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Invasion of the Body Snatchers

'Zombie' parasites are the stuff of Halloween nightmares.

By Nathan Adams

It’s a good time to be a zombie fan. On TV, you can catch AMC’s The Walking Dead. At the multiplex, movie-goers saw Matt Smith and Natalie Dormer search for a zombie cure in Patient Zero. Singer/songwriter Stephanie Mabey racked up more than 6 million views for The Zombie Song on YouTube. No matter the media — television, movies, comic books, novels or video games — zombies are everywhere. Including, as it turns out, in our own backyard.

Texas is home to three known “zombie” parasites, creatures that attack and infect their animal hosts. But these zombies don’t want to eat brains — at least not at first. They want to control them.

“It’s fascinating how these parasites have figured out how to bypass the immune system and interface directly with the host brain,” says Kelly Weinersmith, a parasitologist and adjunct assistant professor at Rice University in Houston. “They are somehow able to get the host to do things that are bad for the host, but good for the parasite.”

creepy larvae

Tales from the Crypt

Weinersmith, together with her colleague Scott Egan, is credited with discovering one of these zombie parasites. Egan studies gall wasps, tiny insects (smaller than 5 mm, about the size of a pencil eraser) that lay their eggs in oak trees. Once these eggs hatch, the larvae alter the tree’s natural behavior, creating small chambers full of nutritious tissue below the surface of the bark. There, protected from predators, the young wasps eat these tissues as they mature to adulthood. Over time, they chew their way through to the surface and then fly away.

At least, some of them do. While studying a tiny, orange-colored species known as the crypt gall wasp, Egan discovered that some wasps weren’t fully exiting the tree. Their heads were stuck, wedged into an escape hole much smaller than normal.

Such behavior is atypical for gall wasps, so Egan reached out to Weinersmith.

“Scott asked me, ‘Could a parasitoid be doing this?’ In the animal kingdom, there’s a strong correlation between abnormal behavior and the presence of a parasitoid.”

The two carefully cut open branches housing trapped gall wasps. Inside the crypt of each stuck wasp they found a second wasp, half the size and iridescent blue. This smaller wasp had attached itself to the gall wasp and appeared to be eating it from the inside. Further research showed that these smaller wasps were somehow able to manipulate their hosts into making a hole too small for the larger gall wasp to exit. Over time, the smaller wasp ate its way through the head of the gall wasp (“They’re really messy eaters,” Weinersmith notes) and flew out into the wild.

Egan and Weinersmith gave this previously undiscovered species the deliciously macabre name of crypt-keeper wasp. The wasp’s formal name, Euderus set, pays homage to Set, the ancient Egyptian god of chaos and evil who, according to legend, could control other evil beings.

“We later learned that Set locked his brother Osiris in a crypt, then came back and chopped him into pieces,” Weinersmith says. “There were a lot of cool connections.”

parasitic larvae

Fish Gotta Swim, Birds Gotta Fly

In the Gulf of Mexico, another mind-controlling parasite moves at a snail’s pace toward its future host — literally. Euhaplorchis sp. A, a type of fluke worm, begins its life attached to the plicate horn shell sea snail. There’s just one problem — to reproduce, Euhaplorchis needs to be inside a bird, and birds aren’t swimming under the Gulf. There are, however, a lot of longnose killifish.

The longnose killifish is a small minnow, about the size of your finger (they can grow as large as 12 cm). Scientists aren’t sure how Euhaplorchis make the transfer from snail to fish. Some theorize the eggs are washed from the snail into the ocean, then inhaled through the gills of the killifish; others think the larvae burrow into the fish some other way. But once inside, their mind-altering takeover begins.

In 2012, Brian Fredensborg and undergraduate student Ashley Longoria of the University of Texas-Pan American published a paper in the Journal of Parasitology, documenting their research into Euhaplorchis. They found that killifish infected by Euhaplorchis swim closer to the surface of the water than uninfected killifish do — and the greater the infection, the closer to the surface they swim. There, the infected killifish is more likely to be spotted (and eaten) by predatory birds. Thus, the parasite passes from the fish to the bird, where Euhaplorchis can reproduce and lay eggs. Those eggs get pooped out, picked up by passing sea snails — and the cycle begins again.

The Phorid is Horrid (if you’re a fire ant)

Researchers at the University of Texas at Austin’s Brackenridge Field Laboratory have been studying a type of mind-controlling parasite called the phorid fly. Female phorid flies seek out colonies of the invasive red imported fire ant, targeting individual ants. Once the target ant is chosen, the phorid swoops down, injects her egg into the ant’s body, then zips away. The entire process takes a fraction of a second.

Over the next 10 days, the egg hatches and the phorid larva moves into the ant’s brain, where it manipulates the ant to leave the nest and seek out a moist place to die. The ant’s head then falls off, and the phorid larva pupates in the safety of the hard shell that once housed the ant’s brain. About a month later, the adult fly emerges.

Another thing phorid flies do to fire ants may be more significant. Phorid flies don’t need to infect the fire ants to influence their behavior — they just need to fly around them. As phorids circle ants looking for victims, some ants will hide, retreating into their nest. Others posture in odd ways, such as piling on top of one another. The presence of phorid flies disrupts the ants’ ability to protect the nest or provision it with food, allowing other, native ant species to take advantage of the disruption and reclaim lost territory. Research is ongoing as to how these parasitic predators may help reduce the impact of invasive fire ants.


Rethinking Zombies

“Parasites are so creepy,” Weinersmith acknowledges. “And if you’d have told me at 14 that as an adult I would be studying them professionally, I would have said you were crazy. But consider what parasites have learned over millions of years studying their hosts, and what can we learn from them. As we understand how parasites affect the behavior of their hosts, we are learning more about how our own systems work, how behavior happens and how diseases work in general.”

This field of study holds great research potential, Weinersmith says, because there’s so much we don’t know, and so much to discover.

“Sometimes we have a tendency to pooh-pooh people getting excited about the natural world,” she continues. “Stuff like this is fascinating — it gets people to notice the world around them.”

So, the next time you step over a fire ant mound, walk past an oak tree or visit the Gulf of Mexico, take a moment to stop and notice the world around you. Because here, there be zombies!


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