The Ecology of Lyme-Disease Risk
Complex interactions between seemingly unconnected phenomena determine risk of exposure to this expanding disease
Societal Attitudes toward Nature
It is ironic that, just as human societies are becoming increasingly aware of the intrinsic value of nature and wilderness, the Lyme-disease epidemic has caused people to fear and mistrust nature within endemic areas. Other emerging tick-borne diseases, such as human babesiosis and human granulocytic ehrlichiosis (HGE), add to the real and perceived risks of encountering a tick while outdoors. Hikers and others entering forests are either annoyed or frightened by the measures they must take to avoid contact with ticks. These include wearing light-colored clothing, tucking pant legs into socks, wearing long-sleeved shirts, applying insect repellents and conducting thorough tick checks after leaving the woods. Parents are often reluctant to allow children to play in the forest, and when such latitude is given, it is often accompanied by the admonition to remain on trails at all times. In sum, our current battery of Lyme-disease prevention strategies relies almost wholly on avoiding ticks and tick bites. Unfortunately, this situation is unlikely to change in the near future.
Some steps are being taken to boost people’s immunity to the bacterium, should they encounter it. At least two vaccines have recently been tested in large-scale field trials with volunteer human subjects, and preliminary evidence suggests that they will be relatively safe and effective in preventing Lyme disease. However, an effective vaccine may not be the panacea that some people expect. On a population-wide level, the risk of Lyme disease remains remarkably low. For instance, in Lyme-disease-endemic counties in the northeastern and north-central states, the number of cases remains about 10 to 30 per 100,000 individuals, and in the rest of the United States, cases per 100,000 are well below 10. Given this relatively low risk at the population level, it remains to be determined whether widespread use of a vaccine is warranted even if it is relatively safe.
For example, the question of who needs to take the vaccine becomes an issue. Should physicians urge that the general population throughout states having high Lyme-disease incidence be given vaccine? Should vaccines be made available only to people who are particularly at risk owing to their profession (for example, landscape gardeners, foresters and park rangers)? Apparently, the effectiveness of the vaccine declines dramatically after one year, and annual booster administrations are needed to maintain an effective response. Will patients and their doctors be responsible about sustaining their immunizations every year? Will vaccinated people believe that they do not have to avoid tick bites, relax their vigilance and become exposed more readily to other, potentially more deadly, tick-borne diseases, such as HGE, from which they are not protected?
It appears likely that, even with a widely available vaccine and a public and health-care community that are well educated about its use, avoidance of tick bites will remain the primary means of preventing Lyme disease. If so, the primary responsibility for prevention will continue to rest with individuals, rather than with the health-care community. Avoiding tick bites requires knowledge about some fundamental aspects of the natural history of Lyme disease, and about the ecological interactions that determine where and when risk is high. Thus, ecological studies on acorn production, on population dynamics of mice, deer, other hosts and their predators, and on the biology of ticks and B. burgdorferi bacteria will continue to be of primary importance in efforts to reduce the incidence of Lyme disease. If ecologists are to use acorn production to predict areas that are likely to be infested with disease-carrying ticks, various measures can be taken to reduce the risk. These measures include boosting public-education efforts in the months preceding an expected increase in local risk, posting warning signs in high-risk areas and alerting health-care specialists whose patients live or work in high-risk regions. The interface between ecology and human health is both broad and deep.