CORVALLIS, Ore. - The alarming increase of physical deformities including extra legs in frogs, toads and other amphibians in the western United States is most likely caused by infection with a certain type of parasite, researchers said in a major study released this week.

The existence and number of those parasites is dependent upon the presence of a group of snails that play an essential role in the life cycle of the parasite, the study said. And burgeoning populations of those snails, in turn, may be due to alterations of habitat, loss of natural wetlands, and high nutrient levels caused by fertilizers or ranch animal grazing.

The study was published in Ecological Monographs, a professional journal of the Ecological Society of America, by researchers from Oregon State University and several other universities and agencies. It was based on an analysis of thousands of amphibians from 11 species over a five-state region of the American West.

The research also pointed away from the use of pesticides as a causative factor in this problem, finding little association between pesticide presence or levels and the number of amphibian deformities.

"This study shows a clear and strong link between amphibian deformities and the presence of parasites, and the snails that form part of the life cycle of those parasites," said Andrews Blaustein, a professor of zoology at OSU and one of the world's leading experts in the study of amphibian ecology and decline. "And we believe the increasing number of parasites and snails can ultimately be traced to human-caused alterations in habitat.

"For example, runoff of nitrogen-based fertilizers into aquatic systems may cause increases in the algal population that the snails feed on," Blaustein said. "The snails carry the parasites. More snails means more parasites infecting frogs and causing deformities."

In every animal population, the researchers said, there are a small percentage of individuals with physical deformities resulting from genetic mutation, trauma or other developmental disturbances. In amphibians, these "background" abnormalities had traditionally occurred at frequencies of 0-5 percent. But at several sites in the western U.S., the level of these deformities has been climbing to 15-20 percent, and in some sites ranges from 50-90 percent.

Most often the deformities take the form of missing digits or portions of a limb, or extra limbs. Researchers have shown in laboratory experiments that such deformities can be caused at high levels, approaching 100 percent, by exposure to the parasitic trematode Ribeiroia ondatrae, whose larvae infect amphibians near the base of hind limbs and form cysts that lead to limb malformations.

What is now becoming more clear is the elaborate life cycle of this parasite and the mechanisms by which that may affect amphibians. The life cycle at various times includes an amphibian or fish; a bird or mammal; and aquatic snails, usually of the genus Planorbella. The tissues of the snails are where the parasite's eggs actually develop and reproduce.

The study surveyed 101 sites across California, Oregon, Washington, Idaho and Montana, and about 60 percent of the sites were in eight counties in Oregon. Scientists looked at deformities in such common species as Pacific treefrogs, western toads, bullfrogs, and others. Careful checks were made for the presence and level of 61 pesticides, orthophosphates and also nitrates.

No relationship was found between pesticide and metabolite compounds and the presence or frequency of malformed amphibians. But presence of the Ribeiroia parasite was a powerful predictor of amphibian deformities - the greater the level of the parasite, the higher the frequency and severity of the deformities.

In their report, the scientists said that parasite numbers may be rising due to the creation of small, permanent farm ponds, the loss of wetlands, and heavy grazing by cattle. The Planorbella snails appear to thrive in some of these situations.

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Andrew Blaustein, 541-737-5356