Meet 5 “zombie” parasites that mind-control their hosts

Zombies may still be a thing of fiction, but some parasites more
or less turn their hosts into the walking dead.

“Some parasites can alter the behavior of their host in ways
that give the parasite a better home, or provide more nutrients, or
cause the host to move to a different environment,” said Janice
Moore^[1], a biologist at Colorado
State University in Fort Collins.

This strategy seems to work, she added: “A parasite that can
alter the behavior of its host, and in doing so improve its own
transmission, is going to be favored by natural selection,” she
said. (See “World War Z: Could
a Zombie Virus Happen?^[2]“)

In honor of Halloween^[3], here’s a selection of a
few zombie parasites.

Females of the Costa Rican wasp Hymenoepimecis
argyraphaga lay their eggs on the abdomens of unlucky orb
spiders called Plesiometa argyra.

But this new web isn’t for the spider: It’s meant to support the
cocoon that the wasp larva will build after finally killing and
eating the spider.

First she injects a toxin into the roach that paralyzes its
front legs. Then the wasp strikes again
in the roach’s head^[4]. Frederic Libersat of
Ben-Gurion University in Israel and colleagues discovered that the
venom targets a specific area of the brain responsible for
initiating movement.

Stripped of its ability to move of its own free will, the
cockroach can be grabbed by the antenna and guided to a burrow,
where the wasp will lay her egg on the victim and entomb them
together.

The wasp larva slowly consumes the cockroach for several days
before pupating in its abdomen, emerging as an adult about a month
later.

As an adult, the lancet liver fluke—a type of flatworm—resides
in the livers of grazing mammals such as cows.

Its eggs are excreted in the host’s feces, which are then eaten
by snails. After the eggs hatch inside the snail, the snail creates
protective cysts around the parasites and coughs them up in balls
of mucus.

These fluke-laden slime balls are then consumed by ants. When
the flukes wiggle their way into an ant’s brain, they cause the insect to
climb^[5] to the tip of a blade of
grass and sit motionless, where it’s most likely to be eaten by a
grazing mammal. That way, the liver fluke can complete its life
cycle. (Read about fungi
that zombify ants^[6].)

Once it finds a fish, the parasite latches on to its gills and
makes its way to the brain. But this isn’t its final stop.

The fluke needs to get inside the gut of a water bird in order
to reproduce. So inside the killifish’s brain, the fluke releases
chemicals^[7] that cause the fish to
shimmy, jerk, and jump.

Jenny Shaw, then at the University of California, Santa Barbara,
and colleagues found that the parasite decreases serotonin and
increases dopamine levels in the fish’s brain. The switch in this
brain chemistry stimulates the fish to swim and behave more
aggressively.

These moves attract the attention of birds, which may eat the
fish—and the flukes. The flukes mate, and their eggs are released
back into the water in the bird’s droppings to be eaten by horn
snails and start the cycle anew.

Hairworms have a perpetual challenge: They infect landlubbing
insects like crickets, but the parasites must make their way to an
aquatic habitat in order to reproduce.

Researchers at France’s Centre National de la Recherche
Scientifique figured out how they accomplish this feat. Hairworms
produce mind-controlling chemicals that cause their cricket host to
move toward light. Because water bodies reflect moonlight, this
often sends crickets toward
lakes and streams^[8].

The crickets jump in and drown, and the hairworms emerge, ready
to find their next victim.