Horsehair Worms' Mysterious Life Cycle Lies in Cysts

Released: 3/20/2001 12:00 AM EST
Source Newsroom: University of Nebraska-Lincoln
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Graphic available: jpeg of researcher holding string of horsehair worms (close-up view) available by contacting Kelly Bartling, Public Relations: kbartling2@unl.edu

Contact: Ben Hanelt, Biological Sciences -- (402) 472-2720

'ENIGMATIC' WORMS' LIFE CYCLE UNRAVELED

Lincoln (Neb.) -- March 19, 2001 -- To biologists, they have been one of the most enigmatic groups of animals in the world.

They're the horsehair worms of the phylum Nematomorpha, one of only four animal phyla known to be completely parasitic. They have been thought of as "enigmatic" because until last year, no one had a clue about their life cycle.

What has been known is as adults, the worms are free-living in streams and lakes, where they gather to mate in tight masses that are almost impossible to unravel. It has also been known that the egg strings they produce become heavy, non-swimming larvae that settle to the bottom of the lake or stream.

The question has been how those aquatic larvae end up inside terrestrial insects such as grasshoppers or crickets, where they feed on the insect's internal organs and reach lengths of as much as 3 meters in some species before emerging to mate.

University of Nebraska-Lincoln doctoral candidate Ben Hanelt found the answer to that question in research performed in Lancaster County, Neb., and at UNL's Cedar Point Biological Station near Ogallala.

"We found that these tiny larvae settle on the bottom of stream or lake and are taken up by almost anything in the water, and they produce cysts," Hanelt said.

Among the creatures that will ingest them, Hanelt found, are snails, fish and the larvae of aquatic insects like midges or mosquitos.

"Nematomorphs will encyst in midge or mosquito larvae and when the midge or mosquito larva emerges as an adult, the cyst is retained," he said. "When crickets or grasshoppers eat a dead midge or mosquito, they ingest the nematomorph cyst."

That, he said, proves to be a fatal meal for the insect. The nematomorph larva, which had ceased development when it encysted, emerges from the cyst and begins to eat the host creature from the inside out. When the larva finishes its meal, it emerges from the dead host as an adult, mates and dies. While crickets and grasshoppers aren't normally found in water (a necessary condition for the adult worm to emege), Hanelt said the speculation is that as the larvae near maturity, the insects become thirsty as well as weak and this can cause them to fall in the water.

In his research, Hanelt collected freshwater snails of a single species at 50 scattered sites in streams in Lancaster County and found nematomorph cysts in snails at 35 of the sites.

He fed some of the infected snails to crickets, waited 30 days and put the now-infected crickets in water.

"Within one or two seconds, the worms start emerging and in one case, nine worms 30 to 40 centimeters (12 to 16 inches) long and about 2 millimeters in thickness came out of one cricket," Hanelt said. "We weighed the worms and we weighed the cricket and they were about equal weight. So when the worms were done with the host, there wasn't much host tissue left."

Hanelt's research also showed that human population density and land use does not affect the distribution of nematomorphs. In fact, one of his collection sites was in a shopping center.

"As urban sprawl increases, the chances for human-worm interaction increases and some people or their pets may get infected," Hanelt said. "It's really a pseudoinfection because humans, dogs and cats really don't get infected the way a grasshopper would. It sure would make you sick -- and there's reason to be concerned about that -- but it's never been reported to be life-threatening."

Hanelt said although human infections are not common, about 150 cases have been reported in the scientific literature.

He said he has heard from people who found horsehair worms (so-called because of their resemblance to a horse hair that has come to life) in their toilet and feared a family member had been infected. "I ask them if they've recently killed a cricket and tossed in the toilet, and that usually turns out to be the case," he said.
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(News release Web site: http://www.unl.edu/pr/releases.html)
(Science news release Web site: http://www.unl.edu/pr/science.html)
By Tom Simons, Public Relations


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