How Climate Change Affects the Spread of Lyme Disease

The warming world can be a hospitable place for blacklegged ticks, which carry the bacteria that cause Lyme disease. They thrive in temperatures at or above 7.2º C (45º F) and at 82% or greater humidity (the warmer and wetter, the better). As climate change steadily bakes the planet—with shorter, milder winters and longer, hotter summers and springs—the range of places with those conditions is growing.

However, climate change is actually making some parts of the world less hospitable to ticks. Extreme weather leads to droughts (which causes ticks to dry out and die) as well as a lack of snow cover (which the species needs to provide insulation as they spend the winter burrowed beneath leaf cover).

Is climate change a net benefit or loss to ticks in their quest to feast on humans (and sometimes infect them with Lyme disease and other illnesses)? The answer is more complicated than the simple formula of “warmth equals ticks equals disease” would suggest.

It’s true that the ticks that pass along Lyme disease—which afflicts more than 475,000 Americans each year—are expanding their geographical range, and climate change is one reason why. “We’re observing that the tick is moving more into Canada, and higher temperatures do appear to be a key factor,” says Dr. Vishnu Laalitha Surapaneni, an assistant professor of internal medicine at the University of Minnesota Medical School. “We also see Lyme disease cases in Norway, as well as in the Arctic.” Warmer weather means the ticks emerge earlier and stick around longer—contributing to the spread of Lyme disease in the process.

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“I’ve taken care of people who have cases early in May, which is too early,” says Surapaneni. “And my colleagues have seen cases as late as December. So I think public awareness of when there’s Lyme disease now is very important.”

“Fall is their peak season,” says Thomas Mather, professor of public health at the University of Rhode Island and director of the school’s Tick Encounter Resource Center. “They come out especially around October and November and only start to slow around Thanksgiving.”

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Parts of Mather’s own state used to be too cool for the ticks to enjoy such a robust season, but not anymore. “In the 1990s, the blacklegged tick was found pretty readily in the southern part of Rhode Island, but not in the northern part,” he says. “And over the following decade we saw it spreading to the north as well.”

That expansion of the ticks in some parts of the U.S., however, is being met by a die-off in others. Blacklegged ticks might be known mostly as a Northeast species, but they actually populate the south, the plains, and the western range as well. All of those regions have struggled with drought in recent years, and all of them are thus less habitable to ticks than they once were.

“Humidity prevents desiccation, and that explains why the tick struggles in the far western range,” says Rebecca Eisen, a research biologist with the U.S. Centers for Disease Control and Prevention (CDC). “When you get into the plains, too, it’s much more arid, and the tick will struggle to survive.”

California, which has been clobbered by alternating heat waves, droughts, and floods related to climate change, actually has all that punishment to thank for its relative lack of Lyme-disease ticks. “You can’t just say annual temperatures are getting warmer [in California] so we should see a rise in ticks,” says Dan Salkeld, a disease ecologist at Colorado State University. “You may get a long, wet winter, and you’ll see a lot of happy and persistent adult ticks, but then you have a heat wave or an abrupt drought or wildfires, and that reduces tick abundance. All of these things are working in concert.”

What’s more, blacklegged ticks are a more fragile species than they seem. That 82% humidity range is more than merely a preference—it’s a must, at least when the species is in its young nymphal stage. “One of my PhD students determined in the laboratory that a blacklegged tick nymph could only survive eight hours at less than 82% humidity,” says Mather. “After that, they start to die. Even if you bring them back into moisture, it’s like a plant that’s reached its wilting point. They just can’t recover.”

More than that of the ticks themselves, the spread of white-tailed deer may be to blame for the northeasterly creep of Lyme-carrying ticks. The ticks, says Surapaneni, depend on the deer for “food and transportation.”

Read More: Nearly 15% of People Worldwide Have Had Lyme Disease, Study Says

In the 19th century, wide-scale deforestation for timber and deer-hunting for food contributed to the collapse of deer populations across the Northeast, explains Mather. “There were vast landscapes of open space you couldn’t imagine now, and there was nowhere for the deer to hide,” he says. “Then there was a huge change that started in the 1920s where we started to see reforestation of these cut-over places, and that continued into the 1970s.” The deer population bounced back, and the results were immediately evident. “It was right then that people started getting Lyme disease in Lyme, Connecticut.”

Across their range, many deer have lost their fear of both humans and the built environment, with lawns and gardens regularly encroached upon by a species once known for its timidity. “The deer is the keystone host for the tick,” says Mather. “And to them, the artificial environment is like a smorgasbord.”

Another tick host that’s proliferating—this time, due to deforestation—is the white-footed mouse. When portions of forest are cut to build housing or roadways, ecosystems are fractured. That eliminates predators that would otherwise feed on the mouse. “You have just these strips of forest, and that has resulted in a boom for the mice,” says Surapaneni.

For humans, the implications of all this are what they’ve long been: if you live in an area likely to be home to ticks, be careful. “Make sure that you have your pants tucked into your socks when you’re going for a walk in trees or wooded areas,” says Surapaneni. “Use insect repellent; check yourself and also your dogs or other pets for ticks because they can pick them up when they go outside. And after you go outside, take a shower and make sure that you remove any ticks because they need to latch on to you for a couple of days before they start to spread the disease.”

Being alert to symptoms of Lyme disease—fever, chills, headache, fatigue, muscle, and joint aches, swollen lymph nodes, and a characteristic rash, which can resemble a bullseye—remains important if preventive measures don’t work. Getting treated, typically with a two- to four-week course of antibiotics, is vital as well.

While climate change may be spreading ticks in some places and suppressing them in others, there’s no doubt that the disease is here to stay. “It’s important,” says Surapaneni, “that the public be educated and that public-health departments that may have never seen a case of Lyme in their area are, too.”