Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures

^{Malcolm Jones, Centre for Microscopy and Microanalysis, the University of Queensland}

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Hookworms live inside 1.3 billion people. They use their powerful teeth to lacerate a patch of the intestinal wall (inset) and drink blood from the wound.

^{© Manfred Kage/Peter Arnold Inc.}

Tapeworms, reaching up to sixty feet long, are the biggest parasites that live in humans.

^{Claire Healy, University of Connecticut}

^{Courtesy of Kirsten Jensen, University of Connecticut}

^{Daniel Brooks}

^{Claire Healy}

^{Courtesy of Kirsten Jensen}

^{Darlyne Murawski}

There are 5,000 known species of tapeworm that live in various animals, and probably many thousands more still await discovery. Each one has a head specially adapted for lodging itself in its host’s body.

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Schistosoma (also known as the blood fluke) infects more than 200 million people. Its eggs hatch in fresh water and the young parasite seeks out a snail.

Inside the snail, the parasite passes through several generations before producing a missile-shaped stage called a cercaria.

^{Ming Wong}

The cercaria then penetrates human skin and becomes an adult that finally ends up in the veins of its human host.

^{Dickson Despommiers}

Trichinella, the cause of trichinosis, is an exceptional parasite: an animal that lives like a virus. Its larvae penetrate individual muscle cells and coil up inside, taking control of the muscle’s DNA in order to make the cells a more comfortable home.

^{Lennart Nilsson/Albert Bbonniers Forlag AB}

The single-celled parasite Plasmodium falciparum causes malaria. Here a new generation of the parasite bursts out of a red blood cell.

^{© Oliver Meckes, Science Source/Photo Researchers, inc.}

Bottom left: Another single-celled parasite, Trypanosoma brucei, is the cause of sleeping sickness.

^{David Roos}

Toxoplasma gondii (shown here nestled inside a host cell) is one of the most successful parasites on Earth: in some regions of the world, 90 percent of people carry it in their bodies.

^{Matthew Gilligan}

^{Matthew Gilligan}

Parasites often choose very particular—and peculiar—places to live. This crustacean invades a fish’s mouth, devours its tongue, and takes the tongue’s place. It then acts like a tongue; the fish can use it to grip and swallow prey.

^{George Benz, Southeast Aquatic Research Institute, and Jeff Braswell, Dupont}

Another choosy parasite is the crustacean Ommatokoita elongata. It lives only in Greenland sharks, which roam underneath the Arctic ice. Moreover, Ommatokoita lives only in their eyes, anchoring itself in the eyes’ jelly with its specially adapted legs.

^{Photo by Marianne Alleyne and Nancy Beckage}

Insects are masters at parasitizing other insects. Parasitic wasps lay their eggs inside caterpillars, and the larvae slowly devour their living hosts before crawling out and weaving cocoons.

^{Elke Buschbeck, Birgit Ehmer/Cornell University}

The insect Xenos peckii makes non-parasitic paper wasps its host. When its eggs hatch, the female stays inside, devouring its hosts’ sex organs, while the male burrows out and flies to another wasp to find a mate. As an adult, the male has only a few hours to live; as a result, it has evolved remarkable eyes to help find a mate. It has 100 miniature eyes, each of which is equipped with its own retina, able to form a full image of its own.

^{Charles and Fanny Brewer-Carias}

Once a parasite has used up its host, it needs to escape. A fungus emerges from an ant.

^{Andreas Schmidt-Rhaesa}

A worm-like parasite called a nematomorph escapes its cricket host.

^{Thomas Dunagan}

Thorny-headed worms, like many parasites, live in two or more hosts during their life. Many of them live initially in insects or crustaceans and then move into predators such as birds. To get into these predators, the parasites make their intermediate hosts stupid and foolhardy—and thus easily preyed upon.

^{Jens Hoeg}

Top: The parasitic barnacle Sacculina carcini invades crabs and fills up their entire body with a network of roots. It forms a sac full of larvae where the crab’s own egg pouch should be (middle), and it forces the crab to care for its young.

^{Todd Huspeni, University of California, Santa Barbara}

^{Todd Huspeni, University of California, Santa Barbara}

Bottom: Snails can also be horrifically victimized when they are infected with the fluke Leucochloridium paradoxum. The parasite’s final hosts are birds. To get their attention, the parasite climbs into the snail’s transparent tentacles. The striped flukes, which can be seen through the tentacles, look like caterpillars, and they catch the eye of hungry birds.

^{David Kjaer/BBCWild}

Cuckoos are a special sort of parasite—they don’t live inside other animals, but they steal parental care. They lay their eggs in the nests of other bird species and trick the foster parents into rearing them. Here a reed warbler contemplates the giant cuckoo nestling that has taken its own offspring’s place.

Only a few parasites of humans are on the verge of eradication. For centuries people have extracted guinea worms from their legs by gently spooling them onto sticks. Public health campaigns have driven down guinea worms to less than 100,000 cases a year and are on the verge of eradicating the parasite altogether.

In 1998 a new campaign was launched to wipe out elephantiasis, caused by microscopic worms that block lymph nodes.

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Parasites sometimes make mistakes, and the results can be lethal. Normally, tapeworms first mature in a cyst in intermediate hosts such as cows or pigs before they move on to humans. But if their eggs should end up in a human body, they will go ahead and form a cyst anyway, often in the brain.

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A botfly laid its eggs on a penetrated his brain.

^{Photo by Michael A. Huffman}

Hosts have had to evolve ways to fend off the ever-present threat of parasites. Chimpanzees eat medicinal plants to fight off invaders.

^{Fox/Everett Collection}

Hollywood has a healthy respect for the sophistication and cunning of parasites. In the television show The X-Files, a fungus attacks people in the same way that some real fungi attack insects.

^{Everett Collection}

In the Alien movie series, a creature fashioned after parasitic wasps plants its young in the chests of human hosts.