Could Reducing Deer Populations Reduce Lyme Disease?

Editor’s Note: This article originally appeared in Entomology Today, a project of the Entomological Society of America with the goal of reporting interesting discoveries in the world of insect science and news from various entomological societies. To learn more, visit www.entomologytoday.org.

Lyme disease has been a growing problem in the United States, with the number of cases increasing throughout the past few decades.

The disease primarily is spread through a bacterium vectored by the tick Ixodes scapularis, also known as the blacklegged tick (or the deer tick, in some regions). The increasing spread of Lyme disease could be due to a variety of factors, such as suburbanization, reforestation or forest fragmentation and changes to human behavior, just to name a few.

Because of the diversity of reasons for the spread of Lyme disease, controlling and reducing the number of occurrences of the disease takes a multitude of approaches. One popular approach is to reduce the population numbers of ticks that vector the disease.

In an article published in the open- access Journal of Integrated Pest Management, Sam Telford, Ph.D., from the Department of Infectious Disease and Global Health at the Tufts University Cummings School of Veterinary Medicine, argues that reducing deer populations is a key component of managing tick populations.

FIRST, A LITTLE BACKGROUND. To understand Telford’s argument, it’s helpful to know about the life cycle of I. scapularis ticks. First, a female mates and feeds in the fall, then overwinters. In the spring, the female lays a batch of eggs, which could contain as many as 2,000 eggs. In the summer, the larvae emerge from eggs and seek hosts to feed on. After feeding, the larvae molt into nymphs during the fall or early the following spring, when they feed as nymphs. If they feed, they will molt into adults during the summer and seek hosts that fall. Larvae and nymphs both feed on small to medium animals, such as mice, chipmunks, shrews or birds. These animals, especially mice, are where the ticks pick up the bacterium that causes Lyme disease. The adults feed only on larger animals, with white-tailed deer being one of their favorite hosts. Unlike the small mammals, deer do not infect ticks with the bacterium that causes Lyme disease.

COULD REDUCING DEER POPULATIONS HELP? If ticks don’t get Lyme disease from deer, why would reducing deer populations help curb Lyme disease transmission?

Telford argues that it goes back to the tick life cycle. If an adult female tick can’t feed, she isn’t able to reproduce. That means a reduction in the number of larval- and nymphal-stage ticks. Lyme disease is primarily spread to humans through nymphal-stage ticks, so having less of those ticks around could lead to less human infections.

“One might suggest reducing the density of mice, chipmunks or certain birds to reduce the chance that larvae and nymphs will feed on them and develop further (and also become infected), but killing one fed adult female deer tick is equivalent to killing 2,000 larvae or several hundred nymphs,” writes Telford. “It might be argued that without subadult development, there would be no adult ticks, but the successful feeding of a single female tick would compensate for great subadult tick mortality.”

SOUNDS GOOD. BUT DOES IT ACTUALLY WORK? While the method requires further study, there is some evidence that reducing deer populations reduces human incidence of Lyme disease. For example, one study found that reducing deer populations in a community led to a 76 percent reduction in tick abundance and 80 percent reduction in resident-reported cases of Lyme disease. In another study, removing deer from an island resulted in less ticks found per hectare.

However, people critical (or unconvinced) of the method would say that the evidence isn’t clear that deer reduction was the cause of Lyme disease reduction in humans. For example, it could have been because of better education, changes in human behavior or something else. Furthermore, some of the places where deer reduction did appear to reduce tick populations were islands or peninsulas, which could have led to different outcomes than studies on a mainland location would have found.

DEER REDUCTION: A TOOL, NOT A SILVER BULLET. In the article, Telford is clear that he isn’t suggesting deer reduction is the best or only option to reduce incidence of Lyme disease in humans. On the contrary, he mentions that it might not be appropriate in some cases where other measures to reduce the disease could be more effective due to monetary cost, ethical reasons, specific ecology or other reasons. In either case (using deer reduction or not), short-term methods (repellents, insecticide applications to leaf litter, permethrin-treated clothing) to reduce the risk of the disease also must be used. However, he does argue that deer reduction shouldn’t be ruled out as an option simply because the evidence for a reduction in Lyme disease is not as strong as that for reducing ticks.

“It is wrong to conclude that deer reduction does not reduce risk based on the published acarological studies, a less than complete surrogate for human exposure studies,” writes Telford. “Indeed, such a conclusion is based on absence of evidence, not evidence of absence of an effect, and harms future attempts to use deer reduction as part of an integrated tick management system.”