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The Surprising Ecology of Lyme Disease

June 26, 2013

This week’s New Yorker has a fascinating and important story by science writer Michael Specter about Lyme disease in which he reports on who has it, treatment options, the ethics of unproven treatments, patients’ rights, and so on. And if you’re anything like us, you read it and thought, “Oooh! Ticks!” We called up a major source in the story, Richard Ostfeld, PhD ’85, now a disease ecologist at the Cary Institute of Ecosystem Studies, near Poughkeepsie and right in the heart of the Northeast’s Lyme disease country.

At Berkeley, Ostfeld primarily studied the ecology of small rodents. Now he’s a leading expert on Lyme disease as it exists in nature. How did this happen? When Ostfeld joined the Cary Institute in 1990, he was out trapping white-footed mice and trying to understand what they had to do with controlling the invasive gypsy moth (lots, it turns out). But while he was doing that, he noted that the mice were just covered in ticks, 20-30 ticks per mouse. (Here, we pause to consider what it must be like counting the ticks on mouse after mouse.)

Ostfeld did some research and found just the sweetest results a scientist can turn up: The relationship on mice and tick-borne disease was strong but not well studied. Of such things careers are made.

A brief digression: Anyone who’s visited Lyme country knows to watch out for the main species that transmits the disease, deer ticks. But, Ostfeld says, there is no such species. When the disease was first being studied in the late 1970s and early ’80s, it was found in an unfamiliar tick, which was promptly named Ixodes dammini, or deer tick. It turns out, however, that it wasn’t a new species at all, but one discovered further south 150 years previously and already named, Ixodes scapularis, the black-legged tick.

It’s even more of a misnomer, Ostfeld says, because at the time there were a number of studies that suggested the main wild reservoir for Lyme disease was deer. Which turns out not to be true: It’s mice. Specifically, white-footed mice (Peromyscus leucopus), found throughout the eastern United States.

So, to review: Lyme disease is found in the white-footed mouse and transmitted by the black-legged tick. Got it?

The mice are not only very good at catching and transmitting the Borrelia bacteria that cause Lyme disease, they’re very bad at destroying ticks when they groom themselves. The mice aren’t merely a reservoir of the disease — they increase the number of ticks.

Which leads us to acorns.

It turns out, you can predict the future incidence of Lyme disease by the number of acorns that fall. White-footed mice eat acorns, so more acorns means more mice and more ticks.

And the mice can live anywhere, Ostfeld says. They live in old growth forests, in trashed urban woodlots, in areas overrun by exotic plants, in fragmented suburban forests and, Ostfeld says, “in my kitchen and basement.” Mice simply aren’t terribly bothered by the human disruption.

As a consequence, the worst places for Lyme disease may turn out to be suburban forests and small woodlots, because there, human activity has driven out other species, in particular foxes and chipmunks. The foxes reduce Lyme risk because they’re very good at eating mice and as the number of mice go down, the number of ticks on each mouse decrease. But the chipmunks might be even more interesting. They may be tick-riddled, “hoovering them up,” as Ostfeld puts it, and thus disease carriers. But due to a quirk of biology they pass the disease on to half as many ticks.

And what are the best places for avoiding Lyme disease? Ostfeld explained that if you graph the number of people newly diagnosed with Lyme disease by age, there’s a big dip right between the ages of around 12 into the late 20’s — a demographic of people who aren’t in out in the woods, but in schools, cars, new jobs, movie theaters, and: “People have interpreted this as being caused by malls,” Ostfeld says. “But that’s a little glib.”

–Brendan Buhler

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