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Critter Chemistry

July 15, 2002

Frog Deformities Explained

Parasites and Pesticides Act in Concert to Cause Limb Defects in Wood Frogs

Bette Hileman

Recently published laboratory and field data indicate that the high incidence of frog limb deformities observed in recent years may be caused by a synergism between parasitic infection and exposure to low levels of agricultural chemicals.

Since the early 1990s, frogs with missing or extra hind limbs have been found in at least 43 states. Researchers had proposed two major theories about the cause of the deformities: chemical contamination or infection with parasitic trematodes that live in snails.

Now, Joseph M. Kiesecker, professor of ecology at Pennsylvania State University, has concluded from experiments on wood frogs that both conditions may be necessary to produce the high levels of deformities often observed [Proc. Nat. Acad. Sci., published online,].

Kiesecker raised tadpoles in 36 enclosures in six outdoor ponds. In half the enclosures, the tadpoles were protected from trematode larvae by a fine screen. Only the unprotected tadpoles developed limb deformities.

For the next experiment, Kiesecker measured the deformity rate in each pond. In the ponds uncontaminated by pesticide runoff, deformities occurred in 5 to 10% of trematode-infected frogs. In contaminated ponds, 20 to 30% of the trematode-infected frogs were deformed.

Kiesecker next raised tadpoles in the lab, exposing them to pure water or to water with very low levels of the pesticides atrazine or malathion or the synthetic pyrethroid esfenvalerate. After four weeks, the tadpoles were infected with trematodes. The tadpoles exposed to the pesticides developed high levels of parasitic infections, but those exposed to pure water had very low levels. He also found that pesticide-exposed tadpoles had suppressed immune systems.

"What we are seeing with amphibians parallels a disturbing increase in disease prevalence in both wildlife and human populations," Kiesecker concludes. "It raises the question: Are we changing the environment in ways that increase disease prevalence?"

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