Lately, Shannon LaDeau and her colleagues have had unwelcome visitors at their office in New York’s Hudson Valley: ticks, crawling up the building and trying to get through doors.
“Which is kind of alarming,” said Dr. LaDeau, a disease ecologist at the Cary Institute of Ecosystem Studies who studies the arachnids and the pathogens they carry.
As winters get warmer, ticks of several kinds are flourishing. Deer ticks, known for transmitting Lyme disease, are moving farther north. The longhorned tick, which came from overseas, has gained a foothold on the East Coast and begun moving west. Gulf Coast ticks have made it to states like Connecticut and Indiana. The lone star tick, which can make people allergic to red meat, is fanning out from the South and has been found as far as Canada.
And even in places long accustomed to them, ticks are becoming more numerous and active for longer stretches of each year.
Why is this happening, and how can you protect yourself? We asked the experts.
What changes are researchers seeing?
Marc Lame, an entomologist and clinical professor emeritus at Indiana University’s School of Public and Environmental Affairs, put it simply: “There are more and different types of ticks around than there used to be, and I don’t see that stopping anytime soon.”
The spread of individual species can be difficult to track. The longhorned tick, for example, was not identified in the United States until 2017, but a recent study confirmed that it was here as early as 2010.
But there is evidence they are traveling across North America quickly: On its northern front in Canada, the deer tick — also known as the blacklegged tick — is believed to be extending its range more than 20 miles per year, said Catherine Bouchard, a research scientist at the Public Health Agency of Canada.
The movement of the lone star and longhorned ticks particularly worries Laura Goodman, an assistant professor in Cornell’s Department of Public and Ecosystem Health, because it takes so few to establish themselves in a new place. Both can breed in huge numbers — a single longhorned tick can lay 2,000 eggs. Female longhorned ticks can even reproduce alone, essentially cloning themselves through a process called parthenogenesis.
Some ticks are also behaving differently. Dr. Bouchard said that when she began studying deer ticks in southern Quebec around 2007, they stopped looking for someone to bite by October. Now, it is not unusual for her to see them in December.
Is this because of climate change?
Yes, in large part.
There are other factors at play, such as deer populations recovering along the East Coast after years of decline. But “there’s a clear scientific consensus that climate change is playing a role,” said Michael Dietze, who leads Boston University’s Ecological Forecasting Lab.
Because temperatures are rising, ticks and the animals they travel on — like deer and mice — are likelier to survive winters, and new territory is becoming hospitable.
Climate change also explains why ticks are emerging earlier in the spring and staying active later in the fall: They can be active whenever the temperature is above about 39 degrees Fahrenheit.
What are the health implications?
As deer tick populations grow, cases of Lyme disease, the most common tick-borne illness in the U.S., appear to have risen.
Data from the Environmental Protection Agency suggests Lyme cases have been increasing for 30 years. The agency warns that changes in how the disease is defined and reported make it hard to confirm that this reflects a real trend. But while scientists may not have exact numbers, Dr. Dietze said rates had “definitely increased.”
Researchers are seeing indications of increases in other tick-borne diseases, too, including anaplasmosis, babesiosis, ehrlichiosis and Rocky Mountain spotted fever. All are treatable if caught early, but they can sometimes be fatal. Also on the rise is alpha-gal syndrome, an allergy to red meat usually caused by the lone star tick.
Experts worry that, as different tick species feed on the same animals in newly overlapping habitats, they could share diseases.
A single tick may also become more likely to carry multiple pathogens — meaning someone could get Lyme disease and receive treatment without their doctor realizing they also need treatment for, say, babesiosis.
Dr. Goodman had been working with Defense Department funding to develop one test for numerous tick-borne illnesses, but the Trump administration halted her grant.
How can you protect yourself?
Try to prevent bites, and keep in mind that ticks don’t only live in the woods. While deer ticks tend to favor wooded areas, they and other species also frequent parks and lawns — and some even approach buildings.
Health authorities recommend wearing long pants and long sleeves when you might be exposed, and tucking your shirt into your pants and your pants into socks or boots. Repellents containing DEET are effective on skin, and permethrin on clothing.
Once you get indoors, check yourself carefully — and check children and pets as well, said Negar Elmieh, a scientist at Canada’s National Collaborating Center for Environmental Health. Ticks may bite anywhere, but they are often drawn to warm, damp areas such as the backs of knees and the groin, underarms, ears and scalp.
If you find a tick, remove it. Companies sell many removal devices, but tweezers work fine. Grasp the tick where it meets the skin and pull steadily, without jerking or twisting. Then disinfect the area and wash your hands.
Most tick-borne diseases, including Lyme, offer a grace period: You can prevent transmission if you remove the tick within a certain number of hours.
But this may not be true for all diseases. So if you have any unusual symptoms, see a doctor.
Maggie Astor covers the intersection of health and politics for The Times.
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