A parasitic worm has emerged as the leading culprit behind the extra limbs, missing legs, and other deformities plaguing some North American frogs. But scientists caution that the infections, reported in tomorrow's Science, may not account for the different patterns of frog deformities seen outside of California, especially in the Midwest.
First spotted by schoolchildren in Minnesota in 1995, the infamous malformed frogs sparked concerns that the amphibians could be falling victim to some type of environmental degradation--a change that might even threaten people. Investigators have pursued three main theories about what might be causing the problems: pesticides or other pollutants, increased ultraviolet light let through by the atmosphere's tattered ozone layer, or parasites (Science, 19 December 1997, p. 2051).
Despite the flurry of activity, however, no lab had grown a batch of frogs under environmentally relevant conditions and produced the deformities seen in wild specimens of the same species--until now. While an undergrad at Stanford 2 years ago, Pieter Johnson, with ecologist Paul Ehrlich as his adviser, investigated ponds about 45 minutes south of Palo Alto where up to 40% of young Pacific treefrogs have deformities, mostly extra, partial, or missing hindlegs. The water tested free of chemical pollutants, but he noticed that the ponds with deformed frogs always had planorbid snails, a host for a kind of flatworm called Ribeiroia trematodes.
By the time he graduated last June, Johnson had dissected hundreds of frogs and found trematode cysts clustered around their extra limbs. But he hadn't done any experiments exposing tadpoles to the parasites. So Johnson teamed up with two friends and spent the summer "working pretty intensely," as he recalls, often in the lab from 10 p.m. until dawn catching parasitized snails. Back in the lab they let the snails infect tadpoles with the worms. The results were "almost painfully textbook," Johnson says. Higher trematode counts led to more deformities, and the mix of multiple legs, partial and missing limbs, fused skin, and other oddities jibed with that seen in the field.
"This is the best experimental evidence showing a cause for the limb deformities in amphibians," says Andrew Blaustein, an ecologist at Oregon State University in Corvallis. Other researchers say that a different pattern of abnormalities seen in midwestern and eastern frogs may point to additional perpetrators. "I do not believe that there's a single cause" for the deformities, says herpetologist Mike Lannoo of the Indiana University School of Medicine in Muncie. Still, many experts are saying that after several years of frustration it's a relief to finally get at least a partial conviction.