Climate change is bringing more ticks to the Northeast. But it’s not the full story.

On a warm sunny day in April, Patti Casey and Eliza Doncaster, with the Vermont Agency of Agriculture, were walking through a beautiful field in Brownsville.

They were suited up in personal protective gear, like they were ready to handle something toxic in a laboratory.

“We have this cool 1-meter square white flag, and we’re either dragging it or flagging it through vegetation, looking for ticks,” Casey explained as she started a pass.

Casey and Doncaster are part of the Vermont Agency of Agriculture’s tick surveillance team.

While most people avoid ticks, this team visits every town in Vermont in the spring and fall every five years to find them — lots of them.

At one point, Casey described dragging a piece of cloth across a field in Bennington and finding dozens of ticks — mostly blacklegged or deer ticks — at the other side.

Abagael Giles Vermont Public

Over the last 20 years, ticks and the diseases they carry, like Lyme disease, have spread rapidly in Northeastern states, including in Vermont.

Scientists say human-caused climate change is one piece of the puzzle, but it’s not the only thing driving their growth here.

The question of why ticks and the diseases they spread appear to be booming is of great interest to scientists at the state.

“We’re starting to accumulate data over time, so that we can see places where ticks are moving into, or perhaps declining,” Casey said. “Are they moving into higher elevations? Areas where there’s more suburban development?”

When they find a tick while sweeping a field, Casey and Doncaster pick it up with tweezers and drop it into a little vial of alcohol.

This vial of ticks will go back to the state laboratory to get tested for diseases.

Blacklegged ticks, also called deer ticks, carry the bacteria that causes Lyme disease.

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But they also carry a suite of other bacteria, viruses and parasites that cause other illnesses.

Natalie Kwit, the state public health veterinarian for the Vermont Department of Health, said people are now contracting these infections in Vermont year-round.

Eliza Doncaster Vermont Agency of Agriculture, Food & Markets, courtesy

“[The tickborne illnesses found in Vermont] in order of most common to least common are Lyme disease, anaplasmosis, babesiosis, Borrelia miyamotoi disease [or hard tick relapsing fever] and lastly Powassan virus,” Kwit said.

When Lyme was first reported in Vermont a little more than 20 years ago, it was fairly rare. Since 2013, the state has seen more than a thousand new cases every year.

The Centers for Disease Control considers Vermont a “ high-incidence state” for Lyme, anaplasmosis and babesiosis.

“What we are seeing, like other states, with these tick-borne illnesses nationally, is an upward trend,” Kwit said.

It’s a problem she and the health department are very worried about — and they’re not alone.

Climate change is one major driving factor, said Rick Ostfeld, a researcher with the nonprofit Cary Institute of Ecosystem Studies in Millbrook, New York.

“There is evidence for the northward expansion of these tick-borne illnesses and the ticks themselves being contributed to by climate change,” he said. “And I would say that’s fairly strong evidence.”

Ostfeld has been doing some of the longest-running surveillance of tick populations in the Northeast.

Vermont now sees about a week more freeze-free days than in the 1970s due to human-caused climate change. And, according to the National Atmospheric and Oceanic Administration (NOAA), the growing season is expanding by almost four days a decade.

And while winter temperatures are getting milder, Ostfeld said that’s less significant for blacklegged ticks than the shrinking of that freezing period.

Ostfeld said it makes it easier for blacklegged ticks to find hosts to feed on at critical stages of their life cycle.

Nymphs are more successful at surviving to adulthood if they can quickly find a rodent to feed on in the spring, and later winters mean adult female blacklegged ticks have a better chance at finding a final host before they hunker down for the season in the duff. That last feeding is particularly important if they are to be successful in laying eggs.

However, Ostfeld said humans are also driving the spread of ticks in other ways.

“One of the main factors that is contributing to the expansion of the ticks is this habitat fragmentation, destruction and suburbanization, which removes some of the natural biodiversity that we have in animal communities,” he said. “So some of the predators disappear.”

Blacklegged ticks aren’t choosy and will happily feed on most mammals. Despite their name, small rodents like chipmunks or white-footed mice as well as shrews are particularly good hosts for them.

They’re also the animals that infect the ticks with pathogens like the one that causes Lyme.

When humans allow suburban sprawl to break up big chunks of forests, predators like bobcats, foxes, weasels and raptors aren’t able to thrive, which means they can’t keep rodent populations in check.

Mice and chipmunks are especially well adapted to these disturbed ecosystems, like small patches of forest or undeveloped land between homes or subdivisions.

“We are inadvertently filtering out the protective species and allowing in the dangerous species when we fragment the landscape and erode habitat quality,” he said.

And while total forest cover in Vermont has increased over the past few decades, a 2014 analysis from Vermont Fish and Wildlife Department found that contiguous blocks of forest have decreased in size, as a result of residential and other development, road construction and other forms of permanent land clearing.

Ostfeld said scientists think this is part of why we’re seeing tick-borne illnesses spread radially, into southern and midwestern states, and not just northwards.

And while forest fragmentation and climate change are two big factors, he said researchers know there are still drivers they don’t understand.

But he said there’s good news: Climate change and sprawl are things that humans can address through public policy.

Eliza Doncaster Vermont Agency of Agriculture, Food & Markets

In the meantime, Natalie Kwit with the Vermont Department of Health said it’s critical that Vermonters — especially those who work outside — take steps to protect themselves and find ticks early.

“I think a simple way to remember it is: ‘protect, check, remove, watch,” Kwit said.

That’s protect yourself by avoiding areas where ticks live and wearing protective clothing and EPA-approved insect repellents, treat your pets for ticks seasonally, and if you work outside, consider treating your work clothes with permethrin.

Check yourself and your clothes every time you spend time outdoors in tick habitat and consider putting your clothing in the dryer on high for 10 minutes when you come in from the woods.

More from Vermont Department of Health on how to protect yourself from ticks

Additionally, Kwit said taking a shower soon after being outdoors is also a proven way to reduce your risk of getting sick.

Lastly, she said, “If you do have an attached tick, remove it promptly.”

Kwit recommends using fine-tipped tweezers for this.

Then, she said, it’s critical to monitor yourself for signs and symptoms of illness or rash for at least 30 days.

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