No man is an island.
He is an ecosystem.
You’ve got company—and plenty of it. Mites make their home in your eyelash follicles, bacteria colonize your skin, and fleas and lice drop by for blood meals.
The habitat was deeply inhospitable—a sheer bluff, knotted and furrowed by subsurface tremors, intermittently flooded, buffeted by winds, burned by the sun. My guide was Cliff Desch, a mild, likable University of Connecticut professor with unruly gray hair winging out over the tops of his ears. We were searching for life on the human body, or more precisely, on the hostile terrain of my own forehead. I took a bobby pin, as instructed, and scraped the crook of it hard across the skin in front of my hairline. Then, like a fisherman emptying his nets, I spread my catch on a glass slide.
The human body, especially the face, is the natural habitat for two species of mites, Desch said, as he placed the slide under a microscope. One species is minutely adapted to the hair follicle. The other ensconces itself in the micro-habitat of the sebaceous gland, less than a millimeter away. Sir Richard Owen, better known for naming another buried life-form, the dinosaur, brought the follicle mite to the attention of the world in the 1840s. He called the genus Demodex, meaning “lard worm” (though mites are actually distant relatives of spiders).
Desch peered through the microscope and said, “Oh wow” and then, “Hunh!” It appeared that my forehead was home to only one species of mite. But quickly, before I could become despondent about inadequacies in my personal biodiversity, he added: “You’ve got the best population I’ve ever seen.”
It occurred to me first that Desch had spent an entire career looking at this sort of thing and second that I had stood under a shower just a few hours earlier, slathering my forehead with soap and blasting it with steaming water. “Look at ’em all,” Desch was saying now, unable to suppress his delight. “Holy moley!”
Well, no man is an island. He is an ecosystem, though we studiously pretend otherwise. Our skin—two square yards [1.7 square meters] of it on the average human body— is a habitat for roughly as many bacteria as there are people in the United States, for fungi and viruses, and on occasion for mosquitoes, fleas, bedbugs and kissing bugs, blackflies and botflies, lice, leeches, ticks, and scabies mites, which tunnel across the backs of an afflicted person’s hands like moles burrowing in the front lawn.
In the developed world we like to think we have tubbed and scrubbed ourselves free of any overly personal connection to the natural world. Even mosquitoes stay mainly on the other side of our window screens. But this is a delusion, as follicle mites, which live on almost everyone, abundantly demonstrate.
I stepped up to the microscope, and they came into focus, lying crisscross like sticks of wood. The adult mites were about a hundredth of an inch [0.25 millimeter] long. Their stumpy little legs wriggled and twitched as in a dream. They had tiny claws and needlelike mouthparts for consuming skin cells. Here and there were eggs shaped like arrowheads and juveniles with angled-back scutes on their underbellies, like fish scales, the better to anchor themselves in my skin. Desch eyed my forehead as if it were the Grand Banks in high season and said, “I think it’s great.” I smiled wanly.
Once upon a time we were all far more at home, though not necessarily any happier, with the idea of being infested. A 15th-century courtier once discreetly picked a louse off King Louis XI of France, and the king graciously remarked that lice remind even royalty that they are human. (Next day an imitator pretended to find a flea on the king, who was by then perhaps tired of being human. “What!” he snapped. “Do you take me for a dog, that I should be running with fleas? Get out of my sight!”)
For almost all our history as a species, being infested was an inescapable fact of life, and our forebears achieved an intimacy with nature that we can scarcely imagine. European lovers of the 17th century sometimes wrote seduction poems about a girlfriend’s fleas. John Donne once petulantly complained that a flea, having bitten boy and girl alike, “swells with one blood made of two / And this alas is more than we would do.” A few gallant French lovers actually plucked a flea from their lady love and kept it as a pet in a tiny gold cage at the neck, where it could feed daily on their own blood. In Siberia, according to one story, an explorer was disconcerted to find that young women visiting his hut tossed lice at him; it turned out to be their way of expressing amorous intentions.
Clearly, this would not be a successful dating strategy today; for one thing, the human flea itself has almost vanished from modern homes. The hardier cat flea has replaced it, but only partly. Body lice, too, are far more scarce; they lay their eggs in our clothing, an elegant adaptation to human hairlessness, but have thus fallen victim to that environmental cataclysm, the rinse cycle.
The more remote our ectoparasites have become, the more horrifying they seem to be. Moreover, science has made this horror seem rational by demonstrating over the past century that several of our ectoparasites are the most dangerous animals on Earth. The diseases they carry have killed us by the hundreds of millions—fleas with bubonic plague, body lice with epidemic typhus, mosquitoes with yellow fever and malaria. They vex and panic us even in the most modernized countries with maladies like encephalitis, transmitted by mosquitoes and ticks, and tick-borne Lyme disease.
We go to sleep at night aware that our very pillows are home to thousands of dust mites—which, as it happens, help keep our homes clean by busily consuming the tens of millions of skin cells we shed each day. But the mites also cause asthma in some people, and when it comes to the beasts that live on and around our bodies, we tend to focus on the negative.
So it takes an almost unnatural objectivity to suggest that our ectoparasites can also be fascinating. Like any species colonizing difficult terrain, they have adapted ingeniously to our flesh. They use sophisticated chemosensors to find us; saws and scalpels to penetrate our skin; siphons and a small pharmaceutical warehouse, including anesthetics and anticoagulants, to steal a blood meal and get away undetected. If we can suspend for a moment the uneasy awareness that all this evolution is geared to extracting our blood, and if we can forget that our parasites mostly use this blood to produce the eggs for their future pestiferous generations, then it is possible to regard them with awe.
They are capable of extraordinary subterfuge. For example, the adult botfly of Middle and South America manages to parasitize us quite gruesomely without ever actually making physical contact. To avoid being swatted by some balky human or other host, she captures an insect, a mosquito for example, glues her eggs to her prisoner’s abdomen, then sets it free.
The mosquito ignores the eggs (as will we for a moment) and goes off to employ subterfuges of her own. Many mosquitoes feed at night, for obvious reasons (“Consider the outcome if you were to approach an elephant with a syringe,” one entomologist says). But this mosquito is a day feeder, finding a victim with her eyes and with sensors attuned to carbon dioxide, warmth, lactic acid, and other bodily emanations.
Having deftly touched down, the mosquito stabs and saws her way into the fine web of blood vessels in the skin. The damaged vessels instantly attempt to plug their leaks with aggregating platelets in the blood. But host and parasite have evolved together, with all the one-upmanship of any arms race. So the mosquito is equipped with a powerful enzyme in her saliva to disable the platelets. The more saliva she pours down one tube in her proboscis, the faster she can suck up blood through another. Humans in turn have an immune response to the saliva, which alerts us with itching and swelling, but only after about a minute. We swat ploddingly—and are likely to kill only the slowest feeders. Thus we do our bit for natural selection, helping ensure that future generations come only from mosquitoes that are quick enough to get away with our blood in a minute or less.
But the co-evolutionary arms race on the human ecosystem is even more disheartening than all this might suggest. The mosquito may leave behind other gifts, along with her saliva. After having been driven out in mid-century, malaria and dengue fever have lately begun to reappear in the United States and other developed nations. Insect-borne diseases are on the increase worldwide, largely because so many species have developed resistance to insecticides and their pathogens have developed resistance to our best medical therapies. In the New World tropics the insects may arrive bearing not just agents of disease but at least one other gift: Let’s say we get bitten by the mosquito that was briefly held prisoner a few days earlier by a botfly. As the mosquito feeds, our own body heat triggers the botfly eggs glued to her abdomen to hatch. A botfly larva promptly crawls into the fresh bite wound, where it matures with time into the ripest sort of traveler’s horror story.
The larva has a segmented, yellow-brown Michelin-man body, belted with rows of raked-back spines for lodging itself mouth-first in the skin. It also anchors itself with two tusklike hooks sticking out from the mouth. Its tail is a breathing tube, which can lift up, periscope-like, just above the surface at the point of entry. As it develops, the larva wriggles visibly and painfully under the skin. Removing the botfly is relatively simple (one remedy involves applying bacon to the breathing hole, so the botfly has to burrow up through it for air). But a Harvard biology student, curious about his own potential as an ecosystem, once nurtured a botfly in his flesh for six weeks. Finally a one-inch-long [2.5-centimeter-long] botfly larva, ready to move on to its pupal stage, started to emerge from his scalp as he sat in the bleachers during a Red Sox-Yankees game at Fenway Park. The Sox lost, and despite the biologist’s heroic efforts to protect it, the botfly died.
But the beasts that live on our bodies are by no means all bad. A normal population of bacteria on the skin, for example, may actually benefit us by preventing infectious bacteria from gaining a beachhead. But if you tell people that a normal population can mean a hundred bacteria per square inch in the barren habitat of the shoulder blades (or millions in the sweltering armpit), they are liable to scrub themselves raw. In the extreme disorder called delusory parasitosis, victims can imagine they are under assault by invisible bugs that spill out of electric sockets, crawl from holes in concrete, and drop down from ceiling tiles. To stop the constant itching, they scratch themselves bloody. They bathe in gasoline and inundate their homes with pesticides. But the bugs keep coming. Such cases have sometimes ended in suicide and once in the murder of a doctor who tried to get his patient to see a psychiatrist.
When real infestations occur, even sensible people often behave irrationally. In the course of their recent evolution, for instance, head lice seem to have developed resistance to most conventional treatments. Distraught families of infested schoolchildren frequently resort to home remedies. Last year in Oklahoma a man applied a highly toxic cleaning solution to a six- year-old’s scalp, causing cardiac arrest and permanent brain damage.
So it’s important to realize that we aren’t under assault, or rather, that the assault is limited and controllable. We possess the ultimate weapon, which is human intelligence—or, anyway, the opposable thumb. In New York City and Boston, professional nitpickers now charge up to $50 an hour to train parents in the most venerable treatment for head lice: removing the eggs, or nits, by hand, having first drowned them in a shampoo of olive oil. It is a very old idea of quality time. “It gives you a lot of bonding when you nitpick,” says Mary Ward, a Boston nitpicker. “You know these people.”
Our ancestors would regard our otherwise unpestilential lives with dumbfounded envy: We don’t spend our days itching and fidgeting; we know which diseases our parasites carry and how to avoid them; and at least in the more temperate corners of the planet, we don’t generally suffer from nightmarish stuff like botflies. Scientists have demonstrated persuasively that our ectoparasites do not transmit the AIDS virus. And though pathogens and parasites can adapt rapidly, our body beasts appear unlikely to cause new plagues in the developed world anytime soon. “We have better hygiene, screen windows, air-conditioning,” says Duane J. Gubler, who heads the division of Vector-Borne Infectious Diseases at the U.S. Centers for Disease Control and Prevention. “Television has made us reclusive, at home at the time when we are at greatest risk of being bitten by mosquitoes.”
We are spared by being couch potatoes, each of us a lonely and underpopulated habitat, perched before our television sets, with only our resident bacteria and those low-key hangers-on, the follicle mites, for company.
I thought about all this as I looked through the microscope in Cliff Desch’s laboratory. I also thought, as so many of us do in moments of aesthetic and personal doubt, about Martha Stewart, who has written “I have always been inspired by nature.” I asked Desch what sort of inspiring things the follicle mites might be doing on her forehead and by extension on riffraff like me.
These mites, he said, aren’t much good at crawling to new territory. But they spread from person to person when we nuzzle, and because a population thrives in the area around the nipples, they also pass to newborns as naturally as mother’s milk.
An immigrant mite makes itself at home on a fresh face almost instantly, crawling mouthfirst into the nearest follicle, with its back to the hair shaft and its stumpy legs to the follicle wall. Since it has no reverse gear, Desch said, it may never come out again. Embedded upside down in our skin, it feeds by using those needlelike mouthparts to puncture epithelial cells and suck up the spilled fluids—with no apparent harm to us. It filters out solids even as small as the mitochondria of the cell, a feat Desch characterized as “near-perfect pre-oral digestion.” The mite’s digestive process yields so little waste that it doesn’t even have an excretory opening. It need never get up to go to the bathroom. The follicle mite is, in truth, a couch potato’s couch potato.
“And to reproduce?” I asked Desch, with some trepidation, thinking that a mite must get lonely tucked away somewhere out on the vast, windswept expanse of the forehead. The nearest neighboring mite population centers, around the wings of the nose and in the eyelashes, are as distant as oceanic islands.
The female, Desch said, may produce a first generation asexually, by parthenogenesis—that is, virgin birth. Then she mates with her sons to produce the next generation, up to a maximum population of about ten mites per follicle. (“Oedipus should have plucked out his eyelashes and left his eyes alone,” I muttered.) All this passes utterly unnoticed, “the extreme,” one biologist remarks, “of an exquisite adaptation in which each of us is infested right now, but asymptomatically.” Some researchers theorize that follicle mites may even benefit us in ways we do not yet understand. In any case, there is nothing, from soaps to systemic medicines, that we can do about it.
I left Desch’s lab thinking that follicle mites are precisely the ectoparasite we deserve—and that we are lucky to have them, riding on our foreheads, a living reminder that our flesh is merely a part of the natural world.
Back home I offered to write my wife an ode to her follicle mites. She handed me a washrag for my forehead and suggested curtly that I keep my infestations to myself. But I knew that in the nature of life on the human habitat, it was already way too late for that.
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