The Ecology of Lyme-Disease Risk
Complex interactions between seemingly unconnected phenomena determine risk of exposure to this expanding disease
This article originally appeared in the July-August 1997 issue of American Scientist.
In the past two decades, people living in the northeastern, north-central and western United States have unwittingly entered a dangerous enzootic cycle—a cycle of disease that typically is restricted to wildlife. Wild mammals and birds host a wide variety of disease agents, with effects ranging from mild symptoms to mortality, but in most cases the pathogen affects only one or a few host species and never causes disease in people. However, as the result of a complicated sequence of events, people have become frequent accidental hosts for ticks and the disease agents they carry, including a corkscrew-shaped bacterium called Borrelia burgdorferi, the agent of Lyme disease. As of 1995, cases of Lyme disease had been reported in 48 of the 50 states and appear to be increasing, both in numbers of people affected and in geographic distribution.
Where does this disease come from, why has it emerged so rapidly, and what can people do to reduce their risk of exposure? It is possible to address these questions not from a medical point of view, but rather from an ecological one. All living organisms—from the B. burgdorferi bacterium and the ticks they infect to the mice and deer on which the ticks feed—form an ecological relationship with their habitats. Understanding the complex interactions between plant and animal species within those habitats may help people to predict the places where they are most likely to encounter disease-bearing ticks and become infected. Thus armed, individuals may ultimately be able to protect themselves from Lyme disease.
Currently, to prevent Lyme disease people wear protective clothing when they are in wooded areas and perform "tick checks" after leaving the woods. One underemphasized means to avoid exposure to Lyme disease, however, is avoiding the most heavily tick-infested habitats at the times of year when ticks are most abundant or most dangerous. Recent research performed in my laboratory, as well as in others, has suggested that such habitats can be predicted, often well in advance. Ultimately, it is the hope of ecologists studying this problem that we can use our expertise in pinpointing these habitats to warn the public away from areas that are likely to contain an abundance of disease-carrying ticks.