Ula Chrobak, Knowable Magazine
Ula Chrobak, Knowable Magazine
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On a warm spring day, disease ecologist Daniel Salkeld is hiking the hills of coastal scrub and chaparral of Marin County, north of San Francisco. It’s his favorite spot to collect ticks.
As he walks, he trails a white flannel blanket attached to a pole, and every 20 meters, he stops, scrutinizes the flannel and picks off any ticks that have latched on. Ticks are passive predators of blood — they wait for an unsuspecting mouse, deer or person to brush past the blade of grass they are clinging to. And luckily for the scientists who track them, they are easily fooled by wool fabric.
Salkeld tallies his haul as he walks and carefully places the ticks in vials for further examination back in his laboratory at Colorado State University. He is curious to know what areas in California are high risk or low risk for tick-borne diseases. Even when his tick count for the day is zero, “that’s a useful insight,” he says.
Elsewhere in North America and internationally, blanket-dragging tick biologists like him are uncovering an unsettling trend: Many tick species are expanding their ranges, swelling in number and picking up new pathogens that can deliver disease to people should a tick latch on and bite.
That’s reason to worry, because ticks are prodigious vectors — they bring more types of pathogens over to people from animals than any other creature. And they’re on the march. In the United States, the annual number of cases of six tick-borne diseases has roughly doubled since 2004, with most of the increase dominated by Lyme disease cases.
From a public health perspective, it’s important to know when ticks have spread to new places, says Rebecca Eisen, a research biologist focused on vector-borne bacterial diseases with the Centers for Disease Control and Prevention. “We want to make sure that people are aware that there is a risk that maybe they didn’t have as they were growing up in these communities,” she says.
It’s also important to predict where the blood-feasting arachnids will move to next. And that’s a lot more difficult than tricking ticks into grabbing onto flannel. Scientists are working to disentangle a patchwork of drivers — such as land development, climate change and the availability of blood to suck from an array of different critters, large and small. All will have consequences for the number of ticks in a given area, as well as for the likelihood that a tick’s saliva will carry at least one of the 18 tick-borne pathogens identified in the US and the 27-plus known globally.
Teasing out all these factors is complicated, says Lucy Gilbert, an ecologist at the University of Glasgow in Scotland. “You can research the system for decades, and there’s still just so much to learn.”
Ticks come in two main varieties: hard ticks, which have visible mouthparts and a hard plate on their back, and soft ticks, which lack the hard plate and have mouthparts hidden on their undersides. Though both types of arachnid can carry disease, hard ticks — the family Ixodidae — are by far the more serious vectors. There are hundreds of species in this family, scattered all over the world, and their origins are ancient, probably stretching back more than 150 million years.
Hard ticks can transmit an array of bacteria, viruses and parasites to human beings, causing a roster of diseases, both familiar-sounding and obscure: Lyme disease, Rocky Mountain spotted fever, Colorado tick fever, babesiosis, tularemia and more. A handful of tick species are the most serious spreaders. The same tick that causes Lyme disease in the eastern United States — the black-legged tick, Ixodes scapularis — can harbor six other pathogens. The black-legged tick and two others — the lone star tick ( Amblyomma americanum) and the American dog tick ( Dermacentor variabilis) — are responsible for most cases of tick-borne illness in the United States.
Ticks are adept at transmitting disease in part because they live a long time, compared with other vectors like mosquitos. Most of the ticks that carry disease live two to three years and feed on the blood of multiple hosts across their four-stage life cycle (which progresses from egg to larva to nymph to adult). That gives them plenty of opportunity to pick up a pathogen that they can later transmit through their saliva when they bite someone.
The fact that ticks can tap the blood of an array of animals makes the ecology of tick diseases tricky to understand, Gilbert says. As an example, she points to the black-legged tick and the castor bean tick Ixodes ricinus, both of which spread Lyme disease. “They just feed on basically anything: lizards, birds, dogs, deer, everything, so it’s incredibly difficult to work out scientifically what are the main [things] driving these disease risks.”
Most ticks that spread pathogens to people have a two-to-three-year life cycle with four stages: egg, larva, nymph and adult. The timing of these is approximated in this graphic; depending on the particular tick species and where the tick is located, the length of stages and the overall length of a tick’s life can vary. Graphic via the Centers for Disease Control and Prevention
Not every host species can nurture pathogens in their blood, however. And this helps to explain certain geographic patterns. In the southeastern US, for example, black-legged ticks favor lizards for their blood meals. But lizards are poor hosts for Lyme disease bacteria — the blood of one species even kills the pathogen — so the reptiles rarely pass it on to ticks. As a result, Lyme disease is rare in the southern US.
But other patterns are trickier to understand. Scientists are trying to learn, for example, why the lone star tick, most common in the southern US, has been expanding into parts of the Northeast and has even made it into Canada. The species, which the CDC describes as “very aggressive,” can transmit at least six diseases, including an infection that afflicts patients with a sometimes-fatal allergic response to red meat.
And they want to know why black-legged tick populations have expanded to spread Lyme disease, first described in Connecticut in the 1970s, into new areas. The tick is now found down the East Coast and across the Midwest and has moved north into Ontario; the number of US counties it’s been found in has more than doubled in the past two decades.
In lockstep, Lyme disease, which is predominantly caused by the bacterium Borrelia burgdorferi, has increased threefold in the US since the late 1990s, with about 35,000 cases officially reported to the CDC each year, though the agency estimates that actual case numbers could be more than 10 times higher. In 2017, Lyme cases in Ontario were three times higher than the five-year average from 2012 and 2016.
Why are tick-borne diseases rising? There are many factors — and competing theories.
Lyme disease diagnoses have risen over the past couple of decades. Experts also believe that the illness is vastly underdiagnosed. The Centers for Disease Control and Prevention estimates that actual cases could be more than 10 times higher than diagnosed cases.
Climate change is probably contributing to expanding tick habitat for several species of the arachnids. It has potential to influence tick range because ticks are exotherms — their bodies don’t maintain consistent internal temperatures like mammals and birds. And since they spend most of their lives away from the animals that they bite to get blood, they’re sensitive to environmental temperature shifts.
Increasing temperatures allow their life cycles to speed up, potentially shortening a three-year life cycle to a two-year one, Gilbert says. And warmer winters increase the likelihood that ticks can survive in habitats that once would have killed them.
For example, Ixodes ricinus, the castor bean tick, is now found at European altitudes more than 1,300 feet higher than it was in the 1950s. And a 2019 study projected that the range of the lone star tick will move northward and westward under some scenarios of global warming. Similarly, a 2021 review of climate modeling studies found that black-legged ticks are expected to expand farther northward with warming temperatures across the Dakotas, northern Minnesota, Canada and Alaska by 2050.
Globally, while some areas may become too dry for the ticks, the overall area of tick habitat is projected to grow as rising temperatures open up higher altitudes and more northerly climes. “This increased risk of tick-borne disease is just one of the many health consequences that accompany a rapidly changing climate,” says Katharine Walter, an infectious-disease researcher at Stanford University.
But there’s more to the story. “We do know that climate change does have an effect on ticks, but we actually know surprisingly little about how important it is relative to other factors like habitat fragmentation or destruction,” says Richard Ostfeld, a disease ecologist with the Cary Institute of Ecosystem Studies in New York state.
The lone star tick, Amblyomma americanum, can spread six pathogens, including one that can cause meat allergies in infected humans. Adult females have a characteristic “star” on their backs. Photo via James Gathany/CDC
The most studied case is that of black-legged ticks and Lyme disease — and here, research suggests that land use changes have been critical.
Though scientists debate the details, a historic pattern of deforestation, reforestation and development in the Northeast seem to have played a key role in the spread of ticks. During the colonial period in America, settlers felled vast acres of trees in the Northeast to make way for farms. They also hunted white-tailed deer — a major source of blood food for ticks — to near oblivion. As the deer fell in numbers, so did the ticks.
But when northeastern farms were abandoned in the 1800s for farms in the Midwest corn belt, the forests took root again, eventually hosting more deer as well. In a 1993 Science paper often still referenced today, researchers pointed to reforestation and the resultant rebound of deer as culprits in the rise of ticks and Lyme disease. The authors posited that Lyme disease infections probably affected people in the region hundreds of years before medical recognition (in fact, Borrelia burgdorferi has existed in the region for millennia), then disappeared with deforestation, then returned with forest regrowth.
But many ecologists now argue that the real story behind the increase in ticks and Lyme in the Northeast is even more complicated, and that forests got a bad rap. That’s because not every animal that a tick feeds off carries Borrelia burgdorferi. White-footed mice and Eastern chipmunks can be Lyme reservoirs, capable of harboring the pathogen and passing it on to ticks, but other animals that ticks latch onto, such as rabbits and lizards, don’t reliably harbor Borrelia burgdorferi. Deer aren’t Borrelia burgdorferi reservoirs either.
So it’s not surprising that some research has found that the wider the range of species in a tick habitat — especially if a lot of those species are bad Lyme reservoirs — the less likely ticks are to pick up the pathogen. Having a variety of potential hosts dilutes the chance that a tick will feed on an infected animal which, in turn, dilutes the chance that a tick bite will transmit disease.
But if rejuvenated forests and deer aren’t the main driver of the rise of Lyme disease in the Northeast, what is? Ostfeld argues that the true culprit is forest fragmentation: building patterns that break up forests into isolated chunks.
The reasoning goes like this: Scientists know that white-footed mice are excellent Borrelia burgdorferi reservoirs: They harbor the bacteria without becoming ill, infect a tick when it bites them and they feed ticks well, helping tick numbers rise. And white-footed mice thrive where northeastern forests become patchy because small patches of forest have fewer rodent predators.
As mouse numbers rise, the odds that a hungry tick will feed on a disease-harboring rodent go up — in turn, increasing the odds that a person gardening in their yard or going for a walk in the forest will get infected with Borrelia burgdorferi from a tick bite.
Ostfeld has evidence for this from his research in maple forestland in southeastern New York. Collecting ticks from patches of forest of different sizes, his team found that in the smallest forest fragments they studied (all less than 1.2 hectares), the density of young, nymph-stage ticks was more than three times higher than it was in larger forest patches. And the nymphs were infected with Borrelia burgdorferi 70 percent of the time, compared to 48 percent of the time in larger patches.
The ability of ticks to survive, reproduce and spread disease depends on what hosts are around for them to bite. Some small mammals, like the white-footed mouse, can harbor pathogens. Other animals like lizards, however, don’t carry pathogens reliably — they are called dilution hosts because they dilute the odds of a tick getting infected. Ticks also need reproductive hosts like deer, which ticks both feed off and mate on. Predators affect the population sizes of the small mammal hosts, as do resources such as food — in turn, affecting the chance that a tick in an area will carry a pathogen and spread disease.
The implications of this for catching Lyme disease are clear, Ostfeld says. “When you fragment that forest and place housing developments and strip malls and agriculture and stuff in the midst of that forested landscape, then you make matters worse.”
Other studies add more insight into the role small mammals play, finding that tick-borne diseases go up when rodent predator numbers go down. In the Northeast and Midwest, for example, the numbers of red foxes — which dine on mice — have declined, and Lyme cases have risen.
In contrast, deer counts don’t reliably predict where Lyme disease occur. In fact, in some places where deer populations have remained constant for years, Lyme cases have climbed.
But nothing, of course, is straightforward with ticks and tick-borne diseases. Should a different rodent that’s a poor Borrelia burgdorferi reservoir predominate in a fragmented patch of green space, fewer infected ticks — and thus a lower Lyme risk — might result, says Eisen. “It really depends on what your host community looks like,” she says.
And when tick numbers are very high, they’ll pose a risk to people even if rates of infected ticks are low, says Gilbert. If Borrelia or some other pathogen is only present in one in 10 ticks but a brief walk in the woods exposes you to 20 ticks, that’s a risky area.
Though scientists have made progress on illuminating the causes behind the spread of ticks and the diseases they carry, it’s not easy to tell where they’ll head next. One state where this difficulty is clear is California.
Californians might be forgiven if they’re less vigilant about checking for ticks after a hike. But in fact, the Golden State is not free of Lyme disease. In one study of the San Francisco Bay Area, researchers found Borrelia anywhere they were able to collect at least 10 ticks. (Lyme cases on the West Coast are caused by a different tick than in the East: Ixodes pacificus, the western black-legged tick.) Health officials have not yet reported Lyme cases to be rising, but under a future scenario of high greenhouse gas emissions, up to a third of the state could become I. pacificus habitat by the end of the century.
Warmer, wetter winter conditions would make it easier for ticks to survive, says Micah Hahn, an environmental epidemiologist at the University of Alaska Anchorage and first author of the study. And even if their environment dries out, Hahn says that ticks can persist by finding a moist patch in an otherwise parched landscape. “If it gets dry, they climb down and burrow until it gets wet,” she says. “They can find tiny, tiny microhabitats even in less suitable areas.”
But projections are hard to make without fine-grained information about just where ticks like to hang out. With that in mind, researchers including Salkeld launched a citizen science research project in 2016 in which volunteers reported where they encountered ticks and sent them to a lab for identification. Based on almost three years’ worth of data on 18,881 ticks, scientists then determined the ideal climate niche for Ixodes pacificus. Then they projected how much of that habitat there would be in a warmer world.
Trees have grown back or have been replanted in many parts of the United States following the loss of vast acreages for farms and timber. In the Northeast, the return of forests has been associated with an increase in ticks and Lyme disease. Today, many ecologists say this story is oversimplified — that the rise of pathogen-bearing ticks is the result of a complex web of interactions between creatures and is influenced by land use and climate change. Note: The first three images are based on data for virgin forests, while the last shows all forest cover.
Their finding: Suitable tick habitat across California, Oregon and Washington is poised to shrink by 2050. But so much hinges on the complicated ecology of the tick life cycle and the accuracy of future climate predictions, says biologist Tanner Porter, the study’s lead author and an infectious-disease researcher at the Translational Genomics Research Institute.
More citizen science projects like this might give humanity a leg up on ticks before they bite, say Porter and Salkeld. Indeed, when the team recently used three years of data from the project to create maps of tick and pathogen prevalence, those maps looked very similar to the CDC’s.
Other researchers are using similar methods to tackle the human side of the equation. Using data submitted through a smartphone app, disease ecologist Maria del Pilar Fernandez of Washington State University has found that people don’t take the same protective measures in their yards as they would if trekking into the woods. Yet while an urban backyard might have fewer ticks overall, there’s still risk — in a study on Staten Island in New York, she found that about 40 percent of ticks sampled from backyards carried Borrelia burgdorferi.
Everyday people across the country could help tackle lots of other tick questions, Salkeld says, by submitting data on ticks they encounter — and then scientists like him can go out and verify the findings, dragging their tick-gathering blankets along trails.
The job might not appeal to many, but Salkeld finds tranquility in the work. “I love it so much,” he says. “It’s a walk in the woods, or wherever we are, for science and data collection.”
This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter.
Ula Chrobak is a freelance science journalist based in Reno, Nevada.
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