A person hikes a snowy forest path.

With winter weather already upon us, you might have decided to store your insect repellent in the back of the closet until next spring. Well, you might want to dig it back out.

Some species of ticks can be active during winter. And if you live in a warm enough area (or are traveling to one), that goes for mosquitoes, too.

Here’s what you need to know about protecting yourself from mosquito and tick-borne illness when the weather gets cold.

The Ticks That Are Active in Winter

In general, the species of ticks that transmit diseases to humans in the U.S. tend to become inactive during the winter. The combination of cold weather and shorter days triggers a kind of hibernation, known as diapause, says Ellen Stromdahl, a retired entomologist from the tick-borne disease laboratory of the U.S. Army Public Health Center at the Aberdeen Proving Ground in Maryland.

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There are two important exceptions to this rule, however: the blacklegged tick (also known as a deer tick) and its cousin that lives on the West coast, the western blacklegged tick. These are the two ticks that transmit Lyme disease in the U.S., and they “are likely to be active when we get a little warmup spell in the winter,” Stromdahl says.

The reason is that some of the adult blacklegged ticks may not have found a meal before the end of the fall. Because the female adults need to feed in order to lay eggs in the spring, those that haven’t found a meal don’t go fully dormant during the winter. Instead, they can become active whenever the temperature rises above freezing (to about 35° F) and when there’s no snow on the ground, according to James Burtis, Ph.D., a postdoctoral associate in the department of entomology at Cornell University in Ithaca, N.Y., who has studied the wintertime biology of blacklegged ticks. 

And blacklegged ticks may carry not only Lyme disease but also a “whole laundry list” of other pathogens, Stromdahl says, including anaplasmosis, babesiosis, the deadly Powassan virus, and more.

What About Mosquitoes in Winter?

In the fall of 2019, you may have heard reports of a surprisingly high number of human cases of eastern equine encephalitis (EEE) virus, a mosquito-borne disease that can be fatal in as many as 30 percent of people who contract it. Although the disease is extremely rare overall, with an average of just seven cases reported across the whole country per year, in 2019 the Centers for Disease Control and Prevention confirmed 36 cases of the illness.

By wintertime, however, the risk from EEE—already very low—is over in most areas. Health officials in Michigan, which had 10 of the 36 cases in 2019, are no longer concerned about the virus. The possibility of another case can’t be 100 percent ruled out in the event of an unseasonable warm-up, says Michigan Department of Health and Human Services spokesperson Lynn Sutfin, but the chances are extremely low.

In Massachusetts, where 12 more of the EEE cases occurred, by mid-November 2019 health officials said that people were no longer at risk of mosquito-borne disease, because of the hard frost. 

Generally, in cold weather—below 50° F, according to the Connecticut Mosquito Management Program—mosquitoes aren’t active.

In warmer southern states, however, some species of mosquitoes may be active even during the winter. In one CDC study from 2011, for example, researchers found that West Nile Virus, the most common mosquito-borne disease in the U.S., could be transmitted by mosquitoes year-round in parts of Texas and Louisiana. According to Thomas Unnasch, Ph.D., a distinguished professor in the College of Public Health at the University of South Florida in Tampa, Florida is the only state where EEE circulates year-round.

Still, even in areas with year-round mosquito activity, the numbers of buzzing bugs will probably be lower than during the warmer seasons, says Michael Reiskind, Ph.D., associate professor of entomology at North Carolina State University in Raleigh. The risk of being bitten is low, and the risk of contracting a disease is even lower.

Protect Yourself From Insects in Winter

Even though blacklegged ticks bite during winter, the risk of contracting an illness is lower than it is during the peak seasons for these pests, in the spring and summer. Still, it’s important to take precautions against the ticks that are out looking for a winter meal.

If you’re hiking, hunting, or even doing yardwork in an area where blacklegged ticks live (see the CDC’s tick map [PDF] if you’re not sure), you should take precautions against ticks on any above-freezing days. 

“The nice thing about winter is you tend to wear more clothing anyway,” Burtis says, which can help keep ticks away from your skin. You can also spray your boots and clothes with an insect repellent, or treat your clothing with the pesticide permethrin, which can disable or kill ticks on contact. And check yourself for ticks at the end of every day that you’ve been out in their habitat. 

Don’t forget to keep your pets protected as well, with a vet-recommended anti-tick medication

If you’re in an area where mosquitoes are still active during winter, it’s a good idea to use repellent when you might be exposed to them—even though the risk is low. Miami-Dade County in Florida advises residents to keep up with mosquito-control measures, including getting rid of standing water on their properties, throughout the entire year. 

And if you travel to the tropics for a winter vacation, where mosquitoes are always a threat, wearing insect repellent should be part of your strategy for keeping yourself safe from diseases such as dengue and malaria.


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