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A New Explanation for the Steep Rise in Lyme Infections

Lyme Disease

Story at-a-glance -

  • Cases of Lyme disease have skyrocketed in North America over the last decade.
  • A new study seems to prove that monitoring deer populations to assess Lyme disease risk is ill-advised. In fact, reducing the number of deer in an area has little or no impact on the number of Lyme infections.
  • The study shows fox and coyote populations determine the rate at which Lyme disease spreads.
  • Up to 90 percent of infected ticks pick up the Lyme bacteria from small mammals, primarily rodents. Red fox prey on these small mammals. Coyotes are natural predators of foxes.
  • Where coyote populations have increased, fox populations have decreased, and Lyme disease has proliferated.

By Dr. Becker

A fascinating new study1 published in a recent issue of the journal Proceedings of the National Academy of Sciences may explain why we are seeing so much more Lyme disease these days.

In fact, over the last decade there has been a greater than 1,000 percent increase in Lyme cases in some locations across North America.

The new study, completed at the University of California, Santa Cruz by ecologist Taal Levi, reveals that using deer populations to estimate Lyme disease risk is a misguided approach.

According to Discovery News:

Lyme disease is caused by a type of bacteria, which is transmitted to people by ticks in the nymph stage. And because ticks rely on deer to reproduce, conventional wisdom has long held that revived deer populations are responsible for a surge in the disease.

The new study suggests deer have nothing to do with rising rates of Lyme infection, and that populations of foxes and coyotes are the ones to watch.

Why the Red Fox is So Important in Controlling Lyme Disease

Programs to reduce deer populations have had little impact on the spread of Lyme disease. And coincidentally, studies indicate the vast majority (up to 90 percent) of infected ticks acquire Lyme bacteria from small mammals -- primarily mice, chipmunks and shrews.

Study author Levi and the other researchers involved in the project used real-world examples and computer models to investigate the relationship between ticks, their infectious small mammal hosts, and the larger animals who prey on those small mammals.

Surprisingly, study results showed no link between the number of deer and the amount of ticks. What was revealing was the connection between the number of coyotes and red foxes, and the number of ticks.

Here’s why:

  • Foxes prey on the mice, chipmunks and shrews responsible for harboring infectious ticks.
  • The coyote is a natural predator of the fox.
  • Coyote populations in the northeast and Midwest have seen huge increases in the last few decades. And not only do coyotes prey on foxes, foxes will not breed in areas where there are coyote populations.
  • As quickly as the number of coyotes has escalated, the number of foxes has dropped. In Minnesota, where coyote numbers are up over 2,000 percent since 1982, the number of foxes is estimated to have dropped 95 percent.
  • The lack of fox populations has allowed proliferation of the small mammals that transmit the Lyme bacteria to ticks.

So when fox numbers are up, the risk of Lyme disease is down. But when the number of foxes plummets, Lyme disease risk soars.

According to study author Levi:

“Small changes in predation can lead to large changes in Lyme disease risk. This argues for top predator conservation, or at least that there are unintended consequences for people of losing them.”

No Easy Answers

Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, NY, believes introducing wolves in areas of high coyote populations would control not only the spread of Lyme infections, but also other rodent-related diseases.

While that is unlikely to happen, the new study illustrates just how important predators are to healthy ecosystems.

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