How far would you go to survive? Consider a parasite--an organism that lives on or inside another organism, or "host." The lancet fluke, for example, is a leaf-shaped flatworm, and this deceptively simple-looking species--Dicrocoelium dendriticum [dye-KROW-sealy-um den-DRIT-ic-um]--has a wildly elaborate life cycle. Adult flukes naturally live in and dine on the livers of grass-eating mammals like cows. The parasites shed their eggs in cow manure (see diagram, right).
Hungry snails--host number two--then devour the egg-infested dung. Fluke eggs hatch in a snail's intestines, and the larvae (immature worm-like forms) penetrate the snail's organs, eventually spawning an army of young parasites. Breaking out of the organs, the parasites now cause the snail to produce fluke-filled slime balls, soon left behind in a slime trail in grass.
Along comes an ant, host number three. It swallows a slime ball teeming with hundreds of baby lancet flukes. Most flukes just hang out in the abdomen, but one or two "scouts" locate and high jack the ant's command center: nerves below the throat that control the ant's movements. The scouts then perform parasitic voodoo on the ant's nervous system.
As night approaches, an infected ant climbs up a blade of grass--instead of heading back to its colony. As the air cools, the ant clamps down on the tip of a grass blade and waits to be eaten by a cow or other grazer. If the ant should sit the whole night without being eaten, the flukes let the ant loosen its grip on the grass--if the ant were to bake in the morning sun's heat, the parasites would die along with the ant.
The ant scurries to the ground and behaves like a normal insect again. But when night falls, the flukes command the ant to again climb a blade of grass. When a grazing cow eats the ant, the flukes settle in the cow's small intestine.
Then they worm their way to the cow's liver, where they live out their lives as adults.
This is merely one extraordinary cycle of a parasite passing through three hosts in order to reproduce and survive. Scientists have only recently begun to probe the amazing realm of nature's grossest and most common creatures. "A parasitic existence is probably the most common form of life on Earth," says Scott L. Gardner, a parasitologist at the University of Nebraska. "Every species of free-living animal or plant can have five or more parasitic species that exist only in or on that particular host."
Parasitologists (scientists who study parasites) now know parasites can exist as infinite forms of single- or multicellular animals, as well as plants and bacteria. By some estimates, parasites may outnumber the nearly 1.7 million identified species on Earth by at least four to one. Parasites can turn nearly every part of an animal--brain, bladder, muscle--into their home. On the gills (breathing organs) of a single fish, for example, up to 10 different species of parasites each finds its own cranny. In the intestines of humans, tapeworms can stretch like ribbons for 60 feet (see sidebar, p. 11)!
Through extremely sophisticated strategies, parasites can use parts of a host's body to suit their own needs. They can feed on almost anything: blood, gut lining, liver, or snot, and can force their host's body to bring them food. By definition, parasites in some way always harm their host.
One harmful parasite--discovered in 1995--is Sacculina carcini, a barnacle, or microscopic animal that usually lives on rocks (see pictures, left). Sense organs on female Sacculina legs catch the scent of her host, a crab; she dances through water until landing on the crab's armor.
Crawling along a crab arm, she looks for a hole from which small hairs sprout. Then she jabs a long hollow dagger through the hole--injecting a blob made up of a few cells. Sacculina sheds most of her body like a husk; the part that lives on now looks like a microscopic slug.
The slug settles in the crab's underside and grows, forming a bulge in its shell and sprouting a set of root-like tendrils that spread throughout the crab's body, even wrapping around its eyestalks. These tendrils draw in nutrients dissolved in the crab's blood. Remarkably, this gross invasion fails to trigger any immune, or disease-fighting response, in the crab. The crab merely continues to wander through the surf.
As the female parasite grows, the bulge turns into a knob. Sacculina will remain as an adult inside the crab for the rest of her life, unless a male larva should find a pin-size opening on the knob. Typically, each female Sacculina carries two males with her for her entire life. They endlessly fertilize her eggs, and every few weeks she produces thousands of new Sacculina larvae.
Eventually the crab turns into a kind of mindless slave serving the parasite. The crab stops molting (shedding outer layers) and growing, which would only funnel energy from the parasite. But the crab doesn't lose its urge to nurture--it simply directs affection toward the parasite. A healthy female crab carries her fertilized eggs in a brood pouch on her underside; she carefully grooms the pouch, scraping away algae and fungi. When normal crab larvae hatch and need to escape, their mother finds a high rock on which to stand, then bobs up and down to release them from the pouch into the ocean current.
The knob that Sacculina forms sits exactly where the crab's brood pouch would be--and the crab treats the parasitic knob like her own offspring! She strokes the knob clean as the larvae grow, and when they're ready to emerge forces them out in pulses. As parasites spray out from her body, she waves her claws to help them on their way. The newborn parasites hunt down uninfected crabs, and the cycle begins all over.
As scientists discover more parasites and uncover the complexity of their life cycles, they are coming to an unsettling conclusion: Far from being along for the ride, parasites may be one of nature's most powerful driving forces. "Every ecosystem on Earth is full of parasites that exert extraordinary control over their hosts," says Gardner. Scientists have only started to discover that parasites just may rule the world.
Where can you find some of the world's most harmful parasites? Inside human beings. The tiny blood fluke (Schistosoma mansoni), for example, emerges from a snail and swims through tropical waters in Africa or South America in search of soft human skin. When the worm senses skin molecules, it swims madly and drills its way in. In the human bloodstream the fluke snakes its way through one vein after another, finally reaching blood vessels around the large intestine. There, female flukes literally slide into males' bodies and reproduce, feasting on human blood and producing eggs for years or even decades. Some eggs end up in the liver, making human hosts permanently sick or even killing them.
Tapeworms also live for years inside human intestines, absorbing half-digested food through millions of delicate, fingery projections. Some tapeworms can increase their size by 1.8 million times in two weeks!
Plasmodium, the world's most lethal parasite, causes malaria. The parasite enters the bloodstream through a mosquito bite, hiding in the human liver before invading red blood cells. Ultimately, millions of infected blood cells explode at once, causing fever and death in 3 million people a year worldwide.
This blood fluke (magnified 550 times) causes diarrhea. At right, filarial worms damage the human immune system and cause a water-bag like condition called elephantiasis.
Test Your Parasite IQ
Directions: If reading "Animal Parasites: As Gross As It Gets" (page 8) wasn't enough of a gross-out, read the questions below and circle the correct answer. Then turn this page upside down to learn more about nature's strangest creatures.
1. What's a parasite? a. An organism that lives on or inside another organism in order to survive b. An organism that's always harmful to another organism c. both a and b 2. The word "parasite" derives from the Greek and originally described someone who was a. a hanger-on. b. an official at temple feasts. c. a farmer. 3. One of these creatures isn't always a parasite: a. head louse b. mosquito c. tick 4. Hookworms start their life in soil but end up inside a human when a. you swallow a larva. b. the worm penetrates the skin. c. both a and b. 5. How to get rid of the guinea worm, a human parasite: a. antibiotics b. surgery c. slowly twirl it out of an infected body with a stick 6. What infected chimpanzees do to get rid of parasites: a. fast to empty the digestive system b. self-medicate c. leave home to avoid infecting others
1. c. By definition, a parasite is a living organism that lives in or on another organism called a "host," and is harmful to that organism,
2. b. Parasite comes from the Greek word parasitos meaning "beside food," and originally described officials at temple feasts. Later the word was used negatively to indicate people who buttered up noblemen for free meals.
3. b. Male mosquitoes are "free living" and not parasites. Most female mosquitoes are parasites because they suck the blood of other animals--order to get enough protein to produce eggs.
4. c. If a person swallows a hookworm, it travels straight down into the intestines. But hookworms, like blood flukes, can directly penetrate the skin and burrow into a capillary. Unlike tapeworms, the hookworm has a mouth ringed with dagger-like teeth and digs into the intestinal lining for food.
5. c. The universal cure to remove quivering stringy guinea worms is for the victim to rest a week, then slowly wind the worm by turning it from an incision in the skin onto a stick, until it crawls free.
6. b. Parasite-infected chimps often swallow certain leaves whole or strip the bark of plants and eat the bitter insides, which seems to clear out worms from the intestines.
Health: Pick a parasite and find out: who its hosts are and how it is transmitted, how it affects its host, and what countries it lives in.
Did you Know?
* Parasites were named by the Greeks (parasitos). Aristotle recognized creatures that lived on the tongues of pigs, encased in cysts as tough as hailstones.
* The Koran tells its readers to stay away from pigs and stagnant water, which are both sources of parasites.
* In 1673, Antony van Leeuwenhoek, a Dutch shopkeeper, put a drop of old rainwater under a homemade microscope. He discovered legions of crawling creatures, some of which were parasites.
Directions: True of false? Circle the correct letter, then rewrite false statements to make them true.
T or F 1. A parasite is an organism that lives on or in another organism. T or F 2. Parasites outnumber Earth's 1.7 million species by 10 to 1. T or F 3. Parasites feed on blood, snot, the lining of intestines, and almost anything else. T or F 4. Inside human intestines, tapeworms can reach 6 feet. T or F 5. Plasmodium is the world's most deadly parasite.
1. True. 2. False. Parasites outnumber the rest of Earth's species by 4 to 1. 3. True. 4. False. Tapeworms can reach 60 feet in length. 5. True.
National Science Education Standards
Grades 5-8: structure and function in living systems * reproduction and heredity * diversity and adaptations of organisms
Grades 9-12: interdependence of organisms * matter, energy, and organization in living systems * behavior of organisms
Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures by Carl Zimmer (Free Press, 2000)
"Do Parasites Rule the World?" Discover, August 2000, p. 80