Deer & Lyme Disease

Reducing deer populations to levels that reduce the potential for ticks to successfully breed should be an important component of any long-term strategy seeking to reduce the risk of people contracting Lyme disease.

Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans through the bite of infected blacklegged ticks, also known as deer ticks. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system. Lyme disease is diagnosed based on symptoms, physical findings such as its rash, and the possibility of exposure to infected ticks.

In 2016, Indiana saw 152 confirmed or probable Lyme disease cases, quadruple the number for 2006 and a substantial increase from the numbers observed in 2008-2012 (Centers for Disease Control). Confirmed cases in Indiana were just 26 in 2006, but jumped to 101 in 2013, stayed flat in 2014 and 2015 (100 and 102), but then jumped to 127 in 2016. Beverly Shores is in Porter County, Indiana, which reported more probable and confirmed cases of Lyme disease over the period than any other Indiana county. The number of cases in Porter County and the increase in such cases are shown in the chart below.

CDC reports of Lyme disease in Porter County

The prevalence of Lyme disease in our area is due to the presence of large numbers of the deer tick’s preferred hosts—white-footed mice and deer—and their proximity to humans. White-footed mice serve as the principal “reservoirs of infection” on which many larval and nymphal (juvenile) ticks feed and become infected with the Lyme disease spirochete. An infected tick can then transmit infection the next time it feeds on another host (e.g., an unsuspecting human).

Deer are a significant part of the life cycle of the deer tick, and hence are a likely culprit for Lyme disease, but in Indiana, deer populations recovered from essentially zero in the 1920s to huge numbers more recently, with the population surge occurring well in advance of the Lyme disease increase. With mice serving as the principal reservoirs of Lyme disease investions, some recent research has focused on reduced mice predators as a explanation for the growth in Lyme disease cases. For example, red fox are important mouse predators. As coyotes have invaded red fox territory, such as in the Dunes, reducing red fox populations, mice populations may have expanded, leading to more Lyme disease (See, e.g., Levi, T., A.M. Kilpatrick, M. Mangel, and C.C. Wilmers. 2012. Deer, predators, and the emergence of Lyme disease. Proceedings of the National Academy of Science 109(27): 10942–10947. But see also Coyotes, Red Foxes, and the Prevalence of Lyme Disease for a skeptical view.)

Several years ago, the New York Times published an article suggesting that deer might not be a principal contributor to Lyme disease. The authors claimed that the relationship between deer populations and tick populations was weak, and so associating high deer populations with Lyme disease problems seemed questionable. Times readers reacted strongly, pointing to evidence that tick populations were far higher outside of deer-fenced areas and that in a number of instances, deer removal from areas suffering from heavy Lyme disease incidence had resulted in substantial improvements in the Lyme disease infection rate. (See Are Deer the Culprit in Lyme Disease?, New York Times, July 29, 2009.) The authors of the original study replied that ticks were common in areas with few or no deer, that deer are common in the upper midwest, but Lyme disease is not nearly as prevalent as in the northeast, and that deer play no role in infecting ticks. None of these responses is impressive, however. Deer may not be necessary for ticks, but that does not mean that Lyme disease is common when deer are absent. The objection that Lyme disease is less prevalent in the midwest is countered by high rates observed in Minnesota and Wisconsin together with rapid growth in infections in Illinois and Indiana. An most importantly, while eliminating deer does not necessarily eliminate ticks, controlling deer populations does appear to be an effective method of reducing Lyme disease infections.

We now have a better handle on the relationship between deer populations and Lyme disease from a carful 13-year study published in the Journal of Medical Entomology. The investigators surveyed permanent residents of a Connecticut community to determine reported cases of Lyme disease, and related the incidence of the disease to deer density as the area's deer population was aggressively reduced. The authors ldquo;found that reducing deer density by ≥87% resulted in a significant reduction in tick abundance, nearly a 50% reduction in tick infection rate, and an 80% reduction in resident-reported human cases of Lyme disease.” Their study “demonstrated that deer populations can be manipulated to reduce human interactions with deer, infected nymphal ticks, and human risk of contracting Lyme disease.”

The researchers drew clear conclusions for deer control:

Reducing deer populations to levels that reduce the potential for ticks to successfully breed should be an important component of any long-term strategy seeking to reduce the risk of people contracting Lyme disease. … Additionally, good hunter access to deer habitat and a wide variety of management tools (bait, unlimited tags, incentive programs) are important components of a successful deer reduction strategy.

Thus the researchers in this study recommend dealing with deer using precisely the sort of program that ERG has implemented in Beverly Shores.

Protection against Lyme disease

To protect against tick-borne illnesses, people should wear repellents and check their skin for black specks after spending time outdoors, particularly in wooded areas and during the late spring and early summer months when nymphs are present.

If a tick is removed within the first 24 to 48 hours, it is unlikely to cause disease. The sooner you get that tick off of your body, the better. You don’t have to get too many ticks on you before you find one that’s infected with something. See Karen Weintraub, A New Culprit in Lyme Disease, New York Times, February 15, 2016

Here is advice from the U.S. Centers for Disease Control concering tick repellants:

Repel Ticks on Skin and Clothing

And, if you aren't already scared enough

From CNN: (click to view) Experts warn of increases in tick-borne Powassan virus.

Beverly Shores Environmental Restoration Group,
P.O. Box 667, Beverly Shores, IN 46301