I’d done everything you’re supposed to do to avoid a tick bite: Tucked permethrin-impregnated pants into permethrin-impregnated socks. Sprayed picaridin on my boots. Once indoors, I removed my outdoor clothing in the garage and immediately took a shower. Did a full-body tick check before going to bed. I’d taken all these precautions just to prune my vegetable garden for half an hour. But I live in the forest in upstate New York, where tick populations are having a banner year. Several days later, I noticed a bite mark on my stomach, a tiny burgundy dot encircled by a pink histamine reaction.
“That’s almost definitely a bite from a nymph tick that you never even saw,” my doctor said the moment she looked at it. She ordered a prescription for a two-week preventive course of the antibiotic doxycycline to prophylactically address Lyme disease, which can cause serious health problems if treatment is delayed. I didn’t yet have the telltale bull’s-eye rash or any other obvious symptoms, such as headaches, fever or extreme tiredness, but the numbers weren’t on my side: approximately one in three nymph deer ticks (also known as black-legged ticks) in my region, as well as about half of adult deer ticks, carry the bacteria that causes Lyme, called Borrelia burgdorferi. If a bacteria-carrying tick has been embedded in your skin for more than 24 hours, transmission is likely.
Lyme disease is a global health epidemic that grows bigger each year. What’s worse is that Lyme is hardly the only serious tick-related disease to worry about now. At least five dangerous pathogens are circulating in deer ticks alone, which expand their range into new territories every year. At the same time, other tick species that can transmit different infections are showing up in ever bigger numbers. It’s a public health concern that’s hard for medical providers to keep up with.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
To understand what conditions are making 2025 a particularly bad year for ticks and tick-borne diseases in the northeastern U.S., I called Thomas Daniels, a vector ecologist. Daniels is director of the Louis Calder Center, a research field station near New York City operated by Fordham University, where, for 40 years, he has studied the black-legged tick, a primary disease vector for Lyme. At the research site, the number of ticks this year is 20 to 30 percent higher than in 2024. Daniels explained why the reason is more complicated than we think—and why popular wisdom about the relationship between warmer winters and tick populations is an oversimplification.
[An edited transcript of the interview follows.]
Why are the tick numbers so high in the northeast U.S. this year?
We’ve been estimating the tick population for 40 years. Some years are hot tick years, and we don’t have good reasons for that. A tick that has a two-year life cycle with three active life stages [larva, nymph and adult] in which they need to feed on a host in each stage means there’s an awful lot of factors that can influence population numbers from one year to the next.
What about the acorn theory, the idea that when oak trees produce a glut of acorns, we end up with a glut of ticks two years later?
Vector-borne diseases like Lyme are diseases of ecology. Oak trees have masting years where they pump out lots of acorns. A mast event means lots of acorns for mice to feed on, which means you get lots more mice. White-footed mice are the primary [host of] Lyme; smaller rodents are capable of maintaining the infection and transferring it to ticks in [the bacteria’s] immature stages. More mice mean more tick larvae. So the acorn hypothesis says that two years after the mast, you have lots of nymph ticks.
Local differences in tick-disease numbers are a function of the rodents that happen to be in the area. But my opinion is the acorn story is so much more complicated. For instance, we didn’t have a masting year two years ago in Westchester County [where the field site is located], and we have lots of ticks this year. People have published papers showing relationships between environmental factors [such as acorn masts] and tick numbers, but if you try to replicate that work, the relationships don’t hold up over time.
We know that climate change is a factor in expanding the range and number of ticks because these arachnids have an easier time surviving when temperatures remain above freezing. Are there more ticks in the Northeast simply because average temperatures are higher and ticks from warmer climates are expanding into places they couldn’t exist previously?
Climate change is having a big effect. But do warmer temperatures explain why in 2025 we have 20 to 30 percent more ticks than in 2024? Not really.
There’s a lot of speculation put forth as to why tick numbers are generally getting higher. There has been a bit of an extension in terms of the season: ticks are becoming more active earlier than they were 20 years ago. But local factors, such as relative humidity, rainfall, soil types, the number of earthworms available, how much leaf litter is available, the impact of invasive species, which ones have impact on host availability, and so on, can have significant effect. It could be more than 100 different things. In any one year, the [tick population size in one area] might be a result of a combination of several of those things, and in the next year, it will be a combination of entirely different things. Our knowledge of the ecology is pretty rudimentary, and then global warming changes what little we do know.
What’s an example of a climate-influenced environmental factor you’re investigating now to understand changes in tick numbers?
The role of invasive species—[the Louis Calder Center is an] 113-acre piece of property, and the forest here isn’t the same as it was 40 years ago. We’re looking at effects of certain invasive plantson tick numbers to see if they are playing a role—if they are more habitable to ticks. An aggressively spreading invasive [for example, a shrub such as Japanese barberry, or ground cover such as garlic mustard or mugwort] might be changing the microhabitats ticks have access to. That’s all preliminary. But climate change means we’re dealing with a moving target, and there’s a lot of factors I wouldn’t have even considered five years ago. Here’s another question I hadn’t thought about until recently: Are the different tick species going to start competing with one another?
Because climate change is global, are tick populations growing and changing in the rest of the world, too?
Yes. Ixodes ricinus [the castor bean tick] is the species that is largely responsible for Lyme in Europe, and it is spreading into new areas. In Russia, a different tick species [Ixodes persulcatus] carries Lyme, and its range is expanding, too. In some places, it might be becoming too hot for ticks, so maybe their range there could be decreasing.
The tick population is high this year, but it also seems that the percentage of ticks carrying diseases is higher than usual, too. Is that accurate?
The percentage of ticks carrying Borrelia burgdorferi [the bacteria that cause Lyme disease] are usually fairly stable for deer ticks: 25 to 30 percent of the nymphs are infected, and usually 40 to 50 percent of adults are infected.
What about other diseases carried by black-legged ticks?
When I started doing this work, we were looking for one thing in black-legged ticks: Borrelia burgdorferi [the bacteria that causes Lyme]. We didn’t have Babesia [another parasite spread by black-legged ticks that causes babesiosis] in New York State. We weren’t looking for what causes anaplasmosis [a disease caused by Anaplasma phagocytophilum bacteria] because we didn’t know about it. So then we were looking at three, then four pathogens instead of just one. Now we’re looking at Powassan [a virus that can cause brain swelling], and we’re at five pathogens that this one tick species can transmit. Any one black-legged tick can have one or more of those agents. So, yes, there is more risk.
There’s also the lone star tick, which can give people an allergy to red meat.
And those numbers are on the rise. We also have the Asian longhorn tick, which has only been in this country for 10 years, as far as we know. We’ve been monitoring it [at the field station] for seven years, and it’s not really biting people here yet. But if it starts, does it have anything it can potentially transmit? That’s a new front.
That’s scary. I’m taking all the preventative steps yet still ended up with two ticks embedded in me (so far) this summer. Is it possible that tick behavior is changing as a result of some type of evolutionary strategy?
[Laughs] I know exactly what to do to protect myself—I take all the precautions—and I’ve had Lyme disease three times. Are ticks doing anything differently? Probably not. They have been around for 100 million years—they know how to find a host and feed and go undetected. This time of year, they are the size of a poppy seed. They may at some point evolve resistance to some of the pesticides and insecticides we use. But for now, they still go up to quest [for a host] and down to rest.
Last question, from everyone who lives in a tick hotspot: Do you think we’ll finally get a vaccine that protects against Lyme disease? I see that one candidate is in phase 3 clinical trials.
I’m assuming you know the story of the vaccine we didn’t get. That vaccine [LYMErix] had an odd action. It was geared toward attacking the outer surface protein of the spirochete [the corkscrew-shaped bacterium], but these pathogens developed a system of changing their outer surface protein when it’s in something warm-blooded, like a host—in an attempt to avoid the immune system. So that vaccine was effective in killing the bacteria when it was still inside the tick but not so much once it enters a host.
Now we understand more about the biology of the spirochete and can better target what’s inside of it. Trying to come up with a vaccine against a bacterium is not as simple as against a virus. I think we’ll get there, and that will be a huge help. But a Lyme vaccine will only target that one pathogen unless we come up with something that could target the tick itself. There are a lot of things in tick saliva to target.
Even if we get there, then we’ll have to contend with antivaccine sentiment, which is much stronger than it was 20 years ago. Still, there is a yearning for something that is going to reduce risk.
