Nature's Zombies: Mind-Altering Infections from Snails to Humans

September 23, 2015 | Sarah Tse

The brood sack of Leucochloridium, a parasitic flatworm, growing within the body of a snail.
Photo credit: Thomas Hahmann/Wikimedia Commons/CC BY-SA 3.0

As you watch the mindless shuffle and unnatural behavior of zombies on The Walking Dead, you might reassure yourself that such a creature could never exist in reality. But you’d be wrong.

A whole host of parasites rely on an arsenal of chemical and neurological mechanisms that radically alter the behavior of their hosts. Once infected, they transform into shells of their former selves and become slaves to the alien life-forms thriving within them.

The strange mind-controlling abilities of these parasites make sense in light of their life histories: they must move from an intermediate host to a final host where they reach maturity and reproduce. As luck, or rather, evolution, would have it, these successive hosts tend to occupy the same food chain. Parasites that thrive once their hosts are consumed by a natural predator are more successful, and those that can actually cause their host to be eaten pretty much win at the natural selection game.

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So how does an enterprising young parasite manipulate its host into going against all instincts and seeking, rather than avoiding, being eaten? After infiltrating their host, these parasites make their way to the brain and meddle with normal neurological responses to neurotransmitters, which are chemicals that convey messages between neurons and muscles. This overrides the host’s normally complex behaviors with simple instructions that will ultimately benefit the parasite, and help it move onto the next stage in its life cycle.

One particularly gruesome example is Leucochloridium paradoxum, a parasitic flatworm that infects snails. The flatworm begins its life when an unsuspecting garden snail eats bird droppings, a typical gastropod breakfast. Little does the snail know that the droppings come with a side of Leucochloridium eggs, which hatch and spread throughout its body.

Once the larvae reach the snail’s eye tentacles, they form massive brood sacs that transform the tentacles into a brilliant, pulsating advertisement that birds simply can’t resist. The parasites also influence the snails to abandon their usual cautious, light-averse behavior, so they venture out into the open where they are more likely to be spotted by a hungry bird.

Finally, the tentacles are consumed and release the larvae into the bird’s digestive system. While the avian rectum may not seem very romantic, it’s the perfect environment for the Leucochloridium to reproduce and lay eggs. The eggs escape via discharge, and the cycle begins anew.

Check out this video of the pulsating brood sacs:

The idea of mind-controlling parasites is creepy, but falls short of horrifying when compared to all the other macabre phenomenon of the natural world. But a human zombie apocalypse is actually closer to reality than one would think: enter Toxoplasma gondii.

This parasite is a protozoan that can infect any warm-blooded animal, but its definitive host, where it sexually reproduces, is the household cat. T. gondii operates in a similar fashion to L. paradoxum, by causing reckless behavior in intermediate rat hosts. Most rats, and humans, show an aversion to the smell of cat urine. The parasite rewires its host’s brain so that it is attracted to the odor, and replaces its normal response of fear and stress with pleasure and sexual attraction.

The zombie apocalypse theory comes from research that has shown similar neurological effects in infected humans. In addition to an attraction to cat pee, human T. gondii infection is associated with behavior and mood-altering disorders like schizophrenia, depression, and bipolar disorder. In fact, one study found that drugs for schizophrenia treatment reduced behavioral changes in infected rats.

So, humans are just as vulnerable to parasitic manipulation as any other organism. It’s an uncomfortable notion that we may not know whether our decisions — particularly the riskier impulses — are our own, or if they originate from foreign intruders that are insidiously manipulating us. How much of our personalities can we actually attribute to our intrinsic identity? Research into the fascinating and frightening world of parasites continues to illuminate our understanding of behavior and its surprising malleability.

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