Parasitologists need years, sometimes decades, to decipher these adaptations. They can’t spend a summer following a troop of monkeys or put radio collars on a pack of wolves. Parasites live invisibly, and parasitologists usually can see what they’re doing only by killing their hosts and dissecting them. These grisly snapshots slowly add up to a natural history.
Steenstrup knew that flukes were extraordinary animals, but little more than that. After one hundred fifty years of experiments, parasitologists can show just how extraordinary they are. Consider the blood fluke Schistosoma mansoni, a tiny missile just emerged from its snail and swimming through a pond in search of a human ankle. If it feels the ultraviolet rays of the sun, it stops swimming and sinks back down into the darkness to hide from the damaging radiation. But if it senses molecules from human skin, it begins to swim madly, jerking around in different directions. When it reaches the skin, it drills its way in. Human skin is far tougher than the soft flesh of a snail, so the fluke lets its long tail snap off, the wound quickly healing as it burrows in. Special chemicals it releases from its coat soften up the skin, letting it plunge into its host like a worm in mud.
After a few hours it has reached a capillary. It has traded the streams of the outside world for the internal ones. These capillaries are barely wider than the fluke itself, so the fluke needs to use a pair of suckers to inch forward. It makes its way to a larger vein, and a larger one still, finally making its way into a torrent of blood so powerful it carries the fluke away. The parasite rides the surge until it finally reaches the lungs. It moves from the veins to the arteries like a snake in a forest canopy. Finding its way back into a lung capillary, and then to a major artery, it is swept through the body once again. It may tour its host’s entire body three times until it finally comes to rest in the liver.
Here the fluke lodges itself in a vessel and finally has its first meal since leaving the snaiclass="underline" a drop of blood. It now begins to mature. If it’s a female, a uterus starts to take shape. If it’s a male, eight testes form like a bunch of grapes. In either case, the fluke grows dozens of times bigger in a few weeks. Now it is time for the parasite to search for a partner for life. If it is lucky, other flukes sniffed out this human host and are lodged in the liver as well. The females are delicate and slender; the males are shaped something like a canoe. They begin to make blood-borne odors that lure members of the opposite sex, and once a female encounters a male, she slips into his spiny trough. There she locks in, and the male carries her out of the liver. Over the course of a couple of weeks, the pair make the long journey from the liver to the veins that fan out across the gut. As they travel the male passes molecules into the female’s body that tell her genes to make her sexually mature. They keep traveling until they reach a resting place unique to their own species. Schistosoma mansoni stops near the large intestine. If we were following Schistosoma haemotobium, it would take another route to the bladder. If we were following Schistosoma nasale, a blood fluke of cows, it would take yet another route to the nose.
Once they find their destined place, the fluke couple stay there for the rest of their lives. The male drinks blood with his powerful throat and massages the female to help thousands of blood cells flow into her mouth and through her gut; he consumes his own weight in glucose every five hours and passes on most of it to her. They may be the most monogamous couples in the animal kingdom—a male will clasp onto its female even after she has died. (A few homosexual flukes will also get together. While their fit isn’t as tight, they will keep reuniting if a disapproving scientist should separate them.)
Heterosexual flukes mate every day of their long lives, and whenever the female is ready to lay her eggs, the male makes his way along the wall of the bowels until he finds a good spot. The female slides partially out of the trough, far enough to lay her eggs in the smallest capillaries. Some of the eggs are carried away by the bloodstream and end up back in the liver, that meaty filter, where they lodge and inflame the tissue, causing much of the agony of schistosomiasis. But the rest of the eggs work their way into the intestines and escape their host, ready to slice open their shells and find a new snail.
Each piece of the parasite puzzle costs years of research. The question of how parasites navigate has taken up just about the entire career of one scientist, Michael Sukhdeo. Sukhdeo teaches these days at Rutgers University in New Jersey. New Jersey may be a long way from Tambura, but he has no shortage of parasites to study in horses, cows, and sheep. I paid a visit to Sukhdeo at his office. He is a stocky man with a sly goatee. A bike hangs from his office wall, fish swim in a tank by his desk, and classic rock comes out of his radio. Sukhdeo, like a lot of parasitologists I’ve met, can slide into gruesome conversation without any warning. I suppose when you spend your days studying creatures that chew up the lining of livers and intestines, there’s no sense in dancing around the uglier basics of life. He started to talk about how grotesque it is when people get elephantiasis, which was common in British Guyana, where he spent much of his childhood. “Everywhere you walked you saw people with huge bulges in their crotch and big swollen elephantine feet,” he said.
Sukhdeo then told me how he himself became infected when he was eleven. He developed a swelling, and his parents took him to a clinic. “When you’re testing for elephantiasis, the microfilaria come out into the bloodstream only at dusk. Nobody knows where they go. So at night we had to go to this clinic to get our blood checked. And there was a girl there, about my age; she was eleven, and she had only one breast. That’s a place where the worms live. She was a beautiful girl; I was in love. We both got checked at the same time. It was twelve Guyanese dollars—six American dollars—for treatment. They couldn’t afford it for their daughter. We offered to pay for them, but they were very proud and wouldn’t even take a loan. And so that girl remained infective—over six American dollars.”
Sukhdeo went to McGill University in Montreal, and there he discovered that while parasites might be grotesque, they were also the most interesting creatures he had ever encountered. “I took a course in human parasitology, and—pow—it was disgusting and really exciting. I had gone through four years of university and nothing had turned me on in just that way. They were just so weird, and there was so little known about them.”
He decided to go on studying parasites in graduate school, and there he realized that people had very little idea of how parasites behaved as actual, living organisms. Many parasitologists have resigned themselves to studying them on an abstract plane—cataloging new species according to their suckers and spines, for example, without ever knowing what those suckers and spines are for.
For his master’s degree, Sukhdeo chose Trichinella spiralis. This tiny nematode comes our way inside the muscle of undercooked pork, where it lives in cysts formed from individual muscle cells. When a person eats the meat, the parasite breaks out of its cyst and makes its way into the intestines, threading itself through the cells of the lining. There it mates and produces a new generation of Trichinella, which leave the intestines and travel through the bloodstream until they lodge in the person’s muscle and form cysts of their own. Humans are only accidental hosts for Trichinella; they are unable to carry the parasite on to the next stage of its life cycle. Pigs are a much more profitable host; a dead pig may be scavenged by a rat, which then dies and is scavenged by another rat, which may be eaten by a pig. Pigs can pass Trichinella to each other by being fed infected meat or by chewing their tails off. In the wild, predatory mammals and scavengers keep the cycle spinning along—ranging from polar bears and walrus in the Arctic to hyenas and lions in Africa.