Pesticides, Parasite May Cause Frog Deformities

Stentor Danielson
National Geographic News
July 9, 2002

Frogs with extra legs or missing legs have been showing up with greater frequency over the past decade, and scientists have been baffled by the cause.

Some researchers have concluded that pesticide runoff from farms is to blame; others say a common parasite is the culprit. Now, a new study suggests that both these factors in combination have disturbed normal development in many frogs, leading to the abnormalities.

The study, published today in the Proceedings of the National Academy of Sciences, was based on tests in both the laboratory and the field that were designed to examine the interaction of parasites and pesticides. The research team, led by Joseph Kiesecker, found that only frogs infected by the larvae of a parasite, the trematode worm, developed deformities, but infected frogs exposed to pesticide runoff experienced much higher levels of deformities.

"It is not uncommon now for 20 to 30 percent of the frogs at many locations to have limb deformities," said Kiesecker, an assistant professor of biology at Penn State University.

Abnormalities have been documented in 52 species of amphibians, mainly frogs, in 46 U.S. states and four Canadian provinces, according to the U.S. Geological Survey. Reports of deformed frogs have been particularly common in New England and the Upper Midwest and on the Pacific coast.

Although there is some disagreement about what levels of deformities occur naturally in frog populations, most researchers agree that current levels are above normal.

Kiesecker and other researchers have warned that the physiological problems seen in frogs may foreshadow similar effects on humans.

Infected by Trematodes

During its life cycle, the parasitic trematode depends on several hosts, including pond snails. Tadpoles in ponds with snails pick up trematode larvae, called cercariae. In some cases the cercariae develop into hard cysts, which interfere with the tadpole's metamorphosis into a frog. When the cysts occur in tissue that later develops into legs, the cysts disrupt the animal's normal development and cause duplicate or missing legs.

The trematode also affects people—although not so dramatically as in developing frogs. It's the same parasite that causes "swimmer's itch," a common ailment in people who swim in ponds and lakes. Eventually, the human immune system defeats the cercariae, leaving the victim with just a rash.

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