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Are Deer the Culprit in Lyme Disease? Byline By THE EDITORS
Jim Cole/Associated Press
In a Room for Debate forum earlier this week on ticks and Lyme disease (“More Ticks, More Misery”), some readers reacted by focusing on the issue of a link between deer and the black-legged tick, commonly called the deer tick. A few sought more explanation from one scientist in the forum, Richard S. Ostfeld, for his observation that “several recent studies in New York and New Jersey have found no connection between populations of deer and ticks.” Excerpts from the readers’ comments on Dr. Ostfeld’s findings are followed by a response co-written by him and another contributor to the forum, Felicia Keesing, a biologist.
Readers: The Evidence of a Deer Link
Looks like Richard Ostfeld needs to provide the rest of us with a reading list of those studies that shows NO relation between deer abundance and tick numbers. …
I remember a case of an island on the North Shore above Boston that had a very high prevalence of Lyme in its human residents. The citizens finally said “enough” and had all the deer residents removed, and Lyme disease did decline. Ostfeld tells us of cases of no relation between deer and ticks; are there no cases where there is such a relation?
— Eric Olson
… While it’s true that the small mammals are the hosts for the disease, it is our overabundance of deer that allows the overabundance of the deer tick (black-legged tick, ixodes scapularis).
The mice help the ticks in the first year of their two-year life cycle, but the deer give the transportation and last blood meal for the adult tick.
Studies have shown that if you reduce the deer density to about 10 per square mile, you dramatically reduce the tick abundance, and Lyme disease is dramatically reduced also.
Studies of this nature include Monhegan Island, ME; Great Island, MA; and Mumford Cove, CT; and there are more. In the first two cases, they removed essentially all the deer and Lyme disease dropped to non-existent. But it is not necessary to remove all the deer, just get them down to 10 per square mile. This is what happened at Mumford Cove, and it dropped the Lyme cases from a peak of 30 per year, down to two or less.
— Socrates
I would like to see some sources for Richard Ostfeld’s allegation that there is no connection between populations of deer and ticks.
I have a 6 acre property here in central Virginia, consisting of woods and fields, surrounded by more woods and fields. 2 acres of that is fenced in. I am a deer hunter and an amateur naturalist, so I pay extremely close attention to deer behavior and population dynamics all year round. The deer generally stay out of the fenced area for lack of anything special to entice them within, and I can walk through the tall grass every day and emerge without a single tick. …
Outside of the fence, tick densities are enormous. When I follow a deer trail or sit down in a whitetail’s regular bedding area, I will find my jeans crawling with ticks within minutes.
— Jackson Landers
Response: Reducing Herds Won’t Reduce Risks
Felicia Keesing is a biology professor at Bard College, and Richard S. Ostfeld is senior scientist at the Cary Institute of Ecosystem Studies.
Millions of people living in Lyme disease zones, including current patients (who can get infected repeatedly), will benefit from a better understanding of risk. With no vaccine currently available, imperfect diagnostic tests, and controversy over appropriate treatment, prevention is a critically important strategy, and avoidance of infected ticks is the most effective means of prevention. Our research is aimed at understanding where the hordes of infected ticks come from and why. This information is fundamental to prevention.
One theme is the role of deer in determining tick abundance. In the first 15 years after Lyme disease was discovered in coastal New England, several studies showed that many adult ticks feed on deer, and researchers surmised that deer were critical to the tick life cycle. When researchers eradicated deer from New England islands, tick populations crashed.
Unfortunately, nature has a way of being more complex than first thought.
The key to the Lyme disease problem seemed at hand. Unfortunately, nature has a way of being more complex than first thought. One complication is that 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 recent studies (e.g., Jordan and Schulze, 2005; Ostfeld et al., 2006; Jordan et al., 2007 — see citations below) 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 play no role in infecting ticks. Ticks become infected with the Lyme disease bacterium by feeding on small mammals such as white-footed mice, chipmunks, and shrews. And mice play the additional role of increasing tick survival — they are at the opposite extreme from opossums, which kill the vast majority of ticks they encounter. When our group compared the importance of deer, mice, and climate in determining the number of infected ticks over 13 years in southeastern New York State, mice were the winners hands down.
Other compelling reasons exist for controlling deer populations, such as reducing vehicle accidents and increasing forest regeneration. But, in many Lyme disease zones, reducing the deer herd is unlikely to substantially affect tick abundance. Reducing mice is more likely to be effective.
This is best accomplished by allowing natural predators like weasels, coyotes, foxes, and owls to do the job. And the best way to increase their numbers is to maximize the size of forest patches. A number of other ways of reducing risk are currently being tested by our group and others, including the use of natural products such as soil fungi to kill ticks without adverse environmental impacts and the use of vaccines against Lyme disease that can be delivered to wildlife.
Citations:
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.
