Extra legs on frogs linked to worm parasite

Extra legs on frogs linked to worm parasite

By PAUL RECER

WASHINGTON (April 29, 1999 7:42 p.m. EDT http://www.nandotimes.com) - A mysterious ailment that causes frogs to grow extra, deformed legs and touched off environmental concerns may be the result of a tiny worm parasite, not ozone depletion or pesticides, new studies in the journal Science say.

Two studies to be published Friday conclude that defects found in frogs throughout in the Western United States may be caused by a trematode, simple parasitic flatworm with a complex life cycle that includes infecting the developing legs of tadpoles.

The worm infection, says Stanley K. Sessions of Hartwick College in Oneonta, N.Y., causes the tadpoles to grow multiple hind legs, a severe malformation that dooms the animal when it grows to a mature frog.

"Every single frog I have looked at with extra legs, and I have looked at hundreds, all have these cysts around the deformity," said Sessions, who co-authored one of the Science studies.

Pieter T.J. Johnson, a recent graduate of Stanford University now doing research at Claremont McKenna College in Southern California, carried Sessions' early work one step further by collecting trematodes from snails in four ponds where deformed frogs were found. Laboratory tadpoles were then infected with the worms.

Johnson found that the more the trematode infections the tadpoles acquired, the more the legs of the adult frog were deformed, multiplied or missing. And he also showed that the deformities developed in the laboratory experiments were the same as those seen in the four ponds in Northern California.

Andrew Blaustein, an Oregon State University ecologist who conducted experiments to determine if ultraviolet radiation was causing the widespread problem, called Johnson's work "the best experimental evidence showing a cause for the limb deformation in amphibians."

Researchers worldwide have noted for years that many species of frogs are in decline, particularly in the Northern and Western United States, Central America and Australia.

Scientists have suspected pesticides, chemical pollution - particularly retinoic acid, and excess UV radiation caused by a thinning of the atmosphere's ozone layer. A great deal of research has been devoted to the issue because the sudden demise of frogs were considered to be a possible warning about an unknown environmental problem.

But the studies by Johnson and Sessions show that at least some of the frog problems are caused by Mother Nature herself.

Johnson said the increased deformities caused by the parasite could be part of a natural biological cycle. However, he said it is too early to hold humans blameless, saying fertilizer runoff may have caused an increase in a water snail that is a key host of the parasite.

Johnson said that even though trematodes are found throughout North America, the parasite probably is not the single cause for a worldwide decline in frogs and other amphibians first noticed two decades ago.

"It is far too early to say that this is the final answer for the amphibian decline," he said. "Something different may also be going on."