Why Ecologists are Going Nuts Over Preventing Lyme Disease

For people who are concerned about contracting Lyme disease
by Greg Lee

acorns

“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 gave a stunning presentation on the ecological factors that affect infected tick populations at a recent conference on Lyme disease hosted by the National Academy of Sciences Institute of Medicine Committee. 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 different 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.

The size of the forest affects which animals transmit infections to ticks
In other studies, mice and chipmunks were shown to have the highest transmission rates of infection to ticks. There are specific landscapes which support increased mice and small rodent populations. 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 crop of 2010 is estimated to be the largest one in 20 years in New York and other surrounding areas. Early indications in Maryland also show an abundant crop. As mice, chipmunks, and deer increase in numbers with greater acorn production, this leads to a greater populations of ticks. In 2011, more acorns mean greater numbers of small rodents that will be carrying greater numbers of larval-sized ticks. This can lead to a greater risk in 2012.

2012 is predicted to be a dangerous year for getting Lyme disease
The larval sized ticks of 2011 grow into nymph-sized ticks in 2012. Since nymph ticks are believed to be the major reason for people contracting Lyme disease, 2012 is predicted to have greater numbers of hard-to-see infected nymph ticks2. 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.

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

Acorn image courtesy of Pearson Scott Foresman of Wikimedia Commons

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