"Only recently have scientists begun thinking seriously that parasites may be just as important to ecosystems as lions and leopards. And only now are they realizing that parasites have been a dominant force, perhaps the dominant force, in the evolution of life."

Natural History "Attack and Counterattack" 9/00 From Parasite Rex, by Carl Zimmer 2000

Malformed Frogs/Declining Frogs: The causes of amphibian declines and malformities continue to include the usual factors: habitat loss/alteration, xenobiotic chemicals (either chemicals different from those causing malformations or the same chemicals present in different concentrations), UV-B radiation, fungal infections and parasites, with different factors being more or less important based on region, season, year and those scientists working on specific projects based on their theories. The answer is complex and not clear at this time as it takes decades to conclusively determine declines and alterations leading to malformities and possible genetic mutations, either in the host, secondary host (here being the amphibian) or the parasite.

Trailing tangles of a dozen limbs or missing legs altogether, the deformed frogs found in rising numbers around the country have become a powerful symbol of what many fear is an ever more poisoned environment. But at least in some areas, scientists report, evidence shows that the frog deformities are being caused not by toxins but by parasitic worms.

In two new studies focusing on Pacific tree frogs in California, separate teams of researchers report in the journal Science that when parasitic worms known as trematodes burrow into a frog's developing limb, the infestation can cause exactly the kinds of deformities seen in frogs in the wild. Scientists say the studies provide the first solid evidence pointing to a culprit behind the long mysterious deformities.

Researchers based this conclusion on experiments in which they showed that they could exactly duplicate the kinds of limb abnormalities and other deformities seen in California by infecting tadpoles with the trematode, which goes by the genus name Ribeiroia. Some experts say that this work, together with other study reports may exonerate certain chemicals suspected of causing the abnormalities, has now elevated parasites to the top of the list of possible causes for the frog deformities across the country.

"We've finally solved one piece of the puzzle," said Dr. Stanley K. Sessions, the lead author of one of the new papers and a herpetologist at Hartwick College in Oneonta, N.Y.

Dr. Andrew Blaustein, an ecologist at Oregon State University who was not involved with the new studies, said: "This is the best experimental evidence so far showing a particular agent causing the limb deformities we see in nature. I think parasites will explain a lot of the deformities."

Researchers interviewed, all emphasized that the new findings did not rule out the possibility that chemical pollutants or increasing ultraviolet light caused by a thinning ozone layer could be causes of deformities elsewhere. 1

1. "Parasites, Not Pollution, May be behind Frog Deformities", The New York Times, by Carol Kaesuk Yoon, 4/30/99

note: Special thanks to Dr. Stan Sessions for his generosity of time and knowledge, and for giving me the opportunity to photograph, collect data of frogs and parasites. Thanks also to to John Behler, Chief Herpetologist Curator at the Bronx Zoo, NY, where I have photographed frogs in decline.

Images include the pacific tree frog (Hyla regilla) and an Echinostome parasite. Parasite image courtesy of Dr. Sessions.