Parasites make frogs grow extra legs.

In the great mystery of what's causing deformities among frogs in North America, two new research papers make a parasite, once just a minor suspect, look a lot more suspicious.

For the first time, laboratory tests have shown that a parasitic trematode, a flatworm, can cause tree frog abnormalities ranging from six extra legs to no hind legs at all, say Pieter T.J. Johnson of Claremont (Calif.) McKenna College and his colleagues. In the April 30 SCIENCE, they finger Ribeiroia as the villain in ponds in Santa Clara County, Calif.

"This is the first study that has isolated the cause of deformed frogs," Johnson argues. He doesn't expect parasites to be the answer everywhere, however.

In the same issue of SCIENCE, another team says that a survey of nearly 400 frogs with extra legs, mostly tree frogs from the Pacific Northwest, reveals a malformation pattern typical of parasite damage. Led by Stanley K. Sessions of Hartwick College in Oneonta, N.Y., the researchers maintain that the pattern rules out another suspect, a group of chemicals called retinoids.

Reports of occasional deformed frogs date back centuries, but alarm ignited in 1995, when Minnesota schoolchildren found deformities in almost half the leopard frogs they caught. So far, abnormalities have turned up in 36 amphibian species from 42 states. Scientists have been exploring three major suspects: pollutants such as retinoids, ultraviolet light, and parasites.

While doing an undergraduate project in 1996, Johnson focused on deformed Hyla regilla frogs in four ponds. Collected eggs developed normally in the lab, so he ruled out genetic factors. Water tests detected no obvious pollutants, and he noticed that a troubled pond flowed into one with normal animals.

Checking 35 local ponds, he found that only his 4 had both tree frogs and a snail notorious as a flatworm host. In the laboratory, Alaria trematodes caused no frog deformities but Ribeiroia did.

Retinoid researcher David M. Gardiner of the University of California, Irvine ranks Johnson's work as "really important because it brings parasites up to the level [of the other hypotheses]." Commenting on the Sessions paper, Gardiner says that he's not convinced deformity patterns can pinpoint parasite damage, however.

The new work doesn't faze James G. Burkhart from the National Institute of Environmental Health Sciences in Research Triangle Park, N.C. He studies pollutants in deformity hot spots (SN: 10/11/97, p. 230). Among potential causes nationwide, "I've never excluded parasites," he says. "That just happens not to be what we have" in Minnesota and Vermont.

Nor will the parasite work dampen interest in UV and pollutant research at the Environmental Protection Agency's Duluth, Minn., office, says Gerald T. Ankley. "It would be premature to say that we've solved the problem."