“White tailed deer are the one indispensable piece in the Lyme [disease] puzzle¹“
Exploding deer populations have been blamed for greater tick populations and increased incidences of Lyme disease. Deer are believed to spread infected ticks. Some strategies for reducing infected ticks on deer include applying pesticides to ticks using deer feeders. Other strategies employ hunters to reduce deer populations.

Could you reduce new cases of Lyme disease by getting rid of white-tailed deer?

In one study, eliminating deer herds results in decimated tick populations
Richard Ostfeld, Ph.D. Disease Ecologist with the Cary Institute of Ecosystem Studies presented the ecological factors that affect infected tick populations at a recent conference on Lyme disease. He cited one study on Monhegan Island off the coast of Maine where hunters were used to reduce a deer herd from a few hundred to zero. The impact of this hunt had a significant impact on two of the four tick stages: egg, larvae, nymph, and adult. The larval and nymph tick populations were reduced to near zero. Aside for humans and their pets, no other host species live on the island². Similar studies produced different results.

In other studies, reducing deer herds did not significantly impact tick populations
In numerous studies in Connecticut, Massachusetts, New Jersey, and New York, hunters were used to cull deer herds from large numbers to very small numbers. Unfortunately, reduced deer herds did result in a significant reduction of nymph tick populations. Incidence rates of Lyme disease were not significantly reduced, either. Why are ticks able to persist despite reduced herds of deer?

Ticks don’t care what animals that they can feed on
The larval and nymph stage of the black legged tick is know to feed on 41 species of mammals, 57 species of birds, and 14 species of lizards. The adult stage is known to feed on 27 species of mammals and 1 species of lizard². Dr. Ostfeld gave several other theories as to why ticks are able to survive despite lower numbers of deer.

Ticks survive by ganging up on the remaining deer
The remaining deer in these study areas were found to have increased concentrations of ticks. Another reasons that ticks are able to survive from year to year is that some animals allow ticks to stay on their bodies.

Some animals are not good at grooming and killing ticks
In a lab study, approximately 50% of larval ticks were able to successfully feed off white footed mice. In another study which counted the number of larval ticks found on wild animals, the average mouse had about 25 larval ticks, the average gray squirrel had about 150, and the average opossum had about 250. Fortunately, opossums were found to be highly effective at grooming and killing attached ticks². Also, the size of the forest affects which mammals carry and transmit Lyme disease to ticks.

Fragmented forests usually have greater populations of mice
In other studies, mice and chipmunks were shown to have the highest transmission rates of infection to ticks. In Dutchess County New York, fragmented landscapes tend to have a greater population of mice. Dr. Ostfeld estimated that forest fragments less than five to eight acres in size have a greater risk of infection by 300% – 400%². What other critical factor supports a growing tick population?

Ecologists show a high correlation between acorn abundance and tick populations
Dr. Ostfeld reported that the acorn production of 2010 was one of the largest crops in 20 years in New York and other surrounding areas. In Maryland, 2010 was the largest acorn crop in over 32 years according to the Department of Natural Resources. As white-footed mice³, chipmunks, and deer have increased in numbers with greater acorns to feed them, these hosts are predicted to lead to greater populations of nymph-sized ticks in 2012.

2012 is predicted to be a dangerous year for getting Lyme disease
Since increased numbers of hard-to-see nymph ticks² are believed to be the major reason for people contracting tick infections, 2012 is predicted to have greater numbers of Lyme disease cases. This can result in a much greater risk for contracting Lyme disease especially in areas of abundant acorn production.

There are many factors which influence the numbers of infected ticks from year to year
Increased populations of deer are only one of many factors in spreading Lyme disease. Greater acorn production leads to more rodents and deer. These rodents end up carrying and infecting more larval ticks. More deer and other large animals carry and infect tiny nymph-sized ticks that end up transmitting Lyme disease to humans.

Next Step: Want to learn more about healing Lyme disease? Click here to find out about our evening lecture at 6pm on Monday April 2nd, “Getting Rid of Lyme Disease” in Frederick, Maryland.

– Greg

1. Piesman, J. F. 2002. Ecology of Borrelia burgdorferi sensu lato in North America. In Gray, J. S., O. Kahl, R. S. Lane, and G. Stanek, editors. (eds.). Lyme Borreliosis-Biology, Epidemiology, and Control pp. 223-249.CABI International. Trowbridge, England.
2. Richard Ostfeld. A Systems Approach in Understanding Tick-Borne Diseases: People, Animals, and the Ecosystem. National Academy of Sciences, The Institute of Medicine Committee on Lyme Disease and Other Tick-borne Diseases: The State of Science Conference. October 11-12, 2010, Washington, DC
3. Richard Ostfeld. Ecological Drivers of Tickborne Diseases in North America. International Conference on Emerging Infectious Diseases. March 11-14, 2012, Atlanta, Ga.