Lyme disease is endemic to the northeastern and northcentral United States. In 2011, as reported by the Centers for Disease Control, there were a total of 24,346 confirmed cases of Lyme disease. The blacklegged ticks that transmit the etiological agent of the disease, the spirochete Borrelia burgdorferi, are often linked to deer—which consequently triggers demands for deer kills.
The fact that “deer tick” is another common name for the blacklegged tick is largely responsible for this enduring association. When it comes to transmission of Lyme disease, however, deer may be more innocent victim than vector. Indeed, many efforts to reduce the incidence of Lyme disease by substantially reducing deer numbers have not worked, as the ecology of Lyme disease has proven to be a complex tale woven with a variety of animals, predator-prey dynamics, habitat structure, tick biology and ecology, and more.
During its two-year life span, the blacklegged tick exists in a larval, nymphal, and adult form. Ticks become infected during the larval and nymphal stages, during which time they engorge on blood from previously infected host animals—primarily the white-footed mouse and eastern chipmunk—known to be effective carriers for B. burgdorferi. While adult ticks do often feed on larger animals (including deer), such animals are not particularly effective hosts for the disease. A number of studies, including a 2012 study from the National Academy of Sciences, found no significant correlations between deer density and human cases of Lyme disease. In fact, according to a 2010 study published in the journal Nature, the key to controlling Lyme disease (and many other diseases) may lie not with eradication—but, rather, abundance.
When it comes to Lyme disease, there are other hosts for the tick, including gray squirrels, raccoons, and even some lizards. However, these animals are not competent “disease reservoirs,” meaning they don’t do a good job of providing the pathogens with safe harbor and a helping hand into the next generation. Thus, they act to dilute the potential for the disease to survive and spread.
The study singled out opossums, in particular, as beneficial. When ticks hitch a ride on opossums, opossums respond by eating them—with great gusto. And these oft-overlooked mammals (who do better in more intact habitats) have robust immune systems that effectively quash the disease. Hence, a plethora of opossums means more B. burgdorferi end up at the end of the line without a ride.
In a post appearing on a New York Times blog, the lead author of the Nature study, Dr. Felicia Keesing of Bard College, along with Dr. Richard Ostfeld of the Cary Institute of Ecosystems Studies, summed it up by saying that “in many Lyme disease zones, reducing the deer herd is unlikely to substantially affect tick abundance. Reducing mice is more likely to be effective. This is best accomplished by allowing natural predators like weasels, coyotes, foxes, and owls to do the job. And the best way to increase their numbers is to maximize the size of forest patches.”