Scientists have hotly debated what lies behind an epidemic of deformed North American frogs, finally settling on parasitic worms as the most likely culprit. Now it appears that the parasites might have an accomplice: pesticides that weaken frogs' immune systems. The experiment is one of the first to suggest that such stressors can make amphibians more susceptible to pathogens.
Since 1995, frogs with missing, malformed, or extra limbs have been spotted all over northern North America, from Vermont to Oregon. Worried about possible implications for public health, scientists rushed to explore whether the cause is chemical pollutants, too much ultraviolet light, or parasites that form cysts at the base of tadpoles' developing limbs. Three years ago, researchers cleared up much of the mystery by showing that in lab studies, a parasitic worm called Ribeiroia can cause the deformities (ScienceNOW, 29 April 1999).
Now scientists have verified that trematodes are actually causing deformities in the wild and added a new twist: The parasites may be abetted by pesticides. Ecologist Joseph Kiesecker of Pennsylvania State University, University Park, placed mesh boxes around wood frog tadpoles in ponds that naturally had Ribeiroia. Those surrounded by mesh that kept out Ribeiroia larvae didn't develop deformities, while those in larger-mesh boxes did.
Kiesecker also found more frog deformities in ponds that tested positive for pesticides than in pesticide-free ponds. Back in the lab, he placed tadpoles in water containing low levels of one of three widely used pesticides--atrazine, malathion, or esfenvalerate--then added Ribeiroia. The parasites formed more cysts in tadpoles living in pesticide-tainted water. These tadpoles also had lower levels of esonophils, a kind of white blood cell that's thought to defend against parasites. Kiesecker, who describes the work in the 9 July issue of the Proceedings of the National Academy of Sciences, saw these effects even at pesticide levels low enough to meet federal standards for safe drinking water.
The study has convinced ecologist Andy Blaustein of Oregon State University in Corvallis that pesticides can potentially exacerbate the deformities. But others caution that Kiesecker hasn't clinched the case. Ecology grad student Pieter Johnson of the University of Wisconsin, Madison, who helped link Ribeiroia to the frog deformities, suggests that pesticides would also weaken or kill the pond snails that transmit Ribeiroia. "You need to do the [lab] experiment with the whole system," says Johnson.