Michelle Hersh, Sarah Lawrence College – Lyme Disease and an Unlikely Relationship

Michelle Hersh / Photo Credit: Quyen Dac Nguyen

“We couldn’t detect an effect, which is really surprising,” said Michelle Hersh in a recent article that appeared in the Poughkeepsie Journal.

Dr. Hersh is speaking about the unlikely relationship between ticks and white-footed mice. Though these ticks carry Lyme disease, the population of white-footed mice appears to be unharmed.

Dr. Michelle Hersh is an assistant professor of biology at Sarah Lawrence College. She is a community ecologist who studies the connections between biodiversity and disease, from how tree seedling pathogens help maintain forest diversity to how wildlife communities affect tick-borne disease transmission. She earned her Ph.D. in ecology from Duke University in 2009, and was a postdoctoral scientist at the Cary Institute of Ecosystem Studies and Bard College from 2009-2012.

Michelle Hersh – Lyme Disease and an Unlikely Relationship

Although you may not suspect it, little mice play a big role in regulating our chances of getting a tick-borne illness.

White-footed mice harbor the pathogens that cause Lyme disease, anaplasmosis, and babesiosis. Newly hatched ticks – called larval ticks – can become infected with any or all of these diseases when they feed on mice. And if we are unfortunate enough to be bitten during a tick’s next blood meal, we can also become infected.

 My collaborator Rick Ostfeld at the Cary Institute of Ecosystem Studies has been monitoring populations of white-footed mice since 1995, recording the number of ticks they carry.

Rick and his crew noticed many mice had huge larval tick burdens – with as many as 270 ticks on a single animal. Since ticks are parasites that feed on blood, we predicted heavy tick loads would lead to a decrease in mouse survival.

To test our prediction, we developed a statistical model that analyzed 16 years of data on over 5,500 mice. And we found that our original prediction was, in a word, wrong.

Larval tick infestations didn’t decrease mouse survival during the breeding season or over the winter, not even in years when we expected mice to be under stress – like when the acorn crop was low, and competition for food was likely intense. They weren’t linked to a decrease in mouse population growth either.

Surprisingly, we found that male mice infested with larval ticks were more likely to survive during the breeding season. In short, ticks seem to be getting a free lunch.

From a disease-management perspective, this is bad news. Not only are white-footed mice reservoirs for Lyme disease and other debilitating tick-bore ailments, but they are also indifferent to feeding ticks. Heavily parasitized animals persist and feed yet more ticks, creating a positive feedback loop that favors disease spread.

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