Thyroid Linked to Some Frog Defects.

Since 1994, scientists across North America have been chronicling an epidemic of frog deformities, much of which seems to trace to something in lake water. Two studies now home in on some of the pollutants responsible and the means by which they wreak havoc.

Toxicologist Douglas J. Fort of the Stover Group, a firm of toxicology consultants in Stillwater, Okla., and his colleagues collected samples from Minnesota and Vermont lakes. Half came from areas where frog-deformity rates were low, the rest from hot spots with high malformation rates.

The researchers then performed experiments on Xenopus laevis frogs--the African species that serves as the amphibian analog of a lab rat.

Frog embryos placed in a growth medium mixed with water from the low-deformity, or background, sites developed normally, Fort's team reports in the October ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. However, those exposed to water from the hot spots came to display the same deformities seen in amphibians in the field. These include bent spines, malformed jaws, fewer-than-normal legs, and too many or too few eyes. Some tadpoles also failed to metamorphose.

Because thyroid hormones orchestrate much of vertebrate development, including the metamorphosis of tadpoles, Fort suspected the water pollutants were interfering with this endocrine system. In follow-up tests, his group showed that extra thyroid hormone usually prevented or ameliorated the toxicity.

In a second paper in the same journal issue, the scientists reported that what makes a hot spot hot is not simply that its water is polluted. Explains Fort, all the sampled lakes were "chemical soups" of pollutants--only their recipes differed.

For instance, he notes, hot-spot lakes "contained chemicals that we did not find in our [background] sites," such as the pesticides maneb, permethrin, and propylthiourea. Though individually many lake pollutants can induce some deformities, the team found that limb deformities and bent spines were triggered only by maneb or propylthiourea.

Natural features of the water at certain sites--such as plant-breakdown products--proved capable of increasing some pollutants' toxicity, notes coauthor James G. Burkhart, a chemist at the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.

The new findings reinforce what Martin Ouellet of McGill University in Montreal has seen. He says, "Our epidemiological data clearly show amphibian deformities occur only in sites subjected to pesticides"--often a soup of 10 to 20 such agents. Although a few studies have linked deformities to parasites (SN: 5/1/99, p. 277), experiments like those by Fort's team affirm that no single agent can explain all frog malformations worldwide, Ouellet adds.

Indeed, frog X rays are revealing several distinct syndromes, Michael Lannoo of Ball State University in Muncie, Ind., told SCIENCE NEWS. Rare symmetrical pairs of extra legs at various sites trace to parasites, he finds, while symmetrically truncated hind legs occur after overexposure to ultraviolet light. Lannoo applauds Fort's team for mixing field observations and tests to investigate what's behind the typically asymmetrical defects at the sites it studied.