Halloween Special: Real-Life Zombies
With Halloween coming up, we thought we’d take a look at nature’s real-life zombies—and how they get that way.
Several parasite species invade their victims’ bodies, turning them into zombies that do their masters’ bidding. Their methods are insidious enough to make James Wan and Stephen King run away screaming like Ned Flanders. Here’s a look at some of these masterful puppeteers. (See pictures of eerie animals ready for Halloween.)
Our first puppet master is a parasite called Toxoplasma gondii, and it likes to reproduce in the intestinal tract of domestic cats. So how does it get there? Well, one way is that mice infected with T. gondii lose their fear of cats. (See “How a Cat-Borne Parasite Infects Humans.”)
That means the rodents become easy pickin’s for the cat, and the parasite gets a ride back home. Recent research has shown that even after the mice have cleared the infection from their bodies, they don’t recover their healthy fear response to cat urine, suggesting that the alterations in their brains are permanent.
About one-third of humans are infected with T. gondii, so it’s a common brain parasite. That’s a phrase that may haunt your dreams, but it’s only really dangerous for newly infected pregnant women and those with compromised immune systems. One study, however, linked the infection to certain human traits, including neuroticism.
If you’re about to stop reading because you’re neurotic about this stuff … oh, forget it. (See more pictures of animal “zombies.”)
Remember catching grasshoppers as a kid? Happy summers, good memories. Who would make these bugs want to kill themselves?
The parasitic hairworm, Nematomorpha, that’s who. Also called the horsehair worm or the Gordian worm because they sometimes mass and look like a knot (i.e., Gordian knot), these critters can also infect crickets and other bugs. In 2005 a team of French biologists studied how the hairworm manipulates its host by studying a group of grasshoppers trapped in a pool.
Hairworm larvae, possibly ingested by grasshoppers in the water they drink, grow to occupy the grasshopper’s body—the entire thing, except the head and legs.
The French researchers concluded that the hairworms release a protein that acts on the insect’s central nervous system, causing it to go to the water, which is where hairworms reproduce. The worms, now up to four times as long as their host, go out and mate, leaving the grasshopper to drown.
The guinea worm has a similar strategy, but it infects people rather than grasshoppers. Though it doesn’t make us drown ourselves, it does make us seek water to relieve the burning sensation caused when the parasites try to exit out of our feet. It’s a nightmare of pain on its way out, but the good news is the worm is really on its way out: International efforts are moving the guinea worm ever closer to extinction. (Related: “The Guinea Worm—A Fond Obituary.”)
Snails infected with the endoparasitic worm Leucochloridium paradoxum go through a pretty horrifying physical transformation in addition to being driven to what looks like suicidal behavior.
The chain of events goes thusly: A snail eats bird droppings infected with the worm’s eggs, which eventually become sporocysts, or sac-like structures that move up into the snail’s eyestalks.
The normal eyestalks become plump, pulsating, striped cones, and the zombified snail, which normally prefers the shade, climbs up into the sunlight where birds see its hideous throbbing eyestalks and think they are delicious larvae.
The bird eats the snail and the parasite enters the bird’s digestive tract, where it develops into adulthood. The parasite releases eggs in the bird’s rectum, and the eggs are then excreted and ingested by other snails. It’s the circle … c’mon, sing! … the circle of parasite life.
Ants don’t have a lot of fans. Still, you’ve got to feel sorry for them when they encounter the fungus Ophiocordyceps unilateralis.
A research team lead by David P. Hughes of Harvard found that O. unilateralis—actually four distinct species of fungi found in Brazilian rain forests—infect ants, getting into their brains and getting them to travel to a spot that suits the fungus. (See “‘Zombie’ Ants Found With New Mind-Control Fungi.”)
A 2009 study led by Hughes found that infected ants were inevitably found clamped to the underside of leaves about 10 inches (25 centimeters) up from the ground, in just the right conditions for the fungus to reproduce and to fall to the ground and infect other ants.
This dastardly fungus also uses the corpses to claim more victims, sometimes shooting spores from its host’s corpse out onto other ants. These cadavers can make “infection pegs” that infect other ants: Imagine a small tree growing out of your head and you’ll get the picture.
Another ant that becomes a parasite’s patsy is the fire ant, which becomes host to the phorid fly. These flies lance the ants and lay eggs inside the ants’ bodies.
The larvae eventually travel up to the ant’s head, where they dine on the ant’s brain and zombify the creature. Sometimes they cause the ant to leave its colony so that other ants don’t attack it. Eventually the little flies decapitate their host and hatch out of the detached head. The whole shebang takes about 45 days.
This last specimen doesn’t seem to mind-control its victims: It just eats their tongues and lives in their mouths as a replacement tongue.
The monster here is cleverly known as a tongue biter, or Cymathoidae, a group of isopods that swim into the mouths of fish through their gills. The tongue biter eats the fish’s tongue and takes up residence in its mouth, facing outward, as though the fish were a big rental truck and the invader was going on the road trip of its life.
While it doesn’t seem to mind-control its host like the others, it’s a lot bigger and, as Carl Zimmer reports, it does take the nutrients out of its host: Fish with tongue biters had lower blood countsthan those that didn’t.
Luckily, these tongue biters can’t hurt or sicken humans—though they may get stuck in your head for a while.