T:he warming world may be a breeding ground for black-legged ticks, which carry the bacteria that cause Lyme disease. They thrive in temperatures of 7.2º C (45º F) or higher and humidity of 82% or higher (the warmer and more humid the better). As climate change steadily bakes the planet, with shorter, milder winters and longer, warmer summers and springs, the range of places with those conditions is expanding.
However, climate change does in fact shape some parts of the world less hospitable for ticks. Extreme weather results in drought (which causes the ticks to dry out and die) as well as a lack of snow cover (which should insulate the species as they hibernate under leaf cover).
Is climate change a net gain or loss for ticks in their quest to feast on humans (and sometimes infect them with Lyme disease and other diseases)? The answer is more complex than a simple “heat equals ticks equals disease” formula would suggest.
It’s true that the ticks that carry Lyme disease, which affects more than 475,000 Americans each year, are expanding their geographic range, and climate change is one reason. “We’re seeing the tick move more toward Canada, and the warmer temperatures appear to be an important factor,” said Dr. Vishnu Laalitha Surapaneni, an assistant professor of internal medicine at the University of Minnesota School of Medicine. “We are also seeing cases of Lyme disease in Norway as well as in the Arctic.” Warmer weather means ticks emerge earlier and stick around longer, which promotes the spread of Lyme disease.
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“I’ve taken care of people who have cases in early May, which is still early,” Surapaneni says. “And my colleagues saw cases back in December. So I think public awareness of when Lyme disease is now is very important.”
“Fall is their peak season,” said Thomas Mather, a professor of public health at the University of Rhode Island and director of the school’s Tick Encounter Resource Center. “They come out especially in October and November and just start to slow down around Thanksgiving.”
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Parts of Mather’s home state used to be too cool for ticks to enjoy such a strong season, but not anymore. “In the 1990s, the black-legged tick was found quite easily in the southern part of Rhode Island, but not in the northern part,” he says. “And over the next decade, we saw it spread north as well.”
However, in some parts of the U.S., that expansion of ticks is occurring at the expense of die-offs elsewhere. Black ticks may be known primarily as a northeastern species, but they actually inhabit the south, plains, and western regions. All of those regions have struggled with drought in recent years, and thus all are less habitable for ticks than they used to be.
“Moisture prevents drying, and that explains why the tick is struggling in the far western range,” said Rebecca Eisen, a research biologist at the US Centers for Disease Control and Prevention (CDC). “When you get into the plains as well, it’s a lot drier and the tick will struggle to survive.”
California, wracked by alternating heat waves, drought and climate change-related flooding, actually has a relative absence of Lyme disease ticks to thank. “You can’t just say that annual temperatures are getting warmer [in California] so we should see an increase in ticks,” says Dan Salkeld, a disease ecologist at Colorado State University. “You can have a long, wet winter and you’ll see very happy and persistent adult ticks, but then you’ll have a heat wave or severe drought or wildfires, and that reduces tick abundance. All these things work together.”
Furthermore, black-legged ticks are a more fragile species than they appear. That 82% humidity range is more than just a preference, it’s a must, at least when the species is in the young nymph stage. “One of my graduate students determined in the lab that a black-legged mite nymph can only survive for eight hours at less than 82% humidity,” Mather says. “After that, they start dying. Even if you bring them back into the humidity, it’s like a plant that has reached the point of wilting. They just can’t recover.”
More than the spread of the ticks themselves, the spread of white-tailed deer may be to blame for the Northeast creep of Lyme-carrying ticks. Ticks, Surapaneni says, depend on deer “for food and transportation.”
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In the 19th century, large-scale logging for timber and hunting of deer for food contributed to the collapse of deer populations throughout the Northeast, Mather explains. “There were vast landscapes of open space that you can’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 logged areas, and that continued into the 1970s.” The deer population bounced back, and the results were immediately apparent. “That’s when people started getting Lyme disease in Lyme, Connecticut.”
In 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. “Deer are a prime host for ticks,” Mather says. “And for them, the artificial environment is like a smorgasbord.”
Another tick host that is increasing, this time due to deforestation, is the white-footed mouse. When parts of the forest are cut down to build housing or roads, ecosystems are broken. This eliminates predators that would otherwise feed on the mouse. “You’ve got these very forest strips, and that’s led to a mouse boom,” Surapaneni says.
The consequences of all this for people are as they have long been; if you live in an area where ticks are likely to live, be careful. “Make sure your pants are tucked into your socks when you go for walks in tree-lined or wooded areas,” says Surapaneni. “Use insect repellent; check yourself as well as your dogs or other pets for ticks, as they can pick them up when they go outside. And shower after you go outside and make sure you remove any ticks because they have to stick to you for a few days before they start spreading the disease.”
Being alert to the symptoms of Lyme disease — fever, chills, headache, fatigue, muscle and joint pain, swollen lymph nodes, and a characteristic rash that can look like a bull’s-eye — remains important if preventative measures don’t work. Treatment, usually a two- to four-week course of antibiotics, is also vital.
While climate change may spread ticks in some places and suppress them in others, there is no doubt that the disease is here to stay. “It’s important,” Surapaneni says, “that the public be educated, and that public health departments, who may never have seen a case of Lyme in their area, be educated as well.”
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