes, estimated how many infected ticks each host species supports in the wild. White-footed mice appear to be the most dangerous reservoir, because they are common and infect between 40% and 90% of the ticks that feed on them. Species such as squirrels and opossums are less infective, so their presence lessens the disease risk, the authors calculate. They speculate that this pattern holds for other arthropod-borne diseases, which if true would provide another rationale for preserving biodiversity. The authors report the results online on 7 January in the Proceedings of the National Academy of Sciences.
Epidemiologist Durland Fish of Yale University in New Haven, Connecticut, applauds the study for crossing the boundary between academic disciplines. "We need to integrate basic ecological principles and basic epidemiology," he says. He cautions, however, that if biodiversity increases, it might add more hosts, which could raise the risk for humans. LoGiudice says that this is possible, and she plans a survey of Lyme disease hosts in six northeastern states to settle the issue.