The Lowdown on Lyme Disease

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AN ECOLOGICAL PERSPECTIVE

The ticks that transmit Lyme disease in the Northeast and upper Midwest feed on a number of animals, but the host most likely to pass the Lyme-disease bacteria to young tick larvae in these regions is the white-footed mouse (see "The Tick Life Cycle," Page 56). The mouse's immune system doesn't kill the bacteria if it is bitten by an infected tick. For reasons researchers do not understand, the mouse carries the bacteria but does not show any symptoms of the disease. When another tick bites the mouse, that tick picks up the bacteria.

Thus the mice "serve as incubators for the disease," explains Rick Ostfeld, an animal ecologist at the Institute of Ecosystem Studies in Millbrook, N.Y. When Ostfeld and his team surveyed Lyme-disease infection rates in numerous small animals and birds in Dutchess County, N.Y., they found that 90 percent of white-footed mice were carriers, compared to only 10 percent of squirrels (infection rates for other small animals were similarly low). In the Midwest, chipmunks and shrews as well as white-footed mice serve as incubators, also called "reservoir" hosts.

"Anything you can do to preserve native mammal, bird and lizard diversity should reduce Lyme-disease risk," Ostfeld says. Newly hatched tick larvae are disease free, and if they feed on animals that are poor reservoirs—most squirrels, for example—fewer nymphs will be infected with the disease.

Maintaining wildlife diversity is difficult in small plots of less than five acres, yet such small woodland "fragments" are ideal habitats for white-footed mice, which Ostfeld says generally range across less than a quarter of an acre. Why not get rid of the mice? Ostfeld cites these problems: "Spatially speaking, mice are very aware of what's going on around them. As soon as you trap some, others move in to take over the habitat.

"The other problem is that mice do positive things for us by keeping pest insects in check, such as gypsy moths, which defoliate trees. They are the main predators of gypsy moth cocoons." Ostfeld adds that mice also are a primary food source for many animals we like, such as owls, hawks and foxes.

In most Western states, Lyme-disease rates are lower than in the East. In the West, tick larvae and nymphs feed particularly on lizards such as western fence lizards, commonly called "bluebellies," and southern alligator lizards. They feed on small mammals, such as wood rats and kangaroo rats, to a much lesser extent. Robert Lane, professor of insect biology at the University of California at Berkeley, has found that a complex of enzymatic proteins in the lizards' blood actually kills Lyme-disease bacteria. As infected ticks feed and the lizard blood enters their bodies, the bacteria are cleared from the ticks' systems.

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