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Nymphs questing through the forest. The phrase conjures up images of a scene from “Game of Thrones.” But encountering a real nymph on its quest offers a potentially harmful brush with climate change.

Immature deer ticks are called questing nymphs. They now inhabit a wide swath of North American forests, but they didn’t always. During early summer, their quest is for blood. The season now starts earlier and lasts longer than it did in the past, which is good for the ticks. But it’s bad for humans, because these ticks carry the bacteria, viruses, and parasites that cause Lyme disease, anaplasmosis, deer tick encephalitis, and babesiosis.

I have collected thousands of nymphs as part of my dissertation research on the invasion of Lyme disease across North America. I’ve witnessed along the way that where these ticks thrive has been heavily influenced by humans.


Deer tick invasion

Encounters with ticks didn’t always cast a dark shadow over North American summers. Cases of Lyme disease first appeared in 1976 in the woodsy suburb of Lyme, Connecticut. At that time, deer ticks were found only in a hotbed encircling Long Island Sound, along with a small area in Wisconsin.

Since the 1970s, deer ticks have rapidly extended their reach north, west, and south. The most recent map shows that deer ticks now roam throughout the eastern coastal states, from Maine to Florida, and across the Midwest. They are now established in 45 percent of US counties. That means the deer tick has more than doubled its reach in the 20 years since the previous map was published.


The spread of Lyme disease has closely followed the spread of the forest nymphs. Lyme disease is now the most common disease transmitted by a vector — a mosquito, tick, or other bug — in United States. More than 30,000 cases are reported each year, and the Centers for Disease Control and Prevention estimates that 10 times as many Americans develop the disease.

Regions where ticks that carry Lyme are established (red) or reported (blue).
Regions where ticks that carry Lyme are established (red) or reported (blue). Alex Hogan/STAT

In part, ticks are following the spread of one of their favorite sources of blood: deer. As deer populations exploded over the last sixty years, thanks to strict hunting laws and the largely predator-free and deer-friendly landscapes in New England and the Midwest, deer ticks followed. However, the steady crawl of ticks north into Canada can’t be explained by deer alone.

Ticks spend the majority of their lives on the forest floor. They are vulnerable to changing local climates and death by freezing, drowning, or desiccation. Warmer winters and longer summers let more ticks survive and thrive further north each year. Warmer temperatures quicken the tick life cycle, too. Tick eggs hatch sooner and ticks spend more time questing for blood, and so are increasingly likely to feast on a human and pass on a disease-causing pathogen. Because more ticks survive and mature more quickly, diseases can be transmitted faster.

Species that thrive under climate change

The barriers we have created — the heated, cooled, and (somewhat) bug-free spaces we inhabit — give us an artificial sense of immunity to the disturbances shaking our fragile ecosystems. Nymphs don’t respect the barriers of urbanization and wealth that protect many Americans from vector-borne diseases. Window screens, socks, and our skin don’t stop the invasion of nymphs, reminding us of our vulnerability to ecological changes brought about by climate change, habitat fragmentation, and deforestation.

As we worry about the ability of some species to run from climate change and escape extinction, ticks, mosquitoes, kissing bugs, and the parasites they carry may thrive under climate change. Where will these crawling and flying disease carriers move? And who will be at risk for what were once called tropical diseases?

The consequences of climate change will vary dramatically across the globe and are difficult to predict. The yellow fever mosquito (which also carries dengue, Zika, and chikungunya viruses), for instance, is predicted to spread rapidly in some areas, including eastern North America and large parts of southeast Asia, and become less common in others areas, like much of Australia.

A changing climate will affect mosquito-borne diseases in subtler ways, too. In a warmer climate, the dengue virus matures more quickly (up to a certain temperature). That means an infected mosquito can more swiftly spread the virus.

The consequences of climate change will be felt most profoundly by people living in or near areas where diseases carried by mosquitoes and other vectors are already common, and where poverty makes it difficult to stamp out these diseases.

A forest nymph brushing against a hiker doesn’t begin to drink blood immediately. She crawls across the skin, searching for a comfortable dinner spot. She grips her prey with spindly legs and uses knife-like mouthparts to slice into human skin. She secretes cement around the wound, binding herself to her host, and then begins to imbibe. Once attached, this offspring of a changing climate can’t be simply brushed off.

Katharine Walter is a graduate student in the Department of Epidemiology of Microbial Diseases at Yale University.

  • Why does the article use the phrase “climate change” when what is precisely being argued is that only warming climate is increasing the tick population?

    • Global warming is driving climate change. The terms are often used interchangeably, but climate change is a broader term that captures changes in temperature, precipitation, the seasonality of weather, and oceans, among other things. But you are correct that the most important climate factor for tick expansion is warmer temperatures.

  • Why doesn’t this article bring up the govt run Plum Island govt research which researched biological type weapons as to the tick? Plum Island is 10 miles away from Old Lyme, Connecticut and around 1975 is when Lyme Disease hit the scene. It is said that some of these inoculated ticks were released on the island. Why isn’t this mentioned?

  • “can’t simply be brushed off…..” you’re right, there— I had one on my neck last month and it could not be pulled off…had to go the Emergency where the doc had a helluva time getting it out with tweezers

  • This is such a propaganda page. As if a half of a degree of average temperature would make the tick spread…. Well the climate change (ex global warming people) will do everything to bombard us with their nonsense. If anyone was truly trying to lower carbon gases they would start by stopping deforestation in the Amazon and other regions. Trees love CO2 and give us oxygen back.
    It seems like it worked well for Al Gore as he just became a billionaire .

    • The temperature in New England, for example, has increased by 2-3 degrees Fahrenheit since 1895. That doesn’t seem like much, but temperatures during the last ice age were only 9 degrees cooler than the 20th century. That small change in average temperature makes an enormous difference to ecosystems, ticks, moose, and deer included.

      Your final comment that trees love CO2 is half true. They do take in huge amounts of CO2, but also emit just slightly less over time. So while you are correct that stopping deforestation helps take CO2 out of the atmosphere, it’s not enough to offset the additional unnatural CO2 emitted by human activity.

  • Alarmism and advocacy disguised as research?
    Yellow fever was indigenous to the Eastern Mid Atlantic states before mosquito controls began.

    I still remember as a child the army of paid workers that searched out every possible mosquito breeding puddles and treated them. Every year after that mosquito sprayers would rumble through the streets of Levittown (built upon a drained swamp) spreading a thick fog of insecticide.

    There were also programs that spread insecticide treated cotton puffs throughout wooded areas. Mice would use the cotton in their nests and kill off the hatching tick nymphs.

    Ticks survive in an amazing array of outdoor spaces from jungle or desert tropics across the country well up into Canada.

    Freezing kills ticks? When? How?
    Ticks are plentiful, healthy and populous well up into the colder regions of North America.

    The small initial ‘hotbed’ for lyme disease is indicative of an invasive disease initialization and subsequent spread.

    The incredibly rapid spread is well supported by the explosive growth of deer populations and winter host mice since the 1970s. Thanks also to the explosive growth of suburban neighborhoods.

    Wet springs are more deadly to ticks than cold winters. I have yet to notice even bitterly cold winters having any effect of ticks.

    Considering that the global warmth increase since the ‘Little Ice age’ is a grand total of .8C; that is hardly enough temperature change to effect anything. Especially since days and seasons have a far larger spread of temperatures.

    • See my reply to Helmut above. A small change in global average temperature has large repercussions for ecosystems.

      But taking your point to heart, changes in temperature, like we’re seeing now, amplify existing stresses. As you point out, ticks are well adapted to a range of climates. As other things change around them–as predators aren’t able to keep them in check, as freezes are less likely to keep their populations down–they are able to gain footholds where they wouldn’t have been able to otherwise. One example you mentioned: Winter host mice. With shorter winters, winter host mice are active for a longer period of the year. That increases the chances ticks will attach to those mice.

  • Deer density is not correlated to Lyme disease and the proposition that deer are a significant cause for the spread is purely speculative. Deer are immune to Lyme so don’t transmit the disease. There are over 125 hosts for the deer tick, know more accurately as the black-legged tick. Bird species carry both the tick and the Lyme disease. Research at:

  • Borrelia, the microbe commonly associated with Lyme disease, has been found in ticks trapped in amber dating to 15-20 million years old. Because ticks have been biting humans since there have been humans, it is likely that Borrelia has been infecting humans for a very long time. This was confirmed by the finding of Borrelia burgdorferi in an a human mummy dating to 5300 years ago, thawed from a glacier in the Italian alps (National Geographic, 2011). Tick populations are changing with global warming, but because no one was testing ticks for Borrelia 40 years ago or hardly even 20 years ago, there is no baseline to define a change. Also, because testing for Borrelia infection is poor and because asymptomatic carriers are unlikely to be tested, no one on earth has any idea of the actual rate of Borrelia infection in humans, past or present. There may be more ticks, but are people actually being bitten more often? Before 1930, most people lived and worked outdoors in fields and woodland. Tick bites were an everyday fact of life. Today, most people spend most of their time indoors. The real question is: Are people getting sicker from Borrelia today than in the past?

    • Good point—yes, the Lyme disease bacteria, Borrelia burgdorferi, is not new and there is evidence it has been infecting humans for millennia. Even though we don’t know exactly how many people were getting sick with Lyme disease in the 1930s, we know that the area in which people are at risk of infection with Borrelia burgdorferi has dramatically increased. And therefore, more people are at now risk of Lyme disease and other deer tick-borne diseases.

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