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Suicide Grasshoppers Brainwashed by Parasite Worms

James Owen
for National Geographic News
September 1, 2005
 
Parasites were the inspiration behind the creature that terrified
moviegoers in Alien. Now sci-fi screenwriters may have a new role
model—parasitic worms that brainwash their victims.

Scientists say hairworms, which live inside grasshoppers, pump the insects with a cocktail of chemicals that makes them commit suicide by leaping into water. The parasites then swim away from their drowning hosts to continue their life cycle.

A team of French biologists made the discovery after monitoring grasshoppers that became trapped in a swimming pool in southern France.

Postmortems of the grasshoppers suggest that worms triggered the insects' death leaps by sabotaging their central nervous systems.

Writing this week in the journal Proceedings of the Royal Society B, the team says their findings help explain how parasites are able to manipulate their hosts' behavior to the parasites' own ends.

Scientists have long argued whether strange behavior in parasite-infested animals is deliberately engineered or simply an accidental side effect of infection.

The new study suggests the former, says David G. Biron of the Laboratory of Genetics and Evolution of Infectious Diseases at France's National Scientific Research Center in Montpellier.

Biron said hairworms control their hosts to "enhance the parasite's chances of completing its life cycle."

Adult hairworms breed in water, where they form writhing masses. It's still not clear how their young get into grasshoppers, though Biron says it may happen when the insects drink larvae-infested water.

Emerging Worms

When the hairworm is full-grown "it occupies most of the host's cavity, with the exception of the head and the legs," he said. "Worms are only ready to emerge once they reach this stage."

Hugh Loxdale, president of the London-based Royal Entomological Society, traveled to France last month to see what happens next.

"It's one of the most horrific things I've ever seen," Loxdale said. "It makes the science fiction film Alien look pretty tame in comparison."

"When the grasshopper hits the water, the worm comes out of its rear end and then swims off to find a mate. The worm is about three or four times longer than the host—it's amazing." (See related video.)

Loxdale hadn't previously heard of a parasite that drives an insect to suicide. But he draws parallels with a fungus that attacks hoverflies (stingless flies that resemble honeybees.)

"The hoverflies very conveniently die on the top of grass stems, which maybe makes the transmission of fungal spores easier," he said. "The spores penetrate the insect cuticle [skin] then grow inside the insect and quickly kill it."

Hairworms, or Nematomorpha, are a little-known group of parasites, which contains around 300 known species worldwide. Biron, the study author, says the organisms target a wide range of land-based insects, including praying mantises.

The French research team studied proteins produced by both the parasite and its host to investigate how hairworms might make grasshoppers hop to their deaths.

Central Nervous System

The team found that hairworms release proteins that influence grasshoppers' central nervous systems, thereby affecting chemical signals to the insects' brains.

"Some of these proteins secreted by the worm [mimic] proteins produced by the grasshopper," Biron added.

This biochemical tampering appears to drive the grasshopper to water just when the hairworm is ready to reproduce.

Study co-author Frédéric Thomas says other parasites may use a similar strategy to manipulate their hosts.

For example, a type of parasitic flatworm targets cockles in New Zealand, driving the marine mollusks to the surfaces of muddy bottoms in shallow waters. There, oystercatcher birds snap the cockles up and eat them, flatworms and all. The shorebirds serve as the final hosts in the flatworms' complex life cycle.

"Parasitic wasps can also make the host weave a special cocoon-like structure to protect the wasp pupae [offspring] against heavy rain," Thomas added.

While revelations about the hairworm's antics may inspire a new generation of sci-fi aliens, the study team says their findings may also help the development of new medical treatments.

Biron says mind-altering human pathogens—such as those that cause rabies, sleeping sickness, and toxoplasmosis—may manipulate their victim in similar ways.

He said further understanding of biochemical communication between a parasite and its host may "ultimately assist researchers in the search for new drugs and vaccines."

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