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Wisconsin's winters are getting warmer, and it's altering our agriculture, economy, health, and way of life. On the heels of Wisconsin's warmest winter ever, Thin Ice explores the impacts.

As Wisconsin winters warm, it’s the end of tick season as we know it

a tick warning is posted below a sign for the hank aaron state trail. it's a sunny late winter day.
Lina Tran
At Three Bridges Park, a sign warns users that ticks may be in the area.

It’s a gorgeous late February day when I visit the Midwest Center of Excellence for Vector-Borne Disease, based at the University of Wisconsin-Madison: Sunny, high 50s. Research specialist Tela Zembsch woke up and thought it was a great day for a walk.

But she added this to the forecast: “The rule of thumb that we always tell people is that when it’s nice enough that you want to go out, so do the ticks.”

Bitter cold winter temperatures keep disease-carrying vectors like ticks and mosquitoes in check. In Wisconsin, the black-legged tick — also known as the deer tick — can spread Lyme disease, which can lead to symptoms including chills, fever, fatigue and a bullseye-like skin rash, and for some, chronic illness. Mosquitoes can spread West Nile virus or Jamestown Canyon virus, which is transmitted by “snowmelt mosquitoes” that emerge with the first signs of spring.

Winter is Wisconsin’s fastest-warming season and the state has regularly had unusually warm winters since the late 90s, giving bugs more time for activity and breeding. Winter temperatures are expected to continue warming 5 to 11 degrees Fahrenheit over the next 25 years, so that by mid-century, winters will be milder and about a month shorter than they are now. That means more chances of getting bitten.

a graphic of the United States winter warming indicates wisconsin has among the fastest-warming winters nationwide
A Climate Central analysis of winter warming indicates that Wisconsin has among the fastest-warming winters nationwide.

This past winter was Wisconsin’s warmest in state history, a stretch of balmy months that led residents to submit tick identification requests to the state Department of Health Services as early as mid-February. Usually, people start sending in their tick photos to DHS for identification in late March or early April, said DHS vector-borne disease epidemiologist Rebecca Osborn.

“It’s quite a bit earlier than normal,” Osborn said. “We’re just seeing that beginning of the tick season inch earlier and earlier and also last longer and longer into the fall. It’s shortening that timeframe where the temperatures are cold enough that the ticks aren’t active.”

That has the DHS conducting surveillance for ticks both late into the season and as early as possible. Osborn said the agency's messaging to the public is also evolving, encouraging residents to think about proper prevention in times of the year that were previously worry-free.

a cage is filled with mosquitoes
Lina Tran
At the Midwest Center of Excellence for Vector-Borne Disease, colonies of mosquitoes are maintained to enable research on pest control strategies.

Other factors like changes in land-use or wildlife populations have also upped the chances of exposure. Ticks don’t carry the bacterium that causes Lyme on their own; they have to draw blood from wildlife hosts like deer or mice first. Improved diagnosing and reporting among doctors have also added to the growing number of cases.

To complicate things, Lyric Bartholomay, co-director of the Midwest Center of Excellence for Vector-Borne Disease, or MCEVBD, said more research is needed on how cold-blooded animals like mosquitoes will respond to the fluctuating, potentially stressful winter conditions that have become more common as the climate changes.

“This is anthropomorphizing, of course,” Bartholomay said. “But should I stay quiescent? Or is it time to become active? It might be stressful to animals that are out there in the environment and should really be resting right now.”

a table shows lyme disease cases rising steeply since 1990.
Data from Wisconsin DHS shows lyme diseases rising steeply since 1990.

In Wisconsin, most vector-borne diseases are spread by ticks, and cases are rising. Lyme disease cases reported annually have more than doubled in the last 15 years. (The black-legged tick can also spread anaplasmosis and babesiosis, but Lyme disease is, by far, the most common.) Plenty more cases go unreported, and one expert estimated the state could have an estimated 30-50,000 cases each year. Nationwide, Wisconsin is a Lyme disease hotspot.

Dr. Victoria Gillet, a Milwaukee primary care doctor, said it’s time to rethink “tick season.” Residents need to be practicing prevention for ticks and mosquitoes nearly year-round. Typically, ticks in Wisconsin are their most active from May to November.

“As opposed to thinking, ‘I need to start thinking about ticks in a certain month,’ [it’s] ‘I need to start thinking about ticks at a certain temperature,’” said Gillet, who is also a member of the advocacy group Healthy Climate Wisconsin. “That’s not a way we’ve had to think before.”

Ticks are active above 40 degrees, particularly when there’s no snow cover, which works like a natural shield.

three small ticks are visible on a pair of white pants outside
Minnesota Department of Health
Light-colored clothes make it easier to spot ticks after spending time outdoors. Two adult female blacklegged ticks (left) and one male blacklegged tick (right) are shown here.

So Gillet is shifting too, preparing herself to see Lyme disease in the clinic early this year. She suggested health care providers will need to make similar adaptations as winters warm.

“I sort of expect that I’m going to be diagnosing Lyme disease in the February-March time of year in a way that I would never have done before,” she said.

Partly because the ticks are out — and because people just might not be thinking about prevention right now. Gillet noted that people who live or work outside, such as unhoused individuals or agricultural workers, are disproportionately exposed and often have poor access to health care.

Listen to the conversation with Dr. Victoria Gillet of Healthy Climate Wisconsin.

The next frontier in tick prevention

Until now, most prevention has focused on what individuals should do: Use repellents, wear light-colored clothing, tuck pants into your socks and check yourself for ticks.

“All those are great things for people to do, but they’re not enough, clearly,” said Susan Paskewitz, an entomologist and co-director of the MCEVBD. “We’re not really making any visible difference in this ever-increasing risk to our population.”

a fridge contains several large glass jars
Lina Tran
A fridge at the Midwest Center of Excellence for Vector-Borne Disease contains colonies of ticks used for research.

So the center is researching a number of community-based solutions. It’s a common strategy for mosquitoes — many Midwestern municipalities run mosquito abatement programs — but the approach is fairly novel in tick prevention.

Currently, they’re partnering with mosquito abatement districts in the Chicago area to see if they can piggy-back on the mosquito work and test strategies for tick control. Such tools could be used in busy public spaces like parks and baseball fields.

“That would allow us to spray a natural product over a larger area and see if that could effectively reduce the tick densities in those areas,” Paskewitz said.

At the center, housed in an upper floor of a UW-Madison science building, researchers maintain busy colonies of ticks and mosquitoes to test these treatments.

A fridge holds big glass desiccator jars, each containing vials filled with ticks at different life stages. The ticks are long-lived and low-maintenance. They need just one blood meal at each life cycle. (They’re also silent. Once, a researcher working with the center tried to capture the sound of a tick walking across a wooden dowel, but they didn’t make a peep.)

Mosquitoes, on the other hand, are more work. They can live up to three weeks, and there’s a constant cycle of chores, from counting tiny larvae to weekly feedings and cage-cleanings.

Sorted into cages by species, the mosquitoes live in a walk-in cooler that is humming. Inside, it’s warm and humid — just how the mosquitoes like it. It makes it easy to see why they're buzzing more in a warmer Wisconsin.

Want to help researchers study tick exposure?

a screenshot shows different options in the tick app: daily log, report a tick, tick activity, remove a tick, tick prevention, and tick 101
The Tick App
The Tick App offers tick safety and identification resources, while inviting users to share their tick encounters with researchers.

The Tick App shares information about tick safety and invites users to log their tick encounters. It’s a project of the Midwest Center of Excellence for Vector-Borne Disease, as well as Columbia University and Michigan State University.

Data from the project is helping researchers better understand the behaviors that shape people’s exposure to ticks.

People in the Midwest, for instance, tend to mow their own lawns, while users on the East Coast were more likely to hire someone to do so. And, East Coast users more commonly used fences to keep animals out, while the Midwesterners were more likely to actively attract wildlife to their yards.

These insights could help tailor public health messaging. “That does seem to be regional and even state-specific, in some cases,” Paskewitz said.

Contribute to their efforts by joining the app community: https://tickapp.us/

Lina is a WUWM news reporter.
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