STANDARDS

NGSS: Core Idea: LS2.A

CCSS: Writing: 3

TEKS: 6.12D, 7.12C, 8.11A, B.10A, B.12A

Zombies Among Us

How parasites take over the brains of living hosts—then direct them to take deadly risks

ANAND VARMA/NATIONAL GEOGRAPHIC IMAGE COLLECTION

AS YOU READ, THINK ABOUT why it would benefit an organism to invade and control an animal’s body.

From the depths of a dark cemetery, zombies rise from the grave. The undead wander the streets, terrorizing townspeople. Although scenes like that only play out in horror movies, something just as creepy happens in real life. Weird organisms take over other creatures’ brains, turning them into zombies.

The nightmare begins when a parasite enters an animal’s body. The invader steals vital nutrients from its host and grows stronger. Once the host is weakened, the parasite finds a way to control the host’s behavior. The parasite can, for its own benefit, force its host to do all sorts of bizarre, self-destructive things. For example, some parasites need to get to water to survive. They’ll squirm into insects and persuade them to take a deadly leap into a lake or pond. The hosts drown, and the parasites swim away.

From deep inside a dark cemetery, zombies rise from the grave. The undead wander the streets and terrify people in the town. Scenes like that play out only in horror movies. But something just as creepy happens in real life. Weird organisms take over other creatures’ brains and turn them into zombies.

A parasite enters an animal’s body, and the nightmare begins. The attacker steals important nutrients from its host. The parasite grows stronger, and the host is weakened. Then the parasite finds a way to control the host. It can force its host to do strange, self-destructive things that benefit the parasite. For example, some parasites need to get to water to survive. They’ll enter insects and get them to leap into a lake or pond. The hosts drown, and the parasites swim away.

Parasites can turn many creatures into zombies—possibly even humans. Toxoplasma gondii can be transmitted to people through cat droppings. If a person accidentally ingests infected cat poop, some alarming behaviors can result. Not only has T. gondii been linked to mental illness and reckless driving, it’s also been connected to lower school achievement in kids. So wash your hands right away when you clean a cat’s litter box!

“Many mind-controlling parasites are single-celled organisms that overpower far more complicated animals,” says Susan Perkins, a parasitologist at the American Museum of Natural History in New York City. “It’s really creepy how these tiny creatures can change the way their host looks and acts.” Take a look at four of these zombifying creatures in action.

Parasites can turn many creatures into zombies, maybe even humans. People can pick up Toxoplasma gondii through cat droppings. If people accidentally eat infected cat poop, they may do some alarming things. T. gondii has been linked to mental illness and reckless driving. It’s also been connected to lower school performance in kids. So wash your hands right away when you clean a cat’s litter box!

Susan Perkins is a parasitologist at the American Museum of Natural History in New York City. “Many mind-controlling parasites are single-celled organisms that overpower far more complicated animals,” she says. “It’s really creepy how these tiny creatures can change the way their host looks and acts.” Take a look at four of these zombie-making creatures in action.

ALEXANDER WILD

BURST BODY: A parasitic worm escapes from the abdomen of a trap-jaw ant, tearing the insect’s body in two in the process!

EXPLODING ANTS

The gruesome scene above took place while insect expert Alex Wild was teaching a bug photography class in Belize. He noticed a cluster of ants that looked strangely misshapen. “They had huge abdomens but tiny heads,” says Wild. A student of Wild’s picked one up, “and it went ‘pop.’” A long, wiggly roundworm had burst out of the ant’s body, killing it instantly.

Another type of parasitic worm, called a horsehair worm, invades crickets and grasshoppers. The worm pumps out chemicals that convince its host to jump into pools of water. The host drowns, but the parasite swims away to continue its life cycle. Although scientists don’t know the exact behaviors caused by the ant-invading roundworm, they speculate that it too may cause the insect to seek out water—before exploding from its stomach.

Insect expert Alex Wild was teaching a bug photography class in Belize. That’s when the gross scene above took place. He noticed a group of ants with strange shapes. “They had huge abdomens but tiny heads,” says Wild. One of his students picked one up, “and it went ‘pop.’” A long, wiggly roundworm had burst out of the ant’s body. This killed the ant instantly.

A horsehair worm is another type of parasitic worm. It invades crickets and grasshoppers. The worm releases chemicals that get its host to jump into pools of water. The host drowns, but the parasite swims away to continue its life cycle. And what exactly does the roundworm cause the ants to do? Scientists aren’t sure. But they think it may also get the insect to find water. Then it explodes from the ant’s stomach.

MATHIEU B.MORIN/ALAMY STOCK PHOTO

PEST PROTECTOR: A ladybug is tricked into guarding a wasp larva’s cocoon.

BUG BODYGUARD

Dinocampus coccinellae wasps use their parasitic powers to turn ladybugs into zombie babysitters. Female wasps seek out a ladybug and implant an egg inside it. An orange, wormlike larva hatches from the egg. The immature form of the wasp begins to grow, feasting on nutrients inside the ladybug.

About two weeks later, the ladybug stops moving and the larva tunnels out of its body. Scientists believe D. coccinellae injects a virus along with its egg. The virus likely spreads to the host’s brain, damaging the ladybug’s nervous system and causing paralysis. Next, the larva weaves a cocoon between the ladybug’s legs. While mostly immobile, the ladybug can still slightly twitch its hard shell. That scares away potential predators looking to eat the larva.

By forcing the ladybug to become an unwitting bodyguard, the parasite stays safe until it can emerge from its cocoon and fly away. Most ladybugs die at this point. But some manage to survive the ghastly ordeal—and may even become zombie parents again!

Dinocampus coccinellae wasps also have parasitic powers. They use them to turn ladybugs into zombie babysitters. Female wasps find a ladybug and place an egg inside it. An orange, wormlike larva hatches from the egg. The immature form of the wasp begins to grow. It feeds on nutrients inside the ladybug.

About two weeks later, the ladybug stops moving. The larva tunnels out of its body. Scientists believe D. coccinellae injects a virus along with its egg. The virus likely spreads to the host’s brain. It harms the ladybug’s nervous system and causes paralysis. Next, the larva weaves a cocoon between the ladybug’s legs. The ladybug can’t move much, but it can still twitch its hard shell a little. If predators want to eat the larva, the movement scares them away. 

The parasite has forced the ladybug to become its bodyguard. Now the parasite stays safe until it can leave its cocoon and fly away. Most ladybugs die at this point. But some manage to survive the awful ordeal. They may even become zombie parents again!

COURTESY OF STANISLAV KORENKO

  • WEIRD WEB: A parasitic wasp forces orb-weaver spiders to create an oddly shaped web to protect the wasp’s developing larva (left).
  • REGULAR WEB: This is what a normal orb-weaver spider’s web looks like (right).

WACKY WEB-BUILDER

In a forest in Italy, the parasitic wasp Sinarachna pallipes hunts for a type of spider called an orb weaver. When the wasp finds one, it delivers a sting that temporarily paralyzes the spider. The wasp uses this opportunity to lay an egg on the spider’s abdomen. Then the wasp flies off. The spider regains its ability to move and builds a web as if nothing happened—that is, until the wasp egg hatches.

For two weeks, the wasp larva clings to the spider’s abdomen, feeding on its blood. Events then take another strange turn: The larva releases chemicals that force the spider to do something unusual. Instead of making a typical flat web with circles spiraling toward its center, the spider weaves a three-dimensional web to protect the developing parasite.

When the structure is complete, the larva eats the spider. Then the parasite retreats into its spider-built den, which shields it from rain, wind, and predators on the forest floor. Tucked inside the shelter, the larva forms a cocoon where it will safely grow into an adult wasp.

The parasitic wasp Sinarachna papllipes lives in a forest in Italy. It hunts for a type of spider called an orb weaver. When the wasp finds one, it stings the spider. For a while, the spider is paralyzed. That’s when the wasp lays an egg on the spider’s abdomen. Then the wasp flies off. The spider starts to move again. It builds a web as if nothing happened, until the wasp egg hatches.

For two weeks, the wasp larva hangs onto the spider’s abdomen. It feeds on the spider’s blood. Then things get even more strange. The larva releases chemicals that make the spider do something unusual. The spider normally makes a flat web with circles winding toward its center. Instead, the spider weaves a three-dimensional web to protect the growing parasite.

When the web is complete, the larva eats the spider. Then the parasite enters its spider-built den. The web protects it from rain, wind, and predators on the forest floor. The larva forms a cocoon, safe inside the shelter. There, it will safely grow into an adult wasp. 

BLICKWINKEL/ALAMY STOCK PHOTO

INFECTED SNAIL: Birds mistake the green worm inside this snail’s eyestalk for a caterpillar and eat it. 

WORMS FOR EYES

What’s grosser than slimy snails? How about ones whose brains are infested with mind-controlling worms? That’s what happened to this garden snail. Flatworms called Leucochloridium paradoxum invaded its body and made a home in its eyestalk.

The worms sneak into the snail’s body through a creepy process (see A Snail Parasite’s Life). After being eaten by a bird, the worms lay their eggs inside the animal’s stomach. The eggs end up in the bird’s droppings. A hungry snail eats the poop, which contains the parasite’s eggs. The baby flatworms hatch, slither into the snail’s brain, and eventually make their way into the eye-tipped tentacles on its head. That causes the snail’s eyestalks to swell and resemble fleshy, squirming caterpillars.

The flatworms convince the snail to travel into dangerous, well-lit areas that the snail would normally avoid. Scientists think the worms do this by releasing chemicals that decrease the snail’s stress levels. Hanging out in exposed locations makes the snail easy prey for birds. The birds think the caterpillar-like eyestalks look delicious, and they bite the feelers right off the snail. The worms go on to infest the birds, and the cycle begins again.

What’s grosser than slimy snails? How about ones with mind-controlling worms in their brains? That’s what happened to this garden snail. Flatworms called Leucochloridium paradoxum entered its body and made a home in its eyestalk.

The worms have a creepy way to sneak into the snail’s body (see A Snail Parasite’s Life). A bird eats the worms, and the worms lay their eggs inside the animal’s stomach. The eggs end up in the bird’s droppings. A hungry snail eats the poop with the parasite’s eggs in it. Then the baby flatworms hatch and crawl into the snail’s brain. Over time, they make their way into the eye-tipped feelers on its head. That makes the snail’s eyestalks swell and look like fleshy, squirming caterpillars.

The flatworms get the snail to travel into dangerous, well-lit areas. Normally, the snail would avoid these places. How do the worms do this? Scientists think they release chemicals that decrease the snail’s stress levels. When the snail hangs out in open areas, it’s easy prey for birds. The birds think the caterpillar-like eyestalks look delicious. So they bite the feelers right off the snail. The worms invade the birds, and the cycle begins again.

EVALUATING INFORMATION: The parasites in the text all likely share the same reason for infecting their hosts. What is it?

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