Tick Populations Booming
As climates change, ticks spread farther north, harming dogs and humans
A few weeks ago, on a pleasantly cool day, this reporter and his dog, an Alaskan malamute named Bear, headed for a small set of trails in an area of woods not far from the New York-New Jersey border. With bicyclists plying their way on the shoulder of a nearby highway and the Hudson River rushing along beyond the wooded landscape, man and dog walked along the well-maintained trails, yielding to other visitors and trying to stay away from the tall grass.
Photo by Uwe Mäurer
Memories of the day were somewhat dampened after returning home. Bear, whose deep malamute hair is a jungle of fluffiness, brought home an intrepid hitchhiker. Crawling in that furry maze, and thankfully not attached to his skin, was a tick, no doubt on the hunt for some dog blood — or human blood, for that matter. Another one was found crawling nearby. This episode plays out across the United States and the rest of the world on a regular basis.
According to experts in the field, ticks have gone through some changes over the past few years.
“I think one of the biggest concerns that you see within the published literature for ticks is that ticks’ geographical regions are expanding,” said Dr. Janet Foley, a professor and researcher at the School of Veterinary Medicine at the University of California, Davis. Foley, who studies the ecology and epidemiology of infectious diseases, also serves as co-director of the Center for Vector-Borne Diseases, an institution on the frontlines of tick and mite research.
“Clearly ticks are expanding farther north,” she said. “[W]e’re finding a lot of tick species moving into new areas. And a lot of that has to do potentially with climate change [and] animal husbandry practices if we’re cutting forests or recreating grasslands.… So as a whole ticks themselves are really becoming an emerging problem, not that they always weren’t anyway, but they are getting worse.”
Foley said the expansion of their range has brought them into Canada, and she called some of them “very, very aggressive human biters” that can potentially transmit disease.
The Centers for Disease Control & Prevention lists on its website the tick species in North America, including the American dog tick, blacklegged tick, brown dog tick, Gulf Coast tick, lone star tick, Rocky Mountain wood tick and western blacklegged tick. To the untrained eye, these insects may seem invisible (in the case of poppy-seed-sized nymphal ticks) or indistinguishable (small insect with eight legs — must be a tick). Encounters with a particular species depend on where one lives, and these geographical ranges may be in the process of changing.
For example, the CDC reports that the American dog tick — perhaps the species found on Bear — is found on the East Coast, Midwest, and parts of the West Coast. This particular tick can transmit Rocky Mountain spotted fever. Another candidate for Bear’s tick would be the blacklegged variety, also known as a deer tick. This is the species that can spread Lyme disease, which the CDC states can cause fever, headache, fatigue and a skin rash. Antibiotics can be used to treat the disease.
Foley and her colleagues are actively involved in tick-related research, both in the laboratory and out in the field. Her particular interest lies in ecology and how tick-born disease and mange, which is caused by mites, take advantage of complex, natural communities.
Photo by Dr. Gary D. Alpert/Harvard University
“What we are doing is diagnostic testing, so surveillance, collecting ticks to see if they are currently infected with a pathogen,” she said. “We dissect them. We remove the DNA, [and] do diagnostic testing that way.… Ticks are notoriously difficult to work with in the lab because they have such a slow life cycle. The western blacklegged tick can take three years to go through its whole life cycle, and that’s a long time to keep them.”
According to Foley, there used to be a commonly held belief that ticks couldn’t survive below a certain minimum temperature, preventing their spread to northern climates. The line seemed to be drawn along the southern border of Canada, although exceptional populations abounded.
Researchers, including Foley, are now rethinking the relationship between tick populations and temperature. “[W]e are seeing them north of [the United States], so the question is, it’s not so much that it gets too cold once, but maybe the average cold is less cold than it used to be,” she said. “Or another thing might be, and I think this is more likely … what if it warms earlier in the spring? What if it actually gets just as cold as it used to but starts giving them a little more time in the spring to come out? Then they don’t have to stay in hibernation as long.”
Foley said many tropical areas around the world have “terrible problems” with ticks. In the United States, she identified Oklahoma, Kansas and north Texas as especially susceptible. Dogs in these areas struggle. “It’s very difficult to find drugs that will protect a dog from ticks,” she said. “Sometimes you’ll see dogs coming into clinics with just thousands of ticks on them.”
Dr. Thomas N. Mather, also known as the TickGuy, serves as director of the University of Rhode Island’s TickEncounter Resource Center. He has focused his research on ecology, tick control strategies, tick-bite protection and disease prevention. The Resource Center is a web-based project that delivers information for everyday use, including research, FAQs and prevention techniques.
This year is the second year of the center’s tick survey, known as Tick Spotters. Even with just two years of data, some trends are emerging.
Mather said that most people thought the recent “winter apocalypse” would have killed ticks on the East Coast. “Instead it seems to have left ticks pretty active this springtime, especially compared to last year,” he said.
“One trend that we’ve been noticing is that the American dog ticks in many places are far more abundant,” Mather added. “We’ve gotten I don’t know how many reports of people saying, ‘Well, I’ve lived in my spot for 15 years, and this is the first year I’ve ever seen ticks. And I’m seeing hundreds of them.’ So that, to me, is something to take note of, and just the sheer number of reports that we’ve been getting, especially of American dog ticks, suggests that that tick has had a little bit of an explosion this year.”
Besides the abundance of American dog ticks, another concern is the blacklegged tick, or deer tick. Mather described the species as “loaded with pathogens.” The CDC reports that the blacklegged tick transmits not only Lyme disease, but also anaplasmosis and babesiosis.
“We’ve already identified at least five fairly common pathogens in these ticks,” Mather said. “And an individual tick can be infected [with] probably all five of those pathogens at the same time, so that’s a problem really. The infection rate for the different pathogens differs, but most of those pathogens that are transmitted by blacklegged ticks are not transmitted by any other tick. So what we try to stress [is] it’s kind of important to not just brush a tick off until you know what it is because you have no idea what disease risk you might be at without knowing the type of tick.”
Mather called pathogenic issues in ticks a “smorgasbord of risk.”
Caution is probably better than fretful pandemonium. For example, having a tick crawling on one’s skin doesn’t mean the pathogen has spread. The tick would have to be “binding” in order to transmit, Mather said. The longer a tick is attached the more likely it has transmitted an infectious dose. The human immune system can diminish a small number of pathogens, the URI researcher added, but once the dose gets higher, transmission may occur.
“[For] Lyme disease, in general the tick has to be attached for over a day,” he said. “I’m sure it’s starting to transmit bacteria sooner, but it’s not an infectious dose typically…. There’s been an awful lot of experiments done to show that most of the time there is at least a one-day window before an infectious dose of Lyme disease has been transmitted from a blacklegged tick. The Rocky Mountain spotted fever, rickettsia, can be transmitted much more rapidly, probably within the first 12 hours of tick attachment.”
The mystery of how the tick found its way onto Bear and another one ended up in the house isn’t a mystery at all. He was walking close enough to an area where ticks were present, and the insects caught a ride on a host, full of fluffy fur and the promise of blood. It was an opportunistic jump from the natural world to a 100-pound canine. One could call it the circle of life, highlighting the amazing adventure of these spunky followers from the brush to the carpet of a suburban home, but the thought of those eight tiny legs and the possibility of pathogenic transmission are simply too creepy.
It’s tough not to think of these ticks as ticking time bombs, with more and more potential to explode in a warming climate.