Sometimes people call me “the tick guy,” but I’m a mammal guy by training. Although I respect ticks, I don’t fear them, especially after developing an immunity that kills them when they try to bite me.
Back in 1991 I joined some colleagues at the Cary Institute in New York on a project devoted to understanding the responses by eastern deciduous forests in the United States to stress and damage. One of the biggest sources of stress and damage is defoliation of oaks and other trees by the spongy moth (formerly called the gypsy moth). Moth caterpillars at outbreak levels can strip millions of trees of their leaves in the height of the growing season, but these outbreaks are sporadic or cyclic; most years spongy moths are sparse and do no visible damage.
I joined the project because the rodents that I study, especially white-footed mice, voraciously attack and eat the pupal stage of the spongy moth, to the degree that dense populations of mice are able to regulate abundance of the moths and prevent outbreaks. As a part of this program, I started to monitor forest populations of mice, using standard mark-recapture techniques. Mice are removed from live-traps; individually marked, weighed, and examined; and released at the point of capture.
The very first morning of trapping, I counted roughly 20 ticks per mouse, with a few having nearly 100 ticks (virtually all of which were blacklegged ticks). This observation led my colleagues and me to ask how fluctuating populations of rodents affect tick abundance, tick infection with zoonotic pathogens, and human risk of exposure to tick-borne diseases like Lyme disease. We tackled the questions using long-term monitoring and experimentation.
Mice are not the only mammals that can get infested by ticks in the northeastern United States. Researchers who spend time in the forest get attacked as well. Our field crews are prohibited from using tick repellents (which I highly recommended for others), because we can’t be repelling the creatures we’re studying in nature. As a consequence, I was frequently bitten by ticks during those early years when I was in the field most weeks from spring to fall.
Fast-forward a few years to field work helping my wife and colleague, Felicia Keesing, with her research. She worked in the savannas of Kenya on interactions between large and small mammals, also using live-trapping to monitor abundance and diversity of the latter. While walking from trap to trap at the Kenya field sites, we often saw what looked from a distance like a cattail inflorescence — a fuzzy brown mass near the top of a long stem rising above the grass canopy. But these weren’t flowers. They were hordes of immature (larval) ticks that had aggregated in a spot where they were likely to encounter a large mammal on which to feed. And indeed, as a largish mammal myself, I can attest to the efficacy of this strategy. Back at the hut after a morning of field work, it would take multiple passes with duct tape to remove the large numbers of ticks on my clothing and skin. Clearly I didn’t get them all, as my lower legs were covered by dozens of tick bites (these ticks apparently began to feed but tended not to remain attached).
I suspect that these encounters with ticks of several species in New York and Kenya eventually trained my immune system to react to antigens in tick saliva, resulting in the death of ticks that attempt to feed on my blood. The saliva of ticks is an underappreciated substance (understandably). It contains chemicals that facilitate the ability of ticks to feed, sometimes for a week or longer, on the blood of a vertebrate host without being detected and groomed off or destroyed. Analgesics dull sensation, anti-inflammatories inhibit irritation, and vasodilators and anticoagulants keep the blood flowing.
Researchers have long known that some domesticated mammals, such as cattle, rabbits, guinea pigs, and laboratory mice, develop resistance to tick bites following repeated infestations, a phenomenon termed acquired tick resistance (ATR). ATR manifests as reduced success in tick feeding and even death of the tick. Treating host animals with specific immunosuppressants can eliminate ATR, although the exact immune pathways by which ATR is achieved are only partially understood. Some wild mammals, including wild rodents, show either a weak or nonexistent ATR, but why different species of host vary in their ability to develop resistance is poorly understood.
For years, I dealt with the threat of tick bites the same way most others do: conducting thorough body checks after time in the woods, mirrors and tweezers at the ready, guided by a wealth of knowledge about and ample experience at removing ticks.
But one summer night in the late 1990s my tick-check strategy clearly failed. I woke up in the middle of the night with a strong burning sensation near my armpit. In the well-lit bathroom I discovered a red welt maybe 3 centimeters in diameter with a tiny black speck in the middle. I removed the speck with my ever-present fine-tipped forceps and identified it as a larval blacklegged tick. The tick was flat, meaning it hadn’t imbibed any of my blood, and multiple attempts to get it to move failed — it was dead. The welt and burning sensation remained for several days.
Over the years since, I’ve had that same experience dozens of times: I find an attached but dead tick somewhere on my body, accompanied by a strong burning sensation that lasts for days. Note that the welts that I experience coincide with the tick bite, are small, and do not expand over time — they are clearly not the erythema migrans, or bull’s-eye rash, that is a frequent early symptom of Lyme disease, appearing several days to weeks after a tick bite.
In fact, I have never been diagnosed with Lyme disease or any other tick-borne disease. By killing ticks very early in their attempts to feed, my apparent ATR seems to protect me from exposure to tick-borne infections.
Since my apparent resistance to tick bites has been described in a few media articles, I’ve been deluged with emails and phone calls from people who have similar experiences, have googled what might be going on, and have come across my name. It seems that ATR in humans might not be uncommon. And maybe we can leverage that to our advantage.
A vaccine against Lyme disease existed about 20 years ago, and a new vaccine is currently in late-stage clinical trials. This vaccine elicits immunity by people to the Lyme disease bacterium, Borrelia burgdorferi.
But ticks can transmit many other pathogens, some of which are increasing rapidly in incidence. In addition to Lyme disease, cases of anaplasmosis (another serious bacterial disease) and babesiosis (a malaria-like illness) are growing explosively in the northeastern United States and elsewhere. A vaccine against ticks would help protect against all these pathogens. I hope that immunologists and vaccinologists will continue to pursue the underlying mechanisms in the hopes of developing an anti-tick vaccine for people in high-risk areas. A superpower like killing ticks is something that should be widely shared.