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In the three-plus decades I’ve been alive, I have never been bitten by a tick. Actually, that may be a lie, and I have no way of knowing for sure. Because even though ticks have harpoonlike mouthparts, even though certain species can latch on for up to two weeks, even though some guzzle enough blood to swell 100 times in weight, their bites are disturbingly discreet. “As a kid, I would have hundreds of ticks on me,” at least several of which would bite, says Adela Oliva Chavez, a tick researcher at Texas A&M University. And yet she would never notice until her aunt would pick them off her skin.
The secret behind tick stealth is tick saliva—a strange, slippery, and multifaceted fluid with no biological peer. It keeps the pests’ bites bizarrely itch- and pain-free, and allows them to feed unimpeded by their hosts’ immunity. As climate change remodels the world, spit is also what’s helping ticks enter new habitats and hosts—bringing with them the many deadly viruses, bacteria, and parasites they so often import.
For all their dependency on blood, ticks almost never eat. In their sometimes-multiyear life span, they may feed only once in each stage: larva, nymph, and adult. Which means, as my colleague Sarah Zhang once wrote, each meal must count for an awful lot. Unlike mosquitoes and other bloodsucking bugs that can get away with a dine and dash, ticks must linger on flesh for days or even weeks—an extended feast that requires them to essentially graft onto the host’s body like a temporary limb.
For the entirety of that process, saliva is key. When a tick first bites, its spit lines the wound with a gluelike substance that cements its mouth in place. Once secure, the tick deploys a fleet of spit-borne compounds that dilate its host’s vessels, while simultaneously battling the bodily compounds that would normally prompt the injury to clot, heal, or tingle with pain or itch. Under most circumstances, such an onslaught of foreign molecules would instantly marshal the body’s immune cavalry. But ticks have workarounds for that too. Their saliva is an anti-inflammatory and an analgesic; it can disable the alarms that cells send to one another, preventing them from coordinating an attack. Spit can even reprogram immune cells so that they never complete their development or receive the cues they need to gather at the scene.
All of these strategies can also ease the way for bacteria, viruses, and parasites that the tick swallows from one host, then deposits into the next. With tick saliva breaching the skin barrier and keeping the immune system in check, all the pathogens have to do is come along for the ride. “Tick saliva is like a luxury vehicle that delivers them to the site of infection and rolls out the red carpet,” says Seemay Chou, the CEO of the biotech start-up Arcadia Science. Studies have shown that multiple pathogens get an infectious boost when chauffeured by spit, spilling more efficiently into the skin of newly bitten hosts. Borrelia burgdorferi, the bacterium that causes Lyme disease, will even slather parts of tick saliva onto itself like a cloak, essentially rendering itself invisible to bodily defense. Ticks’ infectious cargo may even egg each other on: Saravanan Thangamani, at Upstate Medical University, in New York, has found evidence that ticks simultaneously carrying Borrelia and Powassan virus may end up injecting more of the latter into fresh wounds.
Already, ticks spread more pathogens to humans and their livestock than any other insect or arachnid. And the risks ticks pose may only be growing, as warming temperatures and human meddling with wildlife allow them to expand their geographic range and infiltrate new hosts. In North America, lone-star ticks and black-legged ticks have been orchestrating a concerted march north into Canada. At the same time, the percentage of ticks carrying infections is also increasing, Thangamani told me, and for decades now, case counts of Lyme disease and tick-borne encephalitis in several parts of the world have been on a steady rise. As cold seasons shrink, the periods of the year when ticks bite—usually, the warmest months—are expanding too. “Many, many places are getting filled up with ticks,” says Jean Tsao, an entomologist at Michigan State University. “And they’re going to get more.”
It helps that many ticks aren’t picky about whom they carry or bite. Some species, as they push into new places, have picked up new pathogens in the past few years—Bourbon virus, heartland virus—that pose additional threats to us. Many tick species are also relatively indiscriminate about their hosts: Within its lifetime, a single deer tick may “feed very happily on reptiles, avians, and birds,” says Pat Nuttall, a virologist and tick researcher at the University of Oxford. Their spit is intricate enough that it can be tailored to counteract the defenses of each species in turn. Transfer a tick from a rabbit to a human or a dog, Oliva Chavez told me, and it will take notice—and adjust its saliva, quite literally to taste.
Vaccines to combat Lyme and other tick-borne diseases have long been in development. But many researchers think the more efficient tactic is going after the tick itself—a strategy that could, at best, “stop the transmission of several pathogens at once,” says Girish Neelakanta, a tick biologist at the University of Tennessee at Knoxville. Anti-tick immunity is possible: Studies have documented guinea pigs, cattle, rabbits, goats, and dogs developing sustained defenses against the arachnids after they’ve been bitten over and over again—even reactions that can help the animals detect a bite immediately, and sweep the pest away.
But spit is a slippery target for bodily defenses to hit. The substance doesn’t just shut down immune responses. It also reformulates itself constantly so that it can keep evading the host’s defenses—as often as every few hours, faster than most of the immune system can keep track. By the time the body has prepped an assault on one salivary ingredient, the tick has almost certainly swapped it out for the next. “It’s a game that the tick is playing, a catch-me-if-you-can kind of thing,” says Sukanya Narasimhan, a tick researcher at Yale. To outcompete the tick’s tricks, Narasimhan thinks it will be key to develop a vaccine that triggers the body to respond to tick bites fast, “as soon as a tick attaches,” she said, ideally by targeting the saliva’s first ingredients.
As ticks continue their takeover, it’s hard not to develop at least some grudging respect for their pluck. Some scientists even think that studying, or perhaps mimicking, their saliva could lead to other breakthroughs. Copycatting the spit’s immunosuppressive tendencies could be useful for the treatment of asthma, or for drugs that assist in organ transplants; imitating its anticoagulant properties could help keep life-threatening clots at bay. Some tick-saliva ingredients have even prompted investigations into their potential as cancer therapy. Ticks, after all, have been studying mammalian bodies for millions of years, all in hopes of subterfuge; under their tutelage, Chou, the Arcadia Science CEO, hopes to learn more about the molecular pathways that drive the urge to itch.
Ticks aren’t invincible, though, and some of the same global changes now easing their entry into new habitats could eventually hinder their progress. Already, they are fleeing parts of the planet that have grown too hot, too humid, too flooded, too razed with wildfires for them or their preferred hosts to survive, including certain inhospitable pockets of the American South. A tick decline could be good for us. But it would also be a symptom of a planetary scourge that has grown worse. Ticks, undoubtedly, “will continue to adapt,” Thangamani told me. And yet they, too, have their limits—further, but not that much further, beyond our own.
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Katherine J. Wu
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