Meet The Parasites That Turn Their Hosts Into Zombies

Johan J.Ingles-Le Nobel, 'Jewel wasp' via Flickr. CC BY-ND 2.0

Zombies don’t just belong in horror films - they crop up a lot in nature, believe it or not. There are many examples of mind-manipulating parasites that can hijack the host’s nervous system, enslaving their victims in twisted ways. And yes, there’s even one organism that scientists believe can alter the minds of humans- and it’s present in as many as 50% of us. 

Mind Controlling Fungi

It’s not always easy being a fungus residing in a tropical forest. Humidity and temperature fluctuate, and many species can only thrive within a specific range of these variables. But members of the Ophiocordyceps genus have evolved a spectacular mechanism to get around this. They zombify host ants and make them seek out the perfect environment for growth. And to add insult to injury, they also kill the poor suckers and grow a triumphant stalk out of their head.

Numerous different Ophiocordyceps species have been described that differ in their choice of host and killing. O. unilateralis, for example, infects carpenter ants (Camponotus). After parasitizing this usually arboreal species, the fungus manipulates it in such a way that it abandons its tree house and heads for the sheltered underside of a leaf. But not just any leaf- a leaf with ideal conditions for the fungus to grow. Here, the ant bites down on the leaf, usually on a vein, performing what is known as a “death grip.” As the name suggests, this location will be the final destination of the ant as the parasite kills it.

Next, the hyphae (tubular fungal structures) grow inside the host, eventually tearing through the back of the head. Spores are then released from the fungus into a small area below the leaf referred to as an “infectious killing field,” ready to start the process over again.

Penn State, "zombie-ant 1" via Flickr. CC BY-NC 2.0

Suicidal Crickets

Horsehair worms, long stringy endoparasites of various arthropods, are sickeningly interesting. The larvae of these creatures reside in bodies of water throughout America, and start off their bizarre journey after being eaten by a mosquito. These insects then get gobbled up by hungry crickets, who really don’t know what's coming next.

The parasites burrow through the gut of the cricket before settling down in a comfy body cavity. Incredibly, the worm can grow to around a foot in length, inside a little cricket! Quite the contortionist. Then, the worm needs to get back in the water to complete its life cycle, so it makes the cricket commit suicide. Normally, crickets stay away from water for fear of death, but infected hosts actually go out of their way to seek out water and cannonball into it, killing the poor swimmers pretty quickly. The worms then wriggle out of the dead carcass in search of a mate.

Although it is not known how precisely these worms alter the behavior of the cricket, studies have shown that the worms produce large quantities of neurotransmitters when inside the host and also cause the host to produce more neurotransmitters. Somewhere along the line, this causes the cricket to behave in an abnormal and suicidal manner, waving goodbye to terrestrial life in favor of an early watery grave.

Alastair Rae, via Wikimedia commons. 

Interestingly, this is not the only example of parasitic worms inducing risky swimming behaviors. Back in the 1980s, scientists observed small crustaceans called gammarids happily chilling at the surface of bodies of briny (slightly salty) water in France. This was considered out of the ordinary because these animals usually reside at the bottom, avoiding the mouths of hungry predators.

Further investigation revealed that the daredevil individuals were all infected with trematode worms known as acanthocephalans which, as before, manipulate the behavior of the host in order to perpetuate its life cycle. Later, it was revealed that infected gammarids had altered serotonin-related (serotonergic) activity in their brains. This change in brain chemistry could affect neuronal integrity, ultimately leading to modified behavior.

Wasp Slaves

Some people are happy to fork out money for babysitters, but the same cannot be said for many wasp species. The majority of wasps are actually parasitoids, meaning that they borrow the body of another organism and use it as a nursery for their demanding offspring. But they don’t merely act as a cozy cot; the developing little critters actually devour the host, too. While it’s still alive. Yikes.

There are a few truly disgusting examples of parasitoid wasps, starting with Glypatapanteles species. Members of this genus lay up to 80 eggs in a live caterpillar that then hatch and snack on the poor unsuspecting host’s body. The larvae gobble their way through the caterpillar’s skin and attach themselves to a nearby piece of foliage and form a cocoon. Then, in a bizarre twist, the caterpillar becomes a bodyguard. It ignores its daily leaf chomping duties and stands guard over the pupae, violently swinging its head at incoming predators, knocking them away. How the parasites cause the caterpillars to turn into faithful guardians is unknown, but unparasitized caterpillars don’t perform this odd behavior.

The emerald cockroach wasp, or jewel wasp, also turns its host into a submissive zombie by injecting venom directly into its brain with a perfectly aimed sting to the head. It then drags the confused and paralyzed cockroach into a chamber and lays an egg on it before sealing the newly acquired pet inside the nest. The larva eventually hatches and bores inside the host, eating its insides while it is initially still alive. After gobbling much of its organs, the unfortunate cockroach dies. What a way to go.

Fatal Attraction

Last, but certainly not least, there’s my favorite mind-controller:  Toxoplasma gondii. This is found in 22 to 84% of the human population, depending on the country. This organism induces a phenomenon called “fatal attraction syndrome” in rodents. T. gondii can only productively replicate in cats, but it can infect humans and rodents. Amazingly, it’s evolved a way to alter the brains signaling of infected rodents to make them more likely to be consumed by cats. Studies found that individuals with the parasite not only had slower reaction times, but also no longer feared the smell of cat urine.

David Goehring, "Noir, Darkly," via Flickr. CC BY 2.0. 

But what’s perhaps even more interesting is what has been discovered in humans. Like rodents, individuals with toxoplasmosis also had slower reactions, but disgustingly they actually enjoyed the smell of cat urine. Researchers exposed study participants to urine from various animals, and those that were infected liked the smell of cat pee, but no other samples. Uninfected individuals, unsurprisingly, didn’t like the whiff.

It goes on; researchers investigated people involved in road traffic accidents in Prague. They found that those involved in collisions, whether they were a pedestrian or driver, were more likely to be infected when compared with residents not involved in such situations. This indicated that infected individuals were perhaps sometimes more reckless. This research has certainly been met with raised eyebrows, and confounding factors are always a possibility. For example, it may not be that toxoplasmosis affects your personality, but perhaps people with a particular personality are more susceptible to the disease.

[Header image "Jewel wasp" by Johan J. Ingles-Le Nobel, via Flickr]

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