Parasite Rex

Inside the Bizarre World of Nature's Most Dangerous Creatures

Carl Zimmer

• Publisher: Atria Books
• Year: 2000
• ISBN:
  9780743213714
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Combining the literary sensibility of David Quammen with the scientific rigor of Stephen Jay Gould, Carl Zimmer reveals the power, danger, and beauty of the surprising creatures who dominate the earth: parasites. For centuries, parasites have lived in nightmares, horror stories, and the darkest shadows of science. In Parasite Rex, Carl Zimmer takes readers on a fantastic voyage into the secret universe of these extraordinary life-forms—which are not only among the most highly evolved on Earth, but make up the majority of life's diversity. Traveling from the steamy jungles of Costa Rica to the parasite-riddled war zone of southern Sudan, Zimmer introduces an array of amazing creatures that invade their hosts, prey on them from within, and control their behavior. He also vividly describes parasites that can change DNA, rewire the brain, make men more distrustful and women more outgoing, and turn hosts into the living dead. This comprehensive, gracefully written book brings parasites out into the open and uncovers what they can teach us all about the most fundamental survival tactics in the universe—the laws of Parasite Rex.

1
Nature’s Criminals

Nature is not without a parallel strongly suggestive of our social perversions of justice, and the comparison is not without its lessons. The ichneumon fly is parasitic in the living bodies of caterpillars and the larvae of other insects. With cruel cunning and ingenuity surpassed only by man, this depraved and unprincipled insect perforates the struggling caterpillar, and deposits her eggs in the living, writhing body of her victim.

—John Brown, in Parasitic Wealth or Money Reform: A Manifesto to the People of the United States and to the Workers of the World (1898)

In the beginning there was fever. There was bloody urine. There were long quivering strings of flesh that spooled out of the skin. There was a sleepy death in the wake of biting flies.

Parasites made themselves, or at least their effects, known thousands of years ago, long before the name parasite—parasitos—was created by the Greeks. The word literally means “beside food,” and the Greeks originally had something very different in mind when they used it, referring to officials who served at temple feasts. At some point the word slipped its etymological harness and came to mean a hanger-on, someone who could get the occasional meal from a nobleman by pleasing him with good conversation, delivering messages, or doing some other job. Eventually the parasite became a standard character in Greek comedy, with his own mask. It would be many centuries before the word would cross over to biology, to define life that drains other lives from within. But the Greeks already knew of biological parasites. Aristotle, for instance, recognized creatures that lived on the tongues of pigs, encased in cysts as tough as hailstones.

People knew about parasites elsewhere in the world. The ancient Egpytians and Chinese prescribed different sorts of plants to destroy worms that lived in the gut. The Koran tells its readers to stay away from pigs and from stagnant water, both sources of parasites. For the most part, though, this ancient knowledge has only left a shadow on history. The quivering strings of flesh—now known as guinea worms—may have been the fiery serpents that the Bible describes plaguing the Israelites in the desert. They certainly plagued much of Asia and Africa. They couldn’t be yanked out at one go, since they would snap in two and the remnant inside the body would die and cause a fatal infection. The universal cure for guinea worm was to rest for a week, slowly winding the worm turn by turn onto a stick to keep it alive until it had crawled free. Someone figured out this cure, someone forgotten now for perhaps thousands of years. But it may be that that person’s invention was remembered in the symbol of medicine, known as the caduceus: two serpents wound around a staff.

As late as the Renaissance, European physicians generally thought that parasites such as guinea worms didn’t actually make people sick. Diseases were the result of the body itself lurching out of balance as a result of heat or cold or some other force. Breathing in bad air could bring on a fever called malaria, for example. A disease came with symptoms: it made people cough, put spots on their belly, gave them parasites. Guinea worms were the product of too much acid in the blood, and weren’t actually worms at all—they were something made by a diseased body: perhaps corrupted nerves, black bile, elongated veins. It was hard to believe, after all, that something as bizarre as a guinea worm could be a living creature. Even as late as 1824, some skeptics still held out: “The substance in question cannot be a worm,” declared the superintending surgeon of Bombay, “because its situtation, functions, and properties are those of a lymphatic vessel and hence the idea of its being an animal is an absurdity.”

Other parasites were undeniably living creatures. In the intestines of humans and animals, for instance, there were slender snake-shaped worms later named Ascaris, and tapeworms—flat, narrow ribbons that could stretch for sixty feet. In the livers of sick sheep were lodged parasites in the shape of leaves, called flukes after their resemblance to flounder (floc in Anglo-Saxon). Yet, even if a parasite was truly a living creature, most scientists reasoned, it also had to be a product of the body itself. People carrying tapeworms discovered to their horror that strips of it would pass out with their bowel movements, but no one had ever seen a tapeworm crawl, inch by inch, into a victim’s mouth. The cysts that Aristotle had seen in the tongues of pigs had little wormlike creatures coiled up inside, but these were helpless animals that didn’t even have sex organs. Parasites, most scientists assumed, must have been spontaneously generated in bodies, just as maggots appeared spontaneously on a corpse, fungus on old hay, insects from within trees.

In 1673, the visible parasites were joined by a zoo of invisible ones. A shopkeeper in the Dutch city of Delft put a few drops of old rainwater under a microscope he had built himself, and he saw crawling globules, some with thick tails, some with paws. His name was Anton van Leeuwenhoek, and although in his day he was never considered anything more than an amateur, he was the first person to lay eyes on bacteria, to see cells. He put everything he could under his microscope. Scraping his teeth, he discovered rod-shaped creatures living on them, which he could kill with a sip of hot coffee. After a disagreeable meal of hot smoked beef or ham, he would put his own loose stool under his lenses. There he could see more creatures—a blob with leglike things that it used to crawl like a wood louse, eel-shaped creatures that would swim like a fish in water. His body, he realized, was a home to microscopic parasites.

Other biologists later found hundreds of different kinds of microscopic creatures living inside other creatures, and for a couple of centuries there was no divide between them and the bigger parasites. The new little worms took many shapes—of frogs, of scorpions, of lizards. “Some shoot forth horns,” one biologist wrote in 1699, “others acquire a forked Tail; some assume Bills, like Fowls, others are covered with Hair, or become all over rough; and others again are covered with Scales and resemble Serpents.” Meanwhile, other biologists identified hundreds of different visible parasites, flukes, worms, crustaceans, and other creatures living in fish, in birds, in any animal they opened up. Most scientists still held on to the idea that parasites large and small were spontaneously generated by their hosts, that they were only passive expressions of disease. They held on through the eighteenth century, even as some scientists tested the idea of spontaneous generation and found it wanting. These skeptics showed how the maggots that appeared on the corpse of a snake were laid as eggs by flies, and themselves grew into flies.

Even if maggots weren’t spontaneously generated, parasites were a different matter. They simply had no way of getting inside a body and so had to be created there. They had never been seen outside a body, animal or human. They could be found in young animals, even in aborted fetuses. Some species could be found in the gut, living happily alongside other organisms that were being destroyed by digestive juices. Others could be found clogging the heart and the liver, without any conceivable way to get into those organs. They had hooks and suckers and other equipment for making their way inside a body, but they would be helpless in the outside world. In other words, parasites were clearly designed to live their entire lives inside other animals, even in particular organs.

Spontaneous generation was the best explanation for parasites, given the evidence at hand. But it was also a profound heresy. The Bible taught that life was created by God in the first week of creation, and every creature was a reflection of His design and His beneficence. Everything that lived today must descend from those primordial creatures, in an unbroken chain of parents and children—nothing could later come squirting into existence thanks to some vital, untamed force. If our own blood could spontaneously generate life, what help did it need from God back in the days of Genesis?

The mysterious nature of parasites created a strange, disturbing catechism of its own. Why did God create parasites? To keep us from being too proud, by reminding us that we were merely dust. How did parasites get into us? They must have been put there by God, since there was no apparent way for them to get in by themselves. Perhaps they were passed down through generations within our bodies to the bodies of our children. Did that mean that Adam, who was created in purest innocence, came into being already loaded with parasites? Maybe the parasites were created inside him after his fall. But wouldn’t this be a second creation, an eighth day added on to that first week—“and on the following Monday God created parasites”? Well, then, maybe Adam was created with parasites after all, but in Eden parasites were his helpmates. They ate the food he couldn’t fully digest and licked his wounds clean from within. But why should Adam, created not only in innocence but in perfection, need any help at all? Here the catechism seems to have finally fallen apart.

Parasites caused so much confusion because they have life cycles unlike anything humans were used to seeing. We have the same sorts of bodies as our parents did at our age, as do salmon or muskrats or spiders. Parasites can break that rule. The first scientist to realize this was a Danish zoologist, Johann Steenstrup. In the 1830s he contemplated the mystery of flukes, whose leaf-shaped bodies could be found in almost any animals a parasitologist cared to look at—in the livers of sheep, in the brains of fish, in the guts of birds. Flukes laid eggs, and yet no one in Steenstrup’s day had ever found a baby fluke in its host.

They had, however, found other creatures that looked distinctly flukish. Wherever certain species of snails lived, in ditches or ponds or streams, parasitologists came across free-swimming animals that looked like small versions of flukes except that they had great tails attached to their rears. These animals, called cercariae, flicked their tails madly through the water. Steenstrup scooped up some ditch water, complete with snails and cercariae, and kept it in a warm room. He noticed that the cercariae would penetrate the mucus coating the snail’s body and shell, drop their tails, and form a hard cyst, which, he said, “arches over them like a small, closely-shut watch glass.” When Steenstrup pulled the cercariae out of these shelters, he found that they had become flukes.

Biologists knew that the snails were home to other sorts of parasites as well. There was a creature that looked like a shapeless bag. There was also a little beast they called the King’s yellow worm: a pulpy animal that lived in the snail’s digestive gland and carried within it what looked like cercariae, all writhing like cats inside a burlap sack. And Steenstrup even found another flukelike creature swimming free, this one not using a missile-shaped tail but instead hundreds of fine hairs that covered its body.

Looking at all these organisms swimming through the water and through the snails—organisms that in many cases had been given their own Latin species names—Steenstrup made an outrageous suggestion. All these animals were different stages and generations of a single animal. The adults laid eggs, which escaped out of their hosts and landed in water, where they hatched into the form covered in fine hairs. The hair-covered form swam through the water and sought out a snail, and once it had penetrated a snail, the parasite transformed itself into the shapeless bag. The shapeless bag began to swell with the embryos of a new generation of flukes. But these new flukes were nothing like the leaf-shaped forms inside a sheep’s liver, or even the finely haired form that entered the snail. These were the King’s yellow worms. They moved through the snail, feeding and rearing within them yet another generation of flukes—the missile-tailed cercariae. The cercariae emerged from the snail, promptly forming cysts on the snail. From there they somehow got into sheep or another final host, and there they emerged from their cysts as mature flukes.

Here was a way that parasites could appear inside our bodies with no precedent: “An animal bears young which are, and remain, dissimilar to their parent, but bring forth a new generation, whose members either themselves, or in their descendants, return to the original form of the parent animal.” Scientists had already met the precedents, Steenstrup was saying, but they couldn’t believe that they all belonged to the same species.

Steenstrup would eventually be proved right. Many parasites travel from one host to another during their life cycles, and in many cases they alternate between different forms from one generation to the next. And thanks to his insight, one of the best cases for spontaneous generation in parasites fell apart. Steenstrup turned his attention from flukes to the worms that Aristotle had seen living in cysts embedded in pig tongues. These parasites, called bladder worms at the time, can live in any muscle in mammals. Steenstrup suggested that bladder worms were actually an early stage in the development of some other worm not yet found.

Other scientists noticed that bladder worms looked a bit like tapeworms. All you had to do was cut off most of the tapeworm’s long ribbony body, and tuck its head and first few segments inside a shell, and you had a bladder worm. Maybe the bladder worm and tapeworm were one and the same. Maybe they were actually the product of tapeworm eggs that had made their way into the wrong host. When the eggs hatched in this hostile environment, the tapeworms couldn’t take their normal path of development but grew instead into stunted deformed monsters that died before they could reach maturity.

In the 1840s, a devout German doctor heard about these ideas and was outraged. Friedrich Küchenmeister kept a little medical practice in Dresden, and in his free time he wrote books on biblical zoology and ran the local cremation club, called Die Urne. Küchenmeister recognized that the idea that bladder worms were actually tapeworms certainly sidestepped the heresy of spontaneous generation. But it then fell into another sinful trap—the idea that God would let one of his creatures wind up in a monstrous dead end. “It would be contrary to the wise arrangement of Nature which undertakes nothing without a purpose,” Küchenmeister declared. “Such a theory of error contradicts the wisdom of the Creator and the laws of harmony and simplicity put into Nature”—laws that even applied to tapeworms.

Küchenmeister had a more pious explanation: the bladder worms were an early stage in the natural life cycle of the tapeworm. After all, the bladder worms tended to be found in prey—animals such as mice, pigs, and cows—and the tapeworms were found in predators: cats, dogs, humans. Perhaps when a predator ate prey, the bladder worm emerged from its cyst and grew into a full tapeworm. In 1851, Küchenmeister began a series of experiments to rescue the bladder worm from its dead end. He plucked out forty of them from rabbit meat and fed them to foxes. After a few weeks, he found thirty-five tapeworms inside the foxes. He did the same with another species of tapeworm and bladder worm in mice and cats. In 1853, he fed bladder worms from a sick sheep to a dog, which soon was shedding the segments of an adult tapeworm in its feces. He fed these to a healthy sheep, which began to stumble sixteen days later. When the sheep was killed and Küchenmeister looked in its skull, he found bladder worms sitting on top of its brain.

When Küchenmeister reported his findings, he stunned the university professors who made parasites their life’s work. Here was an amateur out on his own, sorting out a mystery the experts had failed to solve for decades. They tried to poke holes in Küchenmeister’s work wherever possible, to try to keep their own ideas about dead-end bladder worms alive. One problem with Küchenmeister’s work was that he sometimes fed the bladder worms to the wrong host species and the parasites all died. He knew, for example, that pork carried a species of bladder worm, and he knew that the butchers of Dresden and their families often suffered from tapeworms called Taenia solium. He suspected that the two parasites were one and the same. He fed Taenia eggs to pigs and got the bladder worms, but when he fed the bladder worms to dogs, he couldn’t get adult Taenia. The only way to prove the cycle was to look inside its one true host—humans.

Küchenmeister was so determined to prove God’s benevolent harmony that he set up a gruesome experiment. He got permission to feed bladder worms to a prisoner about to be executed, and in 1854 he was notified of a murderer to be decapitated in a few days. His wife happened to notice that the warm roast pork they were eating for dinner had a few bladder worms in it. Küchenmeister rushed to the restaurant where they had bought the pork. He begged for a pound of the raw meat, even though the pig had been slaughtered two days earlier and was beginning to go bad. The restaurant owners gave him some, and the next day Küchenmeister picked out the bladder worms and put them in a noodle soup cooled to body temperature.

The prisoner didn’t know what he was eating and enjoyed it so much he asked for seconds. Küchenmeister gave him more soup, as well as blood sausage into which he had slipped bladder worms. Three days later the murderer was executed, and Küchenmeister searched his intestines. There he found young Taenia tapeworms. They were still only a quarter of an inch long, but they had already developed their distinctive double crown of twenty-two hooks.

Five years later, Küchenmeister repeated the experiment, this time feeding a convict four months before his execution. Afterward he found tapeworms as long as five feet in the man’s intestines. He felt triumphant, but the scientists of his day were disgusted. The experiments were “debasing to our common nature,” said one reviewer. Another compared him to some doctors of the day who cut the still-beating heart out of a just-executed man, merely to satisfy their curiosity. One quoted Wordsworth: “One that would peep and botanise/Upon his mother’s grave?” But no doubt was left that parasites were among the strangest things alive. Parasites were not spontaneously generated; they arrived from other hosts. Küchenmeister also helped discover another important thing about parasites that Steenstrup hadn’t observed: they didn’t always have to wander through the outside world to get from one host to another. They could grow inside one animal and wait for it to be eaten by another.

The last possibility still left for spontaneous generation was represented by the microbes. That was shortly put to rest by the French scientist Louis Pasteur. To make his classic demonstration, he put broth in a flask. Given enough time the broth would go bad, filling with microbes. Some scientists claimed that the microbes were spontaneously generated in the broth itself, but Pasteur showed that the microbes were actually carried in the air to the flask and settled into it. He went on to prove that microbes weren’t just a symptom of diseases but often their cause—what came to be known as the germ theory of infection. And out of that realization came the great triumphs of Western medicine. Pasteur and other scientists began to isolate the particular bacteria that caused diseases such as anthrax, tuberculosis, and cholera and to make vaccines for some of them. They proved that doctors spread disease with their dirty hands and scalpels and could stop it with some soap and hot water.

With Pasteur’s work, a peculiar transformation came over the concept of the parasite. By 1900, bacteria were rarely called parasites anymore, even though, like tapeworms, they lived in and at the expense of another organism. It was less important to doctors that bacteria were organisms than that they had the power to cause diseases and that they could now be erased with vaccines, drugs, and good hygiene. Medical schools focused their students on infectious diseases, and generally on those caused by bacteria (or later, by the much smaller viruses). Part of their bias had to do with how scientists recognize causes of diseases. They generally follow a set of rules proposed by the German scientist Robert Koch. To begin with, a pathogen had to be shown to be associated with a particular disease. It also had to be isolated and grown in pure culture, the cultured organism had to be inoculated into a host and produce the disease again, and the organism in the second host had to be shown to be the same as that inoculated. Bacteria fit these rules without much trouble. But there were many other parasites that didn’t.

Living alongside bacteria—in water, soil, and bodies—were much larger (but still microscopic) single-celled organisms known as protozoa. When Leeuwenhoek had looked at his own feces, he had seen a protozoan now called Giardia lamblia, which had made him sick in the first place. Protozoa are much more like the cells that make up our own bodies, or plants or fungi, than they are like bacteria. Bacteria are essentially bags of loose DNA and scattered proteins. But protozoa keep their DNA carefully coiled up on molecular spools within a shell called the nucleus, just as we do. They also have other compartments dedicated to generating energy, and their entire contents are surrounded by skeleton-like scaffolding, as with our cells. These were only a few of many clues biologists discovered that showed the protozoa to be more closely related to multicellular life than to the bacteria. They went so far as to divide life into two groups. There were the prokaryotes—the bacteria—and the eukaryotes: protozoa, animals, plants, and fungi.

Many protozoa, such as the amoebae grazing through forest floors, for instance, or the phytoplankton that turn the oceans green, are harmless. But there are thousands of species of parasitic protozoa, and they include some of the most vicious parasites of all. By the turn of the century, scientists had figured out that the brutal fevers of malaria weren’t caused by bad air but by several species of a protozoan called Plasmodium, a parasite that lived inside mosquitoes and got into humans when the insects pierced the skin to suck blood. Tsetse flies carried trypanosomes that caused sleeping sickness. Yet, despite their power to cause disease, most protozoa couldn’t live up to Koch’s rigorous demands. They were creatures after Steenstrup’s heart, passing through alternating generations.

Plasmodium, for example, enters a human body through a mo...