by Riskin, Dan
In most cases, the mosquito bite has no aftermath, but sometimes a mosquito’s salivary glands are home to parasites called Plasmodium. Those are tiny wormlike creatures, even smaller than a single red blood cell. When a Plasmodium-infected mosquito bites you, she squirts those parasites into your blood. They float through the bloodstream to the liver, embed there, and breed. After some time, they leave the liver, float back into the blood, and then somehow find their way to the insides of red blood cells—where they reproduce, then burst out, destroying the red blood cells. That process gives the human host horrible fevers, liver damage, and sometimes even inflammation around the brain and spinal cord.
The disease caused by Plasmodium is called malaria, and no other parasite (or predator, for that matter) causes as much human pain and suffering as those little wormlike bastards in the salivary glands of mosquitoes. Using those mosquitoes as their delivery system, Plasmodium parasites infect hundreds of millions of people each year, killing thousands of them, mostly children.10
If having worms enter your red blood cells isn’t bad enough for you, Mother Nature has a smorgasbord of other human parasites to choose from, each more disgusting than the last. For example, I’m personally quite grossed out by the roundworms that cause that grotesque swelling of the lower body called elephantiasis.11 You get those worms from a mosquito bite, and they quickly set up shop in your lymphatic ducts. (Lymphatic ducts are tubes that drain excess fluid from swollen tissues and put that fluid back into the bloodstream.) Once the worms find those lymphatic ducts, they grow to somewhere between one and four inches in length and hide, undetected by the immune system for up to thirty years.
Thirty years!
Do you realize how long that is? Hiding a four-inch worm inside your body without the immune system going nuts should be impossible, even for a day. If humans could mimic the cloaking ability of those worms, it would be an absolute game changer for organ transplantation, where rejection by the immune system is the major barrier to survival. I’m not saying we should love these worms (I sure don’t), but you’ve got to respect what they can do.
Eventually, the worms will die of old age, and when that finally happens, the cloaking system shuts down and the human immune system, suddenly realizing there are giant worms in the body, goes nuts. Swelling is intense and immediate, but the lymphatic ducts that should drain that excess fluid are blocked by the corpses of the worms. As a result, the limbs swell up to unimaginable dimensions. Voilà. Elephantiasis.
And just in case elephantiasis and malaria aren’t enough to convince you of Mother Nature’s dark side . . . You can get roundworms that burrow through your body, eat your organs, and kill you. There are amoebas that can get behind your contact lens, eat your eyeball, and make you go blind. There’s even something called a pinworm that lives in your rectum until it’s ready to lay eggs, then waits until you are asleep, sneaks out your anus, and lays eggs so you’ll be itchy in the morning, touch them with your fingers, and accidentally pass them on to other people when you cook breakfast.12
Another noteworthy human parasite is the leech, a blood-sucking animal closely related to the earthworm. A leech has two suckers—one at the mouth and one on the tail—and it uses them in synchrony, to move around like an inchworm. I remember a fellow bat researcher once telling me about the time he was on a bat expedition in the forests of Vietnam and took a moment to squat in some tall grass to poop. As he squatted, the tips of the grass were at eye level, and he could see leeches on those tips, extended upward from their bottom suckers, waving like alien finger puppets all around him. Every time he rustled to adjust his position, the leeches would bend over and inch their way toward him, then pause once more when he got still again.13 Despite his best efforts at stillness, he told me, he had some removals to perform after the job was done. I didn’t ask him for details.
Leeches feed on people, but they feed on all kinds of other animals too. Some may simply attach themselves to an animal’s leg, but there are other species that specialize in swimming up an animal’s nose when it takes a drink from a stream. Then it painlessly attaches itself to the host and starts drinking.VIII Once a leech is attached, it sucks blood until it swells to around ten times its original weight. Then the next time the animal takes a drink from a stream, the leech lets go, floats away, and digests quietly for a few months, leaving its host only a little worse for wear.
Typically, humans do everything they can to prevent being fed upon by a leech, but that’s not always the case. There are instances where a leech can be a huge help to a person. For instance, let’s say a great white shark bites a few of your fingers off, or maybe a dog gets its lower jaw inside your mouth when it’s attacking you and tears your whole cheek off, or maybe you just lose part of your face to cancer (heaven forbid). In those cases—which I did not make up, by the way—surgeons can reattach the fingers, cheek, or grafted piece of flesh from your own forearm, and they use leeches to help them do that.14
You can’t just sew a piece of flesh back into place—you need to make sure there’s a constant flow of blood into and out of the attached flesh, or the cells within it will die. Surgeons reattaching a flap of skin need to make sure arteries from the body pump blood into the flap, and that the veins within that flap link properly to veins within the body to drain blood out again. But lining up blood vessels is hard, and sometimes blood starts to build up in little pockets when the draining isn’t working quite perfectly. Letting that blood pool in the tissues is bad, but by putting leeches on the surface of the reattached flesh, surgeons ensure that blood doesn’t accumulate anywhere. Leeches just suck blood out of the area so that it can’t accumulate. So long as the leeches are switched out frequently for new ones, the whole blood network of the region has time to heal.
There’s something wonderful to me about the fact that leeches do a better job draining human blood from a surgical site than even our most sophisticated technologies can. The sucker of a leech has over one hundred teeth to make a clean cut, but the leech provides its own anesthetic to make the bite less painful, and its own anticoagulant to keep the blood flowing. Best of all, leeches don’t leave scars.
It’s worth mentioning that when humans take advantage of leeches this way, the leeches don’t count as parasites anymore. Remember, a parasite has to impose an overall cost on its host. Having your cheek heal when it’s reattached is a benefit, so leeches used in surgery don’t count as parasites, even though leeches of the same species would count as parasites if I found them on my leg. Parasitism is all about the relationship. In a sense, we’re taking slothful animals that would rather spend their time mooching, and putting them to work for us.
That’s not the only job we’ve given to leeches either. Researchers have started using leeches to help with the conservation of rare mammals in tropical countries as well. Instead of tromping through the forests of Vietnam looking for animals so rare you might go months without seeing one, researchers can go to a patch of habitat, catch leeches there, then check the DNA of the blood inside the leeches to find out what animals they’ve been feeding on. A leech can store blood for months after a meal, so a single day sampling leeches from an area can give hints about all the different animals that have been there in the last few months.15 Using this method, researchers have found the DNA of extremely rare Vietnamese mammals, including the Annamite striped rabbit, the small-toothed ferret badger, the Truong Son muntjac deer, and a strange antelope called a serow. You might never have heard of any of those animals (don’t feel bad—I hadn’t either), but that’s the point. Even conservation biologists working in those very countries know almost nothing about those creatures, and leeches have become one of their few sources of information about them.
There are a lot of different kinds of parasites, but sometimes what looks like a parasite can turn out to be something else entirely. There’s a fish called Krøyer’s deep sea anglerfish that lives in the pitch dark, one or two kilometers below the surface of the ocean. The fema
le is about two feet long, with a long, thin pole sticking out of her forehead. At the tip of that pole, she has a sack containing bacteria. Those bacteria produce light, and the light they produce attracts smaller fish, which the anglerfish then eats.16 You might remember seeing a deep sea anglerfish in the movie Finding Nemo, when she lured Marlin and Dory in with her light organ and then almost ate them.17
Since the bacteria provide a benefit to the anglerfish, you know they’re obviously not parasites, but that’s not where I’m going with this. If you look carefully at a female Krøyer’s deep sea anglerfish, you’ll usually find something sticking out of her belly that looks a lot like a leech. It’s not a leech, though. It’s another fish. At first, that smaller fish was assumed to be a parasite, but it turned out to be a male Krøyer’s deep sea anglerfish.18
Because males and females of the species meet only rarely down there in the dark, they use a strategy that will ensure that the male will be nearby when the female is ready to lay eggs. When they meet, he burrows his mouth into the bottom of her body and then stays there. Eventually her skin grows around him, and they become one self-fertilizing hermaphroditic Franken-fish. Or if you prefer, she becomes a female with a scrotum. However you want to describe the arrangement, she does all the hunting for both of them: she has a bioluminescent lure and a giant mouth, and while he has neither of those things, he feeds on her body fluids, which almost makes him a parasite. But since he makes sperm for her, he’s not a parasite. He’s a pain, for sure, but she gets a benefit from the relationship, so you can’t call him a parasite no matter how tempting it is to do so.19
Anglerfish use a parasite-like lifestyle to cope with the difficulty of finding members of their own species in the abyss of the deep ocean, but it could be worse. If you think it’s hard to find a sex partner in the deep sea, imagine being a parasite inside the body of your host and then having to search for a mate. You could search forever, and there may simply not be any other parasites of your own species in there to find. That’s the potential predicament faced by a tiny flatworm that sometimes infects people. And that’s why when a couple of those flatworms finally do hook up inside a person, they make that romance last.
This particular flatworm, called a schistosome, starts out inside a snail.20 When the parasite first swims out of the snail, it has just eight hours to find a human host before it dies. If it’s lucky enough to find someone, it burrows right through the person’s skin, enters the bloodstream, and then floats to the lungs. There the worm rests for a couple of days, builds itself a jacket of proteins so the human’s immune system won’t recognize it, then slips back into the bloodstream. Eventually, it finds its way to the small blood vessels near the human’s bladder or intestines. This is where the search for a mate must happen, and as you can imagine, the odds of finding a mate there may be very, very low.
If a male and female schistosome do happen to cross paths there, the smaller male curls up inside a groove that runs down the body of the female.IX He’s like a half-inch-long hot dog, and she’s a slightly longer hot-dog bun. With things in place, they start having hot-dog-on-hot-dog-bun sex, and that sex lasts for up to thirty years.
Over that time, they will release roughly three hundred eggs per day into the blood. (Thirty years of that results in more than 3 million eggs.) Half those eggs will be absorbed by the human host, causing all kinds of problems, like blood in the feces and urine, for example, and half will make their way into the host’s urine or feces, then out of the body.21 With any luck (for the parasite), those wastes will get back into the water so that some other snail can be infected, and the life cycle will continue.
With the odds so low that a male and female will ever happen to meet in the same blood vessel at the same time, these flatworms have adopted a strategy of just making zillions and zillions of eggs in the hopes that a few of them have that kind of luck. The good news for schistosomes is that it seems to be working. Two hundred million people have this disease, which is called bilharzia or schistosomiasis. But of course, what’s good news for schistosomes is bad news for humans.
Unlike malaria, which can kill you, bilharzia just wears down the immune system. That makes the people who suffer from it susceptible to other diseases or infections. Fear of bilharzia once prevented me from dipping my feet in a cool river after a long hot day of bat work in Madagascar in 2008.X I could see the locals washing themselves and their clothes in the river, but my knowledge of parasites made that pristine stream look like the jaws of death. I was taking drugs at the time to prevent me from catching malaria, but I was defenseless against the schistosomes. They terrified me.
You might have had experience with schistosomes too, even in Europe or North America. If you’ve ever had “swimmer’s itch,” your skin was penetrated by schistosomes that normally infect birds.22 Fortunately, they were killed by your immune system before they could do any damage. Those itchy welts on your legs, which resulted from the histamines your immune system used in its assault against the worms, serve as a good reminder that even though most parasites can’t infect humans, they truly are everywhere.
Many of the parasites that infect nonhuman species mirror our own, like bird schistosomiasis or gorilla malaria, both caused by parasite species slightly different from the ones that infect us, but many animal parasites are totally different from anything humans ever get (thank goodness). The slothful but successful strategies of parasites have resulted in some breathtakingly disgusting spectacles.
One of my favorites (I’m literally wincing as I type this) is a wasp, called the emerald cockroach wasp, that will fly out of the sky and sting a cockroach, so that the cockroach, though still able to walk, just really doesn’t want to anymore. Next the wasp grabs the roach by an antenna and drags it, kind of like a poorly trained dog on a leash, to its underground lair.23 Then the wasp lays an egg on the cockroach, buries it, and leaves. When the egg hatches, the maggot burrows into the cockroach and begins feeding. To keep the roach alive as long as possible, the maggot eats the roach’s organs in a careful sequence. It also spits cleaning chemicals all over the place to make sure things stay fresh—that is, to make sure bacteria don’t grow in the insect, which would make the maggot’s food source go bad.24 These strategies allow the maggot to feed on that same defenseless cockroach for several weeks, and what’s really sick about it all is that the cockroach stays alive right up until the end, when the maggot molts into an adult wasp, bursts out, and then tunnels its way up through the soil to the surface.
You can hate cockroaches as much as you want, but you have to admit that’s a pretty terrible way to die.
The thing to keep in mind, though, is that this happens every time an emerald cockroach wasp reproduces. They can only reproduce by torturing cockroaches. It’s not like a wasp here or there suddenly goes all evil and starts needlessly torturing other living things. Every time an emerald cockroach wasp hatches, it’s from the corpse of a tortured animal.
This parasitism is so ruthless that the animals that lay their eggs inside a living host have been given their own special name. They’re not just parasites. They’re parasitoids. In fact, there are many wasps, flies, beetles, moths, and other insects that make their living this way. Believe it or not, 10 percent of all insect species are parasitoids. If you just look at the flies, you can find 16,000 parasitoid species—roughly a fifth of all the fly species in the world!25 In other words, laying eggs in a living host isn’t some biological rarity. It’s common. And that means millions and millions and millions of animals around the world are being tortured this way as you read this sentence.
There are many counterstrategies for the would-be victims of parasitoids. Some fight off the egg-laying attacker, whereas others just try to avoid them in the first place. Because parasitoids sometimes track down a host by the smell of its poop, one way to stay hidden is to make sure you’re never very close to your own feces. That’s why the skipper caterpillar of Brazil launches feces at around 4.3 feet per second whene
ver it goes to the bathroom, propelling its poop up to 30 inches away.26 To put that in human terms, that’s a five-foot-tall woman launching her poop 75 feet (from a lying-down position, no less).27
For many years it was assumed that this fecal feat was accomplished by means of a flicking set of brushes, called an anal comb, that could launch the mess, much like your toothbrush can spatter toothpaste all over the mirror if you rub the bristles. But no, nature is far grosser than that. We now know that the fecal flinging is accomplished through a buildup of pressure, just as you would launch a spitball through a straw. I’ve had some pretty explosive diarrhea in my day, but even amid the gastrointestinal fireworks I experienced in Madagascar, I haven’t come close to the performance of a skipper caterpillar.
The only reason this super pooper ever evolved in the first place is the deadly threat of parasites. Parasites are important that way: they shape animal bodies, animal behavior, and animal evolution. When people describe evolution, they usually talk about how hard animals have to work to avoid getting eaten by predators, but parasites may well be even more important in most circumstances.
Sometimes, even when it looks like predators are picking who lives or dies, it can be parasites who are really in control. For example, one particular bat in a barn will get eaten by a raccoon, as opposed to the other bats around it, but that may be because that particular bat was sick from its parasites. For the parasites inside that prey animal, getting eaten could mean they’ll get into the predator. Suddenly sickness isn’t just a failure of the prey animal’s meat robot but the hijacking of that meat robot by the parasite’s DNA. It’s enough to make you look at sickness and health in a whole new light.28
