Lyme disease’s unusual suspects

A new study challenges the widely held view that mice are the main animal reservoir for Lyme disease in the United States. The paper, published online this week in the Proceedings of the Royal Society B, shows that two shrew species are just as important and that chipmunks play a supporting role. Unless those species are taken into account, say researchers, efforts to control Lyme disease are doomed to fail. Lyme disease can cause anything from rash to arthritic and psychiatric diseases. Borrelia burgdorferi sensu stricto, the bacterium that causes Lyme disease in the United States, is transmitted to humans by blacklegged ticks. Researchers have assumed that the vast majority of ticks become infected when, as larvae, they take their first blood meal from a white-footed mouse. Indeed, lab studies have shown that as many as 90% of ticks feeding on an infected mouse pick up the bug, an “extremely high number,” says disease ecologist Dustin Brisson of the University of Pennsylvania.

But other species that transmit B. burgdorferi to ticks less efficiently might also spread the disease if they are more numerous or if they are bitten by a large number of ticks. And some studies had suggested this to be the case. For instance, a field trial published in 2004 showed that vaccinating mice against Lyme disease and then releasing them led to only a small decrease in the number of infected ticks. To get a fuller picture, Brisson and his colleagues pulled together data from their own studies in the Hudson Valley in New York state and from other papers. The team looked at an outer surface protein of B. burgdorferi found in ticks–which can give clues about the vertebrate host–as well as the probabilities that different host species transmit the microbe during a tick bite, the number of larvae feeding on the animals, and population densities. Then they calculated the importance of each of the host species.

White-footed mice account for only a quarter of the total number of infected ticks, the team found. Short-tailed shrews and masked shrews were responsible for another quarter each, and chipmunks for as much as 13%. That means that mice aren’t the “dominant” host at all, says Brisson, and vaccination strategies aimed at mice alone are unlikely to bring Lyme disease under control. Brisson speculates that mice have received a lot of attention in part because they’re conspicuous, easy to catch, and ideal lab subjects. “It’s a nice paper,” says Durland Fish of Yale University School of Medicine, a co-author of the 2004 vaccination study. However, he believes there are even more culprits. In particular, the team barely looked at birds, he notes, even though one study has shown that robins, which often live close to humans, are very good at transmitting B. burgdorferi to ticks in the lab. “The situation is probably even more complex than we think,” Fish says.

December 11, 2007

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