The alacrity with which Maine’s ticks awoke plays out billions of times in forests and fields when winter turns to spring in the Lebanon Valley of eastern Pennsylvania, along Holland’s North Sea dunes, and in bits of woodland from suburban Beijing and the forests of Poland to urban Chicago, London, and Washington, DC. On the scale of human time, the emergence of ticks has occurred at breathtaking speed, fostered in no small part by rising average temperatures. In the Adirondack Mountains of New York, as elsewhere, ticks wake increasingly at odd times of year: after a hard frost in December when daytime temperatures run unseasonably warm, in January amid a blast of balmy air, in an increasingly snowless February when ticks might ordinarily have been locked under a blanket of white.
I see a lot less snow in the Hudson Valley than I did when I attended college along the river in the early 1970s and then moved to the valley from New York City in 1982. Since 1970, annual average temperatures across New York State have gone up 2.4 degrees Fahrenheit in two generations. Winter warming has increased 4.4 degrees Fahrenheit, a stunning change in meteorological terms. In many parts of New York State, trees leaf out eight days sooner and buds bloom four days earlier than in the 1950s. Bees arrive to pollinate ten days earlier than they did in the 1880s. Breeding birds in the state have shifted northward, as have ocean fish along the state’s coastline.
Ticks have long been hearty survivors of trial and calamity. A quarter century after the Chernobyl nuclear power plant meltdown in 1986, former Soviet-bloc researchers found Dermacentor reticulatus, members of the Ixodidae family of hard ticks, thriving within the plant’s exclusion zone. In perhaps the quintessential example of a species’ adaptability, the ticks, the researchers concluded, “can be maintained in areas after a nuclear disaster with radioactive contamination.” A quarter of them, incidentally, were infected with the bug that causes anaplasmosis, a few with the one responsible for babesiosis.
From 1991 to 2012, researchers at the Cary Institute of Ecosystem Studies in Millbrook, New York, captured 54,000 mice and 12,100 chipmunks; they counted 403,000 larval ticks and 44,000 nymphs on the ears of those animals. In the space of a human generation, they found unambiguous changes in the ecosystem in which these creatures lived. As the climate warmed, the cycle of life for ticks was reshaped, their birthing and development occurring earlier with the passing years: “Accelerated phenology,” the researchers called it.
Extended springs and summers mean that ticks arrive earlier in the year, of course. But the implications are greater still, affecting the interplay between tick stages of development. Larvae hatch and feed in late summer, when some will pick up infections if their host, usually a mouse, is infected. They then molt into nymphs and, if infected, go on to infect other animals, or people, when they feed the following year. Earlier springs means infected nymphs have a longer time to feed, giving more of them the chance to infect more animals, to spread and increase each animal’s cargo of Borrelia burgdorferi, which will later be tapped by larvae. More infected animals means more Lyme-infected ticks and an ever-more perfect cycle to sustain an epidemic.
So compelling were the changes in climate seen in this one corner of the world that nymphs and larvae were predicted to hit their peak eight days to two weeks sooner by midcentury, the risk of tick exposure, the Cary researchers concluded, coming “substantially earlier.” The study’s final year, it turned out, was also its warmest. If trends continued, temperatures seen in 2012, the researchers wrote, will be “substantially cooler than normal by the 2050s.” Yes, the hottest year in the study period would be cool by future standards.
A Dutch scientist I had visited in 2016 foresaw a time when ticks were a year-round phenomenon in a country once renowned for its frozen canals. Willem Takken at Wageningen University had studied tsetse flies and malaria in Africa and had turned his attention in 2000 to the arachnids invading his country. Seven to 26 percent of nymph ticks—the ones likeliest to bite people—were infected with Lyme disease. These ticks, carting other pathogens as well, were common along woodland trails, in homeowner gardens, and in the brush along roadsides. In the rapidly warming Netherlands, Takken said, “The idea that ticks go away is no longer sustainable.”
Armored and Impervious
Short of frying them in the hot sun, blacklegged ticks are very difficult to kill, which is why, forty years into an epidemic of tick-borne disease, ticks populate and proliferate. I once washed a nymph down the bathroom sink and returned to find it crawling up the white porcelain bowl; now I close the trap. Try crushing one with your fingernails. Swatting them is out of the question. Holly Ahern, a microbiology professor at a state college in the Adirondack foothills of New York, spent a semester’s project figuring out how to get the guts out of Ixodes scapularis ticks to test them. Granted, she was new to the task. Her students pressed, banged, and mashed them with mortars and pestle, all to little effect. Finally, placing one under a microscope, Ahern cut it with a scalpel below the hypostome, which is a barbed spear near the mouth that the tick inserts at mealtime. The salivary glands, south of the hypostome, are where the payload is in ticks, where all manner of secrets lie, so the goal was to empty them. Then, she used a power drill with a coarse grinder attachment, which worked unless the critter slid sideways and twirled around like a carousel pony. The blacklegged tick and its many relatives have ingenious protective qualities, the upshot of millions of years of evolution. Among these is a kind of shield on its back, sclerotized in entomological parlance, called a scutum for females, conscutum for males. These plates, and more on the tick’s underside, are the very definition of hard-bodied.
Felicia Keesing is a research scientist from Bard College in Annandale, New York, who has studied Ixodes scapularis for years. She described the ticks to me as “tiny little armored tanks…exquisitely adapted to what they do,” which, not incidentally, is “to drink your blood and infect you with parasites.” For her, the problem isn’t so much how to kill them, but how to find them. The ticks feed for just a couple of weeks in their entire two-, sometimes three-year lives, attaching to a host once for each life stage. They can go hundreds of days without a meal. We see them only when they quest for food—climbing up wildflowers, perching on low branches—and when they find a host and attach. “But we don’t know where they are most of the time—the entire part of their life cycles when they’re not on hosts,” she told me. “And we’re not good at getting to them during these long phases.” Among Ixodes’ many attributes, armor notwithstanding, stealth may be its biggest protection.
When ticks make an appearance, it is for a performance only nature could orchestrate. Thanks to the magic of cinematography and microscopy, researchers at Harvard and Charité University of Medicine, Berlin, recorded an Ixodes ricinus tick, the kind seen in Europe, as it pierced the hairless ear of a mouse. The drama might be called exquisite—and entomologists will shudder at this—if it served some useful purpose. Ixodes ticks appear to play no essential role in nature, as least not one that’s known, as I will discuss later. On screen, the tick is seen using spear-like appendages to pierce its victim, extending one of two sharp tips to almost twice its usual length, retracting it, and then extending the other. As the cameras whir, the tick rhythmically alternates this poking and prodding, until, voilà, the epidermis is breached. A toehold is planted. Then the twin spears, called chelicerae, snap into full breaststroke mode, flexing and retracting, to plant the tick’s barbed hypostome—its anchor—on mammalian turf. The researchers demonstrated what to that point had been theorized: “The cheliceral teeth are thought to be adapted to cutting as well as holding, and ticks are said to ‘cut,’ ‘saw,’ or ‘push’ their way into the skin of their hosts,” they wrote of their adventure in cinéma vérité. Yes, indeed.
Ticks, like mosquitoes, are small. But many people who are bitten by ticks, unlike those jabbed by mosquitoes, report that they never felt the bite, never knew when that offending arachnid geared up its chelicerae, inserted its hypostome, and began to sup.
There is a reason for this, another wonder of nature that ought, in my mind anyway, to find a better use. Ixodes tick saliva is a feat in itself, laced as it is with an anesthetic, a Novocain to numb the skin, which makes the host oblivious and neatly heads off any attempt to swipe the tick away. Other molecules in saliva prevent blood from clotting and ending an otherwise hearty meal, while still another salivary substance defuses efforts by the immune system to jettison the parasite. After it is firmly in place, the tick builds a hardened seal around the bite using—what else?—its saliva. Like superglue, this cement locks the tick firmly in place for feeding, usually for three to four days.
During that time, the adult female’s reproductive organs and salivary glands develop, and her outer covering, or cuticle, grows with her girth. When she starts feeding, she is a taut, walnut brown freckle with an orange crescent rim, flat and indestructible. When finished, she has increased her body weight several hundredfold and morphed into a squishy grayish grape with coiled legs, fat and ready to drop off—sometimes with her tiny slim mate still attached—and lay some 2,000 eggs. Of note, the males get their cake but don’t eat it at this stage of life. They may mate several times, apparently preferring a nicely rotund partner, but they feed little, if at all, as adults.
In the second week of September in 2016, I spent an afternoon with three Cary Institute wildlife biologists, trolling for ticks in the brush and beaten leaves of a housing development on the outskirts of Poughkeepsie, a few miles east of the Hudson River in upstate New York. Wearing white coveralls, the explorers carried white corduroy flags on long poles. Slowly, they ran each flag over small patches of earth, hoping in thirty seconds to entice an appearance by the hardest tick to see: the blacklegged larval tick. The larvae are the progeny of those hefty females, their six-legged babies that hatch in summer, ready for their first blood meal.
It was late in the season when we went hunting for larvae and most had fed already or died. During one early season sweep, I was told, more than 2,000 larval ticks had dotted a biologist’s flag, an institute record. Several years earlier, a friend had snagged 500 ticks and days later found several attached to the side of her breast. The few ticks that we snagged that day were so small as to be invisible to my untrained eye. Not too long ago, it did not matter much if a larval tick escaped detection; baby ticks, after all, were considered clean and uninfected. They had not yet latched onto a mouse, chipmunk, or other infected animal and been inoculated with a pathogen.
But there is an emerging bug that has ratcheted up the potential risk of tick-borne disease. Called Borrelia miyamotoi, the spirochete can be passed transovarially, meaning from tick mothers to babies, some born ready to infect with their first meal. The pathogen has been found in ticks in Europe, North America, and Japan, where it was discovered in 1995, a concerning trend for other reasons. B. miyamotoi infection rates, for one, were twelve times higher in female ticks attached to deer than those not, a 2016 study found. This suggests that deer, which normally don’t infect ticks with the Lyme pathogen, may be good instead at inoculating them with B. miyamotoi, tainting the eggs to be laid later. The first human cases of the disease were reported in North America only in 2013, but the bug may have sickened folks far longer. It looks a lot like Lyme disease but usually without a rash. Moreover, there is no good test for this infection. When B. miyamotoi was found as frequently as the Lyme pathogen in ticks around the San Francisco Bay in California, researchers suggested, logically, that previous cases may have been missed. I thought of all of this as I looked around at where we were trolling for larval ticks—in a backyard rimmed with bushes and late season weeds, in which, tucked in two places, I saw balls left behind by children.
These one-acre housing tracts are the proving grounds of ticks, where they test their mettle and earn their stripes. Here, in habitat heaven, is a plethora of passing mammals on which to feed and become infected—chipmunks, shrews, and mice in the eastern United States, for example, squirrels in the American West, Switzerland, and the United Kingdom. Add to this tableau the deer that love nothing more than munching on impatiens, pansies, hosta, and azaleas, all mainstays of middle-class gardens. Ticks are picked up here, let off there, like some suburban transit authority for arachnids. The home-and-garden lifestyle has abetted Lyme and tick-borne disease by merging the animals and flora that support ticks with the people who become infected by them. From 1985 to 2005, the United States saw a 90 percent increase in homes built on one-half to one acre of land. By 2014, the median size of a subdivision was twenty-four acres and sixty homes, a suburban ideal that eats forests, hurts wildlife, and enables—and puts people in the path of—ticks.
Make no mistake, however, this is not only a middle-class disease, just as ticks are not limited to tract developments. Hikers in stands of California oak; forestry workers in Spain; garden laborers, mushroom pickers, children in day camps and on school playgrounds; urban park visitors; the folks who maintain highways and utility lines, who mow and plow and seed. The opportunities are all-too rich, in many parts of our modern, managed world, to come into contact with infected ticks.
Carved-Up Nature
In 2003, a World Health Organization report on climate change included a list of diseases with the key factors that were driving them. Malaria was fostered by deforestation; hantavirus by growing rainfall. Cholera was being fed by urban crowding, river blindness by dams and irrigation. Next to Lyme disease was a curious driving factor: reforestation. Not deforestation. Reforestation. This is the paradox of Lyme disease, and it has received far more attention than it deserves. It is true enough, but not the real story of Lyme disease. Planting trees surely helped bring back the deer that host adult ticks. But reforestation did not create an epidemic. The ecology of Lyme disease, as discussed in chapter 3, is much more complex than that.
In the last half of the nineteenth century, each and every day, 8,400 acres of forest in the United States were cut down, plowed over, or otherwise built on. The blood-letting leveled off in the twentieth century, and some of the nation’s forest cover returned, with a net gain of about 12 million acres from 1910 to 2012. But the regrown forests in the US Northeast and Midwest, and in Europe as well, the woodlands that allowed deer once again to thrive, were not like the wilderness that preceded them. They were changed in quality, character, and size. They were, and are, subpar fragments of their richer, original whole.
The United States Forest Service analyzed forests in the contiguous forty-eight states from 2001 to 2006, and in a five-year period found a net loss of 1.2 percent of woodlands. More disturbing, however, was a 4.3 percent decline of what it called “interior” forest, wild land in parcels big enough, at least forty acres, to sustain processes essential for forest health. Pictures taken from satellites in space revealed ever more forested land broken up by parking lots and suburban tracts, in what the Forest Service researchers called “clustering patterns.” In this version of modern day forests, species are lost, gene pools isolated, and habitats degraded. The loss of forest is one thing, the slicing and dicing, another. “The fragmentation of forest area into smaller pieces changes ecological processes and alters biological diversity,” the Forest Service report said. And that allows ticks to thrive.
In 2000, a student at the University of Michigan in Ann Arbor named Brian Allan studied tick populations in different sized patches of forest, from about two to eighteen acres, in suburban upstate New York. The results were an indictment of the toll of middle-class development on human health. Taking a census of ticks, Allan found remarkable differences depending on woodlot size. Compared to bigger parcels, the smallest patches had three times the number of nymph ticks and seven times the number infected with the Lyme pathogen. Moreover, the share of infected ticks in the smallest patches was frighteningly high—seven in ten on average—which was the highest recorded to that point. The reason for this suburban curse goes back to what these islands of nature can support and what they cannot. Mice and other small mammals abound. Preda
tors that might eat them do not. And this arrangement works very well for ticks.
In 2012, Taal Levi of the University of California, Santa Cruz, led a team that did two things to challenge the deer-foments-Lyme theory that has been linked to regrown forests. First, researchers analyzed twenty-one studies from four European countries and seven American states and found very mixed results in real-world tests. In some cases, deer culling led to tick reduction; sometimes it didn’t. Sometimes, as in Finland, the Lyme bug was found in ticks where no deer lived. Deer enclosures in Lyme, Connecticut, meantime, greatly reduced young ticks but produced mixed results for adults.
Then, the team used a sophisticated statistical model that compared county-level Lyme disease cases, deer abundance, and populations of coyotes and foxes in four Midwestern and Northeastern states. The problem, researchers concluded, was not one of too many deer. There were, instead, too few red foxes, a primary predator of the four small mammals—white-footed mice, American chipmunks and two kinds of shrews—that infected 80 to 90 percent of ticks. In New York, Lyme disease cases rose along with increasing numbers of coyotes and declining numbers of foxes. Why? Coyotes eat foxes; foxes consume mice. On the deer question, meantime, the researcher found a link, though weak, between the size of the deer herd and the toll of Lyme disease cases in Virginia. But the clear opposite was noted in Wisconsin and Pennsylvania, where more cases sprang up in areas with fewer deer. The findings, published in the Proceedings of the National Academy of Sciences in 2012, aligned with Allan’s research into tick-infested forest fragments. Many mice, few predators, and generally less diversity meant more ticks and more disease.
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