Lyme disease is a bacterial infection caused by the spirochete Borrelia burgdorferi, transmitted to humans through the bite of an infected blacklegged tick (Ixodes scapularis). It is the most commonly reported vector-borne illness in the United States, with an estimated 476,000 people diagnosed and treated annually. Initial symptoms include a characteristic bullseye-shaped rash called erythema migrans, along with fever and fatigue. If left untreated, the disease can affect the joints, nervous system, and heart. Although the blacklegged tick is named for the deer it frequently feeds upon, deer themselves do not typically become sick from Lyme disease, yet they play a large, indirect role in human risk.
The Role of Deer in the Tick Cycle
Deer serve as the preferred host for the adult stage of the blacklegged tick. Adult ticks seek a large mammal for their final blood meal, and the white-tailed deer is their host of choice.
During the late fall, adult female ticks attach to a deer, where they feed, mate, and become engorged with blood over several days. After feeding, the female drops off and can lay thousands of eggs, which hatch into larvae the following summer. Deer are crucial to the reproduction and proliferation of the tick population, as a higher deer density directly correlates with a denser population of ticks. While deer do not infect the ticks with the Lyme bacteria, their presence predicts dense tick populations.
Deer as Bacterial Reservoirs
The scientific distinction between a “host” and a “reservoir” is what explains the nuance of deer and Lyme disease. A host is an animal that provides a blood meal to the tick, which deer do extensively for adult ticks. A reservoir, however, is an animal that harbors the Borrelia burgdorferi bacteria in its blood and can transmit it to a feeding tick.
Deer are considered poor reservoirs for the Lyme disease bacteria because their immune system appears to effectively clear the infection. Research suggests that the blood serum of white-tailed deer has a natural defense mechanism that actively kills the Lyme disease spirochete. The true reservoirs for the Lyme bacteria are small mammals, particularly the white-footed mouse, shrews, and certain species of birds. Larval and nymphal ticks acquire the bacteria by feeding on these small, infected animals, which then allows them to transmit the infection to humans during their subsequent blood meals.
Preventing Tick Exposure
Given the presence of deer correlates with a higher number of ticks, reducing personal exposure is the most direct way to prevent Lyme disease. When spending time outdoors, especially in wooded or grassy areas where ticks thrive, wearing permethrin-treated clothing can repel ticks upon contact. Using EPA-registered insect repellents that contain DEET, picaridin, or oil of lemon eucalyptus on exposed skin also offers protection.
After returning indoors, it is important to perform a thorough full-body tick check on yourself, children, and pets, as well as showering within two hours to wash off unattached ticks.
Landscaping choices can also create a tick-safe zone around the home, such as removing leaf litter, keeping grass mowed short, and placing wood chips or gravel barriers between lawns and wooded areas to discourage tick migration.
If a tick is found attached, removing it promptly with fine-tipped tweezers can greatly reduce the risk of infection, as the bacteria is typically not transmitted until the tick has been attached for more than 24 to 48 hours.