One of the most common concerns cited by residents regarding the presence of urban and suburban deer, is the specter of Lyme disease. Residents worry that areas of deer concentration may increase the risk of transmission of the disease to humans. However, whether or not high deer densities increase the risk of Lyme disease is questionable.

Lyme disease is a systemic, tick-borne disease caused by a spiral bacterium, Borrelia burgdorferi. Following transmission by a bite of an infected tick, Lyme disease spirochetes initially multiply and disseminate in the skin surrounding the bite. If untreated, spirochetes can invade the blood stream where they multiply and cause flu-like symptoms.

Lyme disease is transmitted to humans by the black-legged tick, Ixodes scapularis, aka "deer tick." Black-legged ticks are the only vectors of Lyme disease in Indiana. The black-legged tick follows a generalized life cycle that is relevant to the transmission of the disease to humans. 1) Once a female tick lays eggs on the ground, larva form; 2) Six-legged larva feeds on a white-footed mouse and then drops to the ground and molts. The larva may have acquired the Lyme disease agent in the blood meal; 3) The eight-legged nymph feeds on a small mammal, pet or human and may transit the disease. The nymph then drops off the ground and molts; 4) The eight-legged adult feeds and mates on a deer, then drop off to the ground. Males die soon thereafter and females begin to develop eggs. Tick larvae are infected when they feed on infected white-footed mice. White-tailed deer are not reservoirs of Lyme disease because they do not support the disease in the blood at levels high enough to infect ticks. However, deer are the preferred host for adult black-legged ticks and are therefore critically important to tick reproduction.

Lyme1 Small rodents, such as mice, are reservoirs for B. burgdorferi. The tick becomes infected from feeding on a mouse and remains infected as it changes to nymph and then adult. The spirochetes are transmitted by infected nymphs to other mice and to humans, which are inadvertent hosts. Deer are important hosts for adult ticks, but are not effective reservoirs for B. burgdorferi. (From: Nature 390, 553-554 (11 December 1997)


The problem is, reducing the deer herd doesn't translate into reduced exposure to Lyme disease. First, adult black-legged ticks feed on raccoons, skunks, opossums, and other medium-sized mammals. When deer are scarce, ticks don't necessarily become scarce, because they have alternative hosts. Indeed, several studies on mainland sites in New York and New Jersey found no correlation between deer and ticks. Second, ticks and Lyme disease are rare or absent in parts of the United States (the Southeast, most of the Midwest) where deer are abundant. Third, ticks are only dangerous if they are infected, and deer are hosts to the ticks, but play no role in infecting ticks - that's the role of the white-footed mouse (chipmunks and shrews too).*

Model simulations of the relationship between deer and ticks show that reduction in deer density results in a small reduction of the black-legged tick. In order to drastically reduce the host tick numbers, deer would need to be almost entirely removed from the landscape, since one deer serves as a host for many ticks. Experiments on island deer populations indicate that with a drastic reduction of deer numbers, the host tick numbers will also decline. In a free-ranging population, where deer are not constrained to one geographic area, it is unlikely that reduction of deer numbers would decrease the presence of black-legged ticks, thereby decreasing the risk of Lyme disease.**

Another important point to consider is that the population-level risk of Lyme disease in Monroe County is relatively low. Based on data from the Indiana State Department of Health, the number of confirmed cases in Monroe County has held steady at "less than five" per year over the last several years.*** Indeed, from 1990-2003, only four cases of Lyme Disease were reported in Monroe County.**** To be fair, various factors confuse the diagnosis and reporting of the disease, and there are strong arguments that the disease is either highly under-reported or over-reported (mis-diagnosed).

Biologists at IU have been trapping and testing black-legged ticks for 7 to 8 years. They found healthy populations in southwest Monroe County near Springville, but had never found the black-legged tick in the Bloomington area until spring 2010, at which time they found a small population at Griffy Lake Nature Preserve. These populations are currently being tested for Lyme disease.

For more on Lyme Disease prevention, please visit:


* See, Jordan RA and TL Schulze. 2005. Deer Browsing and the Distribution of Ixodes scapularis (Acari: Ixodidae) in Central New Jersey Forests. Environmental Entomology 34: 801-806; Jordan, RA, TL Schulze, and MB Jahn. 2007. Effects of Reduced Deer Density on the Abundance of Ixodes scapularis (Acari: Ixodidae) and Lyme Disease Incidence in a Northern New Jersey Endemic Area. Journal of Medical Entomology 44: 752-757; Ostfeld RS, CD Canham, K Oggenfuss, RJ Winchcombe, F Keesing. 2006. Climate, Deer, Rodents, and Acorns as Determinants of Variation in Lyme-disease Risk. PLoS Biology 4: 1058-1068.

** Ostfeld, RS. 1997. "The Ecology of Lyme Disease Risk." American Scientist, 85: 338-346.

*** The State accounts for cases in a "less than" fashion in the interest of privacy.

****Indiana Department of Health, http://www.in.gov/isdh/20468.htm