Study adds twist to deformity mystery

When schoolchildren on a field trip found frogs with missing legs in a Minnesota pond 12 years ago, the mystery captured the imagination of biologists and the public. Soon reports of amphibians with missing or extra limbs poured in from sites around the country, including New England. In some places, as much as half the frog population was affected.

What could be causing the deformities? Some scientists pointed to pesticides, which might be endangering people, too. Those who blamed parasites won out, however, when researchers created deformed frogs in the lab by subjecting them to infections of the wormlike ribeiroia ondatrae.

But research published this month in the journal EcoHealth argues that a parasitic invasion can't explain the plague that has hit ponds in Vermont's Lake Champlain Basin, where up to 30 percent of leopard frogs have missing or deformed legs. The paper is part of a growing consensus among amphibian biologists that the mutant-frog conundrum is too complex to be pegged to a single cause.

"This study closes the door on the parasite idea for the New England area," said the study's lead author, David Skelly of Yale University.

Skelly said he and a team of researchers spent six years searching in Vermont for infections of ribeiroia ondatrae, a tiny critter that breeds in the sex organs of snails. The parasites can burst through the bodies of snails and attach themselves to tadpoles, distorting the tadpoles' development and leading to extra or deformed limbs.

Skelly and his colleagues never found evidence of the parasite.

Instead, they noticed that tadpoles living near commercial farms were twice as likely as those living elsewhere to develop deformities. Usually, they were missing limbs, a problem not associated with the parasite.

The discovery, Skelly said, breathes new life into the theory that chemicals may play a role.

"Vermont is a heavily agricultural state, and there is a huge cocktail of different chemicals applied to feed corn and apple orchards," said Skelly, who is now analyzing the types of chemicals commonly used on the farms.

Other amphibian biologists, however, said different explanations for the limping frogs were more compelling. Frogs, like people, become stressed in crowded conditions, said Stanley Sessions , a biology professor at Hartwick College in Oneonta, N.Y., and one of the original proponents of the parasite hypothesis. In experiments due to be published this year, Sessions and his students placed thousands of tadpoles in tanks in his lab and watched as they began attacking each other and biting one another's limbs.

"We have met the enemy and it is us," Sessions said of the discovery, which he called a shock.

Scientists said that while millions of research dollars are riding on the frog controversy, it probably will not be resolved soon.

"The number of stressors on amphibians is enormous," said Richard Wassersug , an evolutionary biologist at Dalhousie University in Halifax, Canada. "If we're trying to protect aquatic environments we cannot look at one single factor."