Can Deer Get Lyme Disease? The Truth About Deer, Ticks, and Lyme Disease
Deer are important hosts for adult blacklegged ticks.
They are not considered major reservoirs for the Lyme disease bacterium.
Research suggests reducing deer alone may not be enough to lower Lyme disease risk.
Can deer get Lyme disease? Many people assume deer spread Lyme disease because they are often covered with blacklegged ticks (Ixodes scapularis). The answer is more complicated. White-tailed deer are important hosts for adult ticks, but they are not considered major reservoirs for Borrelia burgdorferi, the bacterium that causes Lyme disease.
Although deer can be exposed to Borrelia burgdorferi, they generally do not efficiently transmit the bacteria to feeding ticks. Instead, deer provide blood meals for adult ticks, allowing tick populations to reproduce and expand. White-footed mice and several other small mammals are much more efficient at infecting ticks with the Lyme disease bacterium.
This distinction has fueled decades of debate over whether reducing deer populations could lower the risk of Lyme disease. Although some studies have reported fewer ticks and fewer Lyme disease cases after substantial reductions in deer numbers, other research has shown that eliminating deer alone is often insufficient because ticks continue feeding on mice, birds, and other wildlife.
To better understand the effectiveness of deer reduction, investigators from the Center for Vector Biology & Zoonotic Diseases at The Connecticut Agricultural Experiment Station conducted a three-year study in Redding, Connecticut, a community where Lyme disease is endemic. Their findings highlight the complex relationship between deer, ticks, and Lyme disease transmission.
Does reducing deer reduce Lyme disease?
A 2014 study found that reducing the deer population “dramatically reduced I. scapularis abundance and Lyme disease cases on a Connecticut peninsula.” [1] However, a study conducted two years later found that deer reduction was not effective in controlling I. scapularis in non-insular areas.
To evaluate several tick management approaches, including deer reduction, investigators from the Center for Vector Biology & Zoonotic Diseases at The Connecticut Agricultural Experiment Station conducted a three-year study in Redding, Connecticut, an area endemic for Lyme disease. Four residential neighborhoods participated in the study between January 2013 and September 2015. [2]
The investigators found that reducing the number of deer did not have a significant impact on tick populations. Deer reduction alone lowered the number of I. scapularis ticks feeding on white-footed mice by only 3%. As deer became less available, many ticks simply shifted to other hosts, and the percentage of ticks infected with B. burgdorferi actually increased.
[bctt tweet=”Can we reduce the threat of Lyme disease by eliminating the deer?” username=”DrDanielCameron”]
Why didn’t deer reduction work?
The investigators planned to reduce the deer population to fewer than 4.0 deer/km2 using professional sharpshooters.
“Deer were concentrated with bait on cooperating homeowners’ properties and euthanized with a single 0.223 caliber bullet to the center of the brain, consistent with American Veterinary Medical Association and Sikes and Gannon (2011) standards for humane euthanasia and in compliance with The Connecticut Agricultural Experiment Station’s Institutional Animal Care and Use Committee (#P18-13) and Connecticut Department of Energy and Environmental Protection’s Volunteer Authorization (#1315006b),” writes Williams.
“Deer were removed from January-March of each year, largely after the end of the regulated hunting season,” added Williams.
However, the investigators fell well short of their target. Rather than reaching fewer than 4 deer/km2, the population remained above 17 deer/km2. “Due to hunter interference and resulting safety concerns, we were forced to suspend deer removal efforts prematurely and did not achieve target densities,” explains Williams.
The findings suggest that unless deer populations can be reduced to very low levels across an entire community, deer reduction alone may not substantially decrease Lyme disease risk.
Can deer spread Lyme disease?
Although deer are essential hosts for adult blacklegged ticks, they are generally not considered important reservoirs for Borrelia burgdorferi. Instead, white-footed mice and several other small mammals are far more efficient at infecting feeding ticks with the Lyme disease bacterium.
Adult ticks commonly feed on deer before laying thousands of eggs. The next generation of ticks may later acquire B. burgdorferi when larval ticks feed on infected mice or other reservoir hosts. Because of this complex life cycle, simply removing deer does not eliminate Lyme disease from an area.
Were fungus treatments and bait boxes more effective than deer reduction?
In contrast to deer reduction, investigators found that combining broadcast applications of the entomopathogenic fungus Metarhizium anisopliae with fipronil-based rodent bait boxes substantially reduced the number of larval I. scapularis feeding on Borrelia-infected white-footed mice.
This combined approach reduced larval I. scapularis parasitizing infected white-footed mice by 94%. Even so, it did not completely interrupt the Lyme disease transmission cycle. Williams concluded that although the strategy substantially reduced tick numbers, it was “not to levels sufficient to interrupt the B. burgdorferi pathogen transmission cycle.”
The authors suggested that placing additional bait boxes around larger residential properties could improve effectiveness. However, they estimated that installing a second ring of bait boxes would cost approximately $1,000 per season, making widespread use impractical for many homeowners.
What is an entomopathogenic fungus?
Scientists have been investigating alternatives to synthetic pesticides for controlling ticks around homes. One promising approach involves entomopathogenic fungi—naturally occurring fungi that infect and kill ticks and other arthropods.
“Natural product-based acaricides or entomopathogenic fungi have emerged as alternatives to kill host-seeking ticks for homeowners who are unwilling to use synthetic chemical acaricides,” writes Eisen from the CDC’s Division of Vector-Borne Diseases in the Journal of Medical Entomology. [3]
However, Eisen cautions that these approaches generally have limitations. “As compared with synthetic chemical acaricides, these approaches appear less robust in terms of both their killing efficacy and persistence.”
What is fipronil?
Fipronil is an acaricide and insecticide used in rodent bait boxes to target ticks while limiting environmental exposure. As mice enter the bait boxes, small amounts of fipronil are applied to their fur, killing attached ticks without harming the rodents.
“Fipronil belongs to the phenylpyrazole family and has acaricidal and insecticidal properties,” writes Bonneau from Virbac, France, in Parasites & Vectors. [4] “Fipronil’s putative mode of insecticidal action is interference with the passage of chloride ions through the gamma-aminobutyric acid (GABA)-regulated chloride ion channel, which results in uncontrolled central nervous system activity and subsequent death of the insect.”
Frequently Asked Questions
Can deer get Lyme disease?
Although deer can be exposed to Borrelia burgdorferi, they are not considered major reservoirs for the Lyme disease bacterium. Unlike white-footed mice, deer generally do not efficiently infect feeding ticks.
Do deer spread Lyme disease?
Not directly. Deer provide blood meals for adult blacklegged ticks and help maintain tick populations, but they are not the primary source of Lyme disease bacteria. White-footed mice and several other small mammals play a much larger role in infecting ticks.
Does reducing deer reduce Lyme disease?
Research has produced mixed results. Significant reductions in deer numbers have lowered tick populations in some isolated settings, but studies in mainland communities have found that deer reduction alone often fails to substantially reduce Lyme disease risk because ticks continue feeding on other wildlife.
Why are deer important in the tick life cycle?
Adult female blacklegged ticks commonly feed on deer before laying thousands of eggs. Deer help support tick reproduction but are only one part of the Lyme disease transmission cycle, which also depends on reservoir hosts such as white-footed mice.
Clinical Takeaway
The evidence suggests that reducing deer alone is unlikely to eliminate Lyme disease in most communities. Although deer are essential hosts for adult blacklegged ticks, mice and other wildlife continue to maintain the Lyme disease transmission cycle.
Integrated tick management strategies—including habitat modification, personal protective measures, and targeted interventions aimed at reservoir hosts—are likely to have a greater impact than deer reduction alone.
Understanding the relationship between deer, ticks, and reservoir hosts is essential for developing effective strategies to reduce Lyme disease risk.
Related Articles
Learn more about tick ecology, Lyme disease prevention, and reducing your risk of tick bites:
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Backyards at risk of migrating deer ticks
Hundreds of infected ticks found in one yard in Canada
Where don’t ticks thrive?
How do you get Lyme disease?
References
- Kilpatrick HJ, LaBonte AM, Stafford KC. The relationship between deer density, tick abundance, and human cases of Lyme disease in a residential community. J Med Entomol. 2014;51(4):777-784.
- Williams SC, Little EAH, Stafford KC III, Molaei G, Linske MA. Integrated control of juvenile Ixodes scapularis parasitizing Peromyscus leucopus in residential settings in Connecticut, United States. Ticks Tick Borne Dis. 2018;9(5):1306-1316.
- Eisen L, Dolan MC. Evidence for personal protective measures to reduce human contact with blacklegged ticks and for environmentally based control methods to suppress host-seeking blacklegged ticks and reduce infection with Lyme disease spirochetes in tick vectors and rodent reservoirs. J Med Entomol. 2016;53(5):1063-1092.
- Bonneau S, Reymond N, Gupta S, Navarro C. Efficacy of a fixed combination of permethrin 54.5% and fipronil 6.1% (Effitix) in dogs experimentally infested with Ixodes ricinus. Parasit Vectors. 2015;8:204.
Dr. Daniel Cameron, MD, MPH
Lyme disease clinician with over 30 years of experience and past president of ILADS.
Symptoms • Testing • Coinfections • Recovery • Pediatric • Prevention
