Agricultural pollutants may conspire with parasites to derail frog development in many regions of North America, according to a new study.
Malformed frogs with bent spines and extra or missing hind limbs—have been documented since the 1700s. However, since 1994, when Minnesota school children found deformities in many of the frogs they caught, unusually large numbers of malformed frogs have turned up from California to eastern Canada.
Scientists have fiercely debated possible causes, with ultraviolet light, chemicals, and parasites being top contenders
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Researchers have found patterns in the wild that support all these possibilities, says new-study author Joseph M. Kiesecker, an ecologist at Pennsylvania State University in State College. One big question is whether several factors together underlie the deformities.
To investigate that issue, Kiesecker has simultaneously looked at exposure to parasites and pesticides. He used porous plastic film to set up six enclosures in each of six natural ponds in rural Pennsylvania.
All the ponds hosted parasitic flatworms called trematodes. Half the ponds were largely pollutant free; the remainder received pesticide runoff. In eachnd, half the enclosures were made of a film porous enough to permit parasite larvae from the surrounding pond water to enter; the remaining enclosures had a less porous, parasite-blocking film. Kiesecker placed 10 wood frog hatchlings of the species Rana sylvatica in each of the 36 enclosures.
After a month or so, he harvested frogs in their early stages of metamorphosis from tadpoles and examined them for deformities and trematode infections.
He found that limb deformities occurred only in frogs from the parasite-infiltrated enclosures. Pesticides, however, increased the prevalence of deformities. Within polluted ponds, 29 percent of these parasite-exposed frogs harbored deformities, but in pollution-free ponds, only 4 percent of these frogs were deformed.
"My findings support the hypothesis that parasite infection explains . . . limb deformities," says Kiesecker. Pesticide exposure probably exacerbates the situation by compromising the immune system's defense against infections, he adds.
To test this, Kiesecker conducted laboratory experiments with three common pesticides at concentrations within Environmental Protection Agency limits for drinking water. Tadpoles exposed to two of these pesticides had significantly fewer eosinophils, white blood cells important for staving off parasitic infections.
Kiesecker describes his findings in an upcoming Proceedings of the National Academy of Sciences.
This is the first demonstration from a field study of a link between chemicals and parasites, says David M. Gardiner at the University of California, Irvine. This research "has demonstrated yet again the dangerous effect of exposure to low doses of chemicals," he says.
Adds Kiesecker, the pesticide concentrations tested in the laboratory are likely to be common in the environment. The combination of pesticides and parasites may help explain amphibian population declines in many regions, he says.
Kiesecker, J.M. In press. Synergism between trematode infection and pesticide exposure: A link to amphibian limb deformities in nature? Proceedings of the National Academy of Sciences.
Milius, S. 1999. Parasites make frogs grow extra legs. Science News 155(May 1):277. Available at http://www.sciencenews.org/sn_arc99/5_1_99/fob4.htm.
Raloff, J. 1999. Thyroid linked to some frog defects. Science News 156(Oct.
2):212. Available at http://www.sciencenews.org/sn_arc99/10_2_99/fob1.htm.
David M. Gardiner, Department of Developmental and Cell Biology and Developmental Biology Center, University of California, Irvine; Irvine, CA 92697-2275
Joseph M. Kiesecker, Department of Biology, The Pennsylvania State University, Mueller Laboratory, University Park, PA 16802
From Science News, Vol. 162, No. 2, July 13, 2002, p. 19.