Parasitic puppeteers

JOE HANSON: Zombie stories like the Walking Dead, World War Z, Night of the Living Dead are fun. Armies of clumsy, mindless, but surprisingly fast undead brain-eaters.

But could they actually exist? To find out, you need to start looking for real world zombies. Or... zom-BEES. If this tiny fly happens across a honeybee, it hops on its back, stabs it in the gut, and injects its eggs. It's kind of rude, really. But those eggs will eventually hatch into maggots that eat the bee from the inside out. Before the bee dies, these wriggly passengers do something to their host's brain. The bee starts to move erratically, it flies out at night, just like... a zombie.

And that's just one of dozens of strange parasites in nature that seem to be able to turn unsuspecting creatures into half-dead mind-controlled puppets.

Every one in a while, the odd worker ant will drift from the beaten path and blindly begin to climb the stalk of a plant. It walks out on a leaf, bites down with its last ounce of strength, and dies. Soon after, the zombie master bursts out of its sarcophagus—stalks of a fungus named Cordyceps—raining spores down below to infect the next passing ant.

When it's time to lay her eggs, the beautiful emerald jewel wasp turns cockroaches into zombie midwives. When they first meet, the wasp introduces itself by stinging the roach in the abdomen and temporarily paralyzing it. But then it goes to work like a neurosurgeon. The wasp probes its stinger into a very specific part of the roach's brain, injecting chemicals that block the roach's instinct to run away. It's not dead, or frozen, it just has no will to run. Now the wasp master can grab the roach's antennae and pull its new pet home like a cockroach cocker spaniel... where it promptly lays eggs inside it so it can be eaten from within.

One tiny worm infects crickets. It slowly feeds on them from within, growing larger and larger and larger. When it's time to leave the "nest" these worms release chemicals that hijack the cricket's nervous system and cause them to jump into the water and drown. The foot-long worm then wriggles out, to breed in the water, and start the cycle all over again.

Then there's the tiny roundworm that turns the abdomens of South American ants bright red and makes them stick their rear ends in the air like tasty berries, perfect for birds to pluck up and go poop out elsewhere, which delivers the worm to new hosts.

Not even plants are safe. This wild mustard can be manipulated by a fungus to produce fake flowers, complete with fake nectar, which then trick bees into landing and transferring spores to new plants.

These examples and lots more are some of nature's creepiest biochemistry. What's amazing is that even our fancy pharmaceuticals don't act as specifically as many parasites' chemical cocktails do. The emerald wasp's venom, for instance, blocks receptors for one special neurotransmitter, and only in the neurons that would make the cockroach run away. Nature's parasites were doing neurobiology long before humans ever picked up a medical textbook. And for parasite cases like those ripe red ants, evolution is acting on a whole group of animals: the worms, the ants, and the birds that nibble on them.

These elegantly evolved relationships are everywhere in the world of zombie parasites, but perhaps none is weirder than the case of the psychedelic zombie escargot.

Let's say a garden snail eats some bird poo, which is already gross, but this poo is full of parasite eggs. These worm eggs hatch inside the snail, living and feeding and filling the snail's eye stalks with their babies. The zombie snail crawls out into broad daylight—its mind taken over—eyes pulsing full of baby worms like some sort of horror film half-caterpillar half-disco ball. Yum yum, and the cycle continues. 

Luckily, that could never happen to us. Unless you own a cat. The single-celled protozoa Toxoplasma gondii can only sexually reproduce inside the intestines of a cat. But it can take up residence in a variety of mammals. If it finds its way into the brain of a mouse, the rodent suddenly wants to be friends with felines, like it's under the spell of some parasitic love potion, following the smell of cat urine to its untimely demise.

The Toxo undergoes sexual reproduction inside the cat, gets dropped off at the litter box, and the cycle begins again. What's weird is that as many as one in three humans may be infected wth Toxo. They aren't offering themselves as dinner for cats or anything, but some scientists think these people show personality differences.

VIDEO: I really love cats, and I just want to hug all of them but I can't, because it's crazy, I can't hug every cat.

HANSON: That evidence is thin, but it makes you think. Or maybe it makes the parasite think.

So could human zombies we see in movies actually exist? Well, no. After we die, our bodies stop producing ATP, and the protein machinery that moves our muscles is frozen in rigor mortis. And considering that the trillions of bacteria in our intestines have to eat something, real zombies would be less like the movies, and more like gooey statues. But if the complex neural machinery of mammals can be overtaken by a simple, single-celled organism, what else is possible?