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.

In tropical climates trematodes cause schistosomiasis, a disease that kills millions of people. The World Health Organization estimates that 120 million people worldwide suffer from schistosomiasis.

Kiesecker's team took tadpoles from Centre County, Pennsylvania, and placed them in six local ponds—three affected by pesticide runoff and three pesticide-free. In each pond, the tadpoles were separated into two groups. One group was placed inside a fine mesh that kept out cercariae.

Only the tadpoles that were exposed to cercariae developed deformities. "We learned from the first field experiment that tadpoles have to be exposed to trematode infection for limb deformities to develop," Kiesecker said.

Pesticide Problems

Kiesecker's team then compared the rate of infection between trematode-exposed tadpoles in the different ponds. The team discovered that rates of infection were much higher in the ponds that received pesticide runoff.

This result parallels the finding of a study in 2000 in which frogs from the same pond—that is, those experiencing the same environmental conditions—were found to have similar deformities.

To examine the effects of pesticides on cercaria development, the team conducted lab experiments on four groups of tadpoles—three groups exposed to three common pesticides and a control group. The pesticides were Atrazine, the most commonly used pesticide in North America; Malathion, a common household pesticide that also is used to control insect pests in agricultural fields; and Esfenvalerate, a synthetic pyrethroid pesticide. The tadpoles were all exposed to cercariae.

When they counted the number of cysts that formed in the tadpoles, the researchers found much higher levels in the tadpoles exposed to pesticides. The team also took blood samples before and after the experiments to determine whether the tadpoles' white blood cell count—a measure of immune system health—was affected.

"The tadpoles that we exposed to pesticides had fewer of this particular kind of white blood cell compared to the tadpoles that we did not expose to pesticides, suggesting that pesticides make these animals more susceptible to parasitic infections," Kiesecker said.

Pesticides have been found to have additional harmful effects on frogs. A study published in April in the Proceedings of the National Academy of Sciences found that Atrazine interfered with the sexual development of male frogs in the Midwest, reducing their levels of testosterone to below the levels found in female frogs.

"Atrazine-exposed frogs don't have normal reproductive systems," said Tyrone Hayes, the leader of a team from the University of California at Berkeley. "The males have ovaries in their testes and much smaller vocal organs."

Human Impacts?

Kiesecker said society can learn a lot from the experiments because "amphibians are particularly sensitive to environmental changes that appear to be associated with the recent emergence of new diseases and resurgence of old diseases that infect humans."

Especially disturbing, he added, is that the concentrations of two of the pesticides that caused the deformities in frogs, Esfenvalerate and Atrazine, were low enough for the water to be considered safe for human consumption under Environmental Protection Agency standards.

"Frogs may be a sentinel species that is warning us about the interplay between human-caused environmental change and disease susceptibility," he said, adding: "Hopefully, people will listen."

In other recent research on this problem, a study published in the July 1 issue of Environmental Science & Technology indicates that frog deformities may also occur as a result of exposure to ultraviolet (UV) radiation. At levels close to 60 percent of normal sunlight, frogs experienced deformities.

A survey of ponds in the Duluth, Minnesota, area showed that frogs in only three of 26 ponds were at risk of UV-induced deformities, because wetlands absorb a significant portion of the radiation. However, Steve Diamond of the Environmental Protection Agency's Duluth office and leader of the UV study said there may be cause for concern if human activities cause UV levels to rise.

Join the National Geographic Society

Join the world's largest nonprofit scientific and educational organization, and help further our mission to increase and diffuse knowledge of the world and all that is in it. Membership dues are used to fund exploration and educational projects and members also receive 12 annual issues of the Society's official journal, National Geographic. Click here for details of our latest subscription offer: Go>>

© 1996-2008 National Geographic Society. All rights reserved.