Can You Predict a Bad Lyme Disease Season?

Gazing through the window on a rainy New England afternoon, I can only see a few scattered white patches, sad remnants of our last winter storm. The yellowish grass is a reminder that spring follows winter, sure as can be. Good science aims to discern patterns that are less obvious, and one fascinating idea is about to be put to the test.

Spring is always welcome, but to anyone who lives in this part of the country, the promise of outdoor adventure is tempered by a sobering reality: ticks. The prevalence of tick-borne illness in the United States is on the rise, and a recent story on NPR’s Morning Edition suggests that this year we should brace for the worst. It featured ecologist Rick Ostfeld predicting that 2017 would be a banner season for ticks, specifically the blacklegged tick, which carries Lyme disease as well as other serious, increasingly common infections like Babesiosis and Anaplasmosis.

A blacklegged tick, the carrier of lyme disease, teeters on a blade of grass. Image credit: CDC.

A blacklegged tick, the carrier of lyme disease, teeters on a blade of grass. Image credit: CDC.

Ostfeld’s forecast is based on the fact that 2016 saw an explosion in the population of white-footed mice in the Hudson Valley region where he’s been measuring various pieces of the local environment for a quarter century. In fact, he predicted the mouse surge, too. In the fall of 2015, I paid a visit to Ostfeld’s research site, at the Cary Institute for Ecological Studies, about two hours north of New York City. As we walked through the woods, he pointed to generous piles of acorns beneath trees, rolling down hillsides and strewn across paths. In some places, they were so thick, I had to tread carefully to avoid slipping.

Acorns are a bounty for squirrels, chipmunks and smaller critters, especially white-footed mice. “These things are a remarkable little package of protein, lipids and all sorts of nutrients,” Ostfeld explained, holding up a handful of the nuts. “It’s a bumper crop, so we expect the mouse population in 2016 will be extremely high. And that means the infected nymphal tick population in 2017 should be extremely high.”

Dr. Rick Ostfeld counting acorns in order to predict future mouse and tick populations. Photo credit: author and Gary Streiker.

Dr. Rick Ostfeld counting acorns in order to predict future mouse and tick populations. Photo credit: author and Gary Streiker.

Last year, the first part of the prophecy came true. On the carefully measured forest plots where Ostfeld conducts most of his research, the surge of mice was dramatic. Over the past 25 years, a typical count was 39 mice per hectare. The population fluctuates dramatically, however, and in a lean year it’s about 5 mice per hectare. Last summer, Ostfeld’s research team was counting close to 300. Closer to home, he and his wife Felicia Keesing, a fellow biologist, came home from a short vacation to find their kitchen strewn with mouse droppings and even a few dead mice.

Researchers monitoring white-footed mouse population. Photo credit: author and Gary Streiker.

Researchers monitoring white-footed mouse population. Photo credit: author and Gary Streiker.

To understand why this matters to ticks, you need to appreciate the complex lifecycle of these gruesome, tiny bloodsuckers. When their eggs hatch, in the summer, larval ticks latch on to birds or small mammals, including mice. Mice often incubate the bacteria which causes Lyme disease, Borrelia burgdorferi, and carry other pathogens as well. As they feed, the larval ticks become infected.

Colorized electron microscope image of three corkscrew-shaped Borrelia burgdorferi bacteria. Image credit: CDC.

Colorized electron microscope image of three corkscrew-shaped Borrelia burgdorferi bacteria. Image credit: CDC.

After eating their fill for a few days, the ticks drop off and molt into the next stage, when they’re referred to as nymphs. After resting through the winter, they emerge mid-Spring to feed again. Mice and other small animals are still the preferred meal, but humans are an alternate snack. Bitten by an infected tick, a person may contract Lyme disease. This makes spring and early summer the riskiest time for people, although in milder winters the ticks come out sooner.

Life cycle of ticks. Image credit: CDC.

Life cycle of ticks. Image credit: CDC.

Nymphs and adult ticks also feed on deer, which leads some to argue that lowering deer populations is a viable way to limit the spread of Lyme.

Some advocate that managing the deer population is the best way to stop the spread of Lyme disease. Photo credit: CDC.

Some advocate that managing the deer population is the best way to stop the spread of Lyme disease. Photo credit: CDC.

Not everyone is convinced that the acorn connection will hold. Kirby Stafford, an entomologist who studies tick control methods for the state of Connecticut, notes that prior predictions of tick infestations, including a warning in 2012, did not come to pass.  “As for this being a big year for ticks, it’s hard to tell. The adult tick activity this mild winter is not directly linked with upcoming summer tick activity,” Stafford says.

Another big caveat: Ostfeld is counting mice on his research plots, but we don’t know if there was a surge throughout the region. He does note that oak tree “masting,” the periodic shedding of large numbers of acorns, tends to be synchronized across thousands of square kilometers.  

Even if this year doesn’t see a surge in ticks, it doesn’t mean there’s no pattern. We just can’t see it yet. There are hints the patterns go even deeper. Why did 2015 see a bumper crop of acorns? No one really knows, but acorns start developing nearly two years before they fall, so it probably had something to do with conditions in the spring of 2014.

If we understood all the pieces, then in theory we might have predicted a tick surge a full three years in advance. And if we knew what caused the spring of 2014, what then? How far back could we go?

Ecology is a tough puzzle, perhaps even harder in a world of fast-changing climate. Will shorter winters help ticks breed faster, or spread more widely? It’s certainly possible, but then… maybe warmer weather leads to more drought, which is bad news for ticks, which rely on moisture.  

We’ll check back on this prediction, by summer. In the meantime, be careful when you go outside.

Watch tonight’s PBS NewsHour to see our report on another piece of the Lyme disease equation: deer. They’re spreading fast through suburbia. What can stop them?

 

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Banner image credit: CDC.