Ticks from distant countries are turning up in the United States after hitching rides on international travelers, a new study found.
The study, published Friday in the journal iScience, documented seven nonnative ticks that were carried by humans to Connecticut from 2019 to 2023. The little arachnids came from Belize, Costa Rica, Germany, Guatemala, Poland, Scotland and Tanzania.
“The main finding is that we are facing an increasing risk of invasive ticks in the United States,” said Goudarz Molaei, a study author who is a medical entomologist with the Connecticut Agricultural Experiment Station. He added that the rate of unintentional introductions of nonnative species appeared to be rising.
That could increase the number of tick-borne pathogens in the United States — which, the study notes, was already on the rise — and heighten disease risks for people across the country.
In recent decades, ticks of all kinds have been flourishing in the United States. Some originated abroad, like the Asian long-horned tick.
The arachnids have been traveling domestically, too. Ticks from the Gulf Coast are moving north. So is the lone star tick, a southern bug whose bite may trigger an allergy to red meat.
And deer ticks, which are known for spreading Lyme disease, are moving farther north.
The United States is likely to be friendly to ticks that come from other continents, too. “Because of the hospitable environment currently, there is no limit for these ticks to start establishing populations,” Dr. Molaei said.
The study found additional cause for alarm. Two of the ticks documented by the researchers — nymphs from Costa Rica and Guatemala — tested positive for Rickettsia amblyommatis, a type of bacteria that could be linked to spotted fever, a potentially fatal disease.
The research does not necessarily suggest that the seven nonnative species are proliferating widely. Still, the findings help to provide a sketch — or an early warning — for entomologists and medical professionals who focus on tick-borne diseases.
“I do want to know, when these species pop up, to keep an eye out for them,” said Megan Linske, a tick ecologist in Connecticut who was not involved in the study. “Because some of them look very similar to our species. Some of them have very similar behavior and host dynamics.”
Ticks these days are generally more active, and more numerous, than they used to be. Climate change is partly to blame: When winters are warmer, ticks and the animals they depend on, like deer and mice, are more likely to survive. And rising temperatures allow ticks to be active for longer parts of the year.
Ticks have also been helped by the spread of the invasive Japanese barberry plant, which they tend to like. The ever-growing footprint of human settlement also results in more people being bitten.
According to the study, nonnative species — just like the domestic ones — may spread more easily in the United States because of changing temperatures.
“No matter how north we are in the United States,” Dr. Molaei said, “it does appear that climate change is making the environment more conducive, more inhabitable for invasive ticks — even those ticks that are coming from rather tropical areas.”
It is fitting that the study was conducted in Connecticut. Lyme disease is named for towns in the southern part of the state where, in 1975, a cluster of illnesses prompted a search for the pathogen that, we now know, is transmitted by ticks. Since then, the state has become a sort of epicenter for tick research.
The ticks documented in the study were not collected in the wild. All came from humans who found the pests attached to their bodies following international travel, and who then submitted the arachnids to health care professionals or to the researchers’ laboratory.
The results, then, do not amount to a comprehensive survey of nonnative tick species in the state, said Dr. Linske, whose work for the Connecticut Agricultural Experiment Station focuses on outdoor tick surveillance.
As always, health care professionals recommend checking for ticks on your person and on your pets, whether you’ve been to Tanzania or just to your own backyard. Doing this regularly can improve your chances of catching a tick before it transmits a pathogen, or even before it bites.
And while studies like this one should prompt caution, as well as additional research, Dr. Linske said, they are not a cause for abject fear.
“I always encourage people to go outside, enjoy the outdoors, even when you’re in other countries,” she said. “Go enjoy the outdoors, but just be aware that ticks are everywhere.”
Jacey Fortin covers a wide range of subjects for The Times, including extreme weather, court cases and state politics across the country.
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