Agricultural pesticides have been linked with some frog deformities in Minnesota, according to two studies.
The research is the culmination of 18 months of laboratory analyses of Minnesota pond water, and it suggests that combinations of chemicals appear to be causing malformations of the frogs' limbs, eyes, mouths and other parts.
Scientists who conducted the work said they are still far from understanding the mystery of the deformities and whether the causes could affect humans. The studies were published in the October issue of Environmental Toxicology and Chemistry, a national journal.
"At this point we can't say that this is something that applies only to frogs," said Jim Burkhart, co-author of the studies and a biochemist at the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.
Water and sediment samples were taken from six Minnesota ponds, three of which had produced high proportions of deformed frogs. The locations weren't named. In analyzing the water, scientists found many chemicals, several of which are products of various pesticides.
Burkhart said the chemicals are active, but exactly how they function is difficult to determine because there are other chemicals and natural materials in the water, such as plant breakdown products.
In the other study, scientists took Minnesota pond water and used it to raise frogs. The water from cleaner sites did not cause problems, but water and sediment from ponds in which deformed frogs were found caused the same kinds of malformations, Burkhart said. Deformities included defects of the skull and face and abnormal development of the mouth and eyes. Filtering the water to remove some of the chemicals reduced the deformities significantly, according to the study.
The most obvious deformities seen in Minnesota frogs have been twisted spines or malformed hind limbs, including missing, contorted or shortened legs and other problems such as webbed skin or missing digits. Maneb, a fungicide, and propylthiourea, a pesticide breakdown product, induced some types of hind limb defects in the lab frogs.
Burkhart said the most intriguing finding was that some of the compounds appear to be more toxic in natural waters in Minnesota than in the solutions prepared in the labs. Compounds segregated and mixed with lab water produced deformities, he said, but when they were mixed with "clean pond" water, the deformities were much worse.
"The question we're asking is, what is the interaction between some of these manmade compounds with the nontoxic compounds of natural origin at some of these sites?" he said.
Doug Fort, a co-author of the studies and a toxicologist at the Stover Group, a private research and development firm in Oklahoma, said the research is more suggestive than definitive.
He said several of the chemicals suspected of causing deformities appear to have one similarity: They affect the thyroid, the primary gland responsible for growth, maturation and development in most animals, including humans. When thyroid hormones were added to some of the experiments, he said, the incidence of hind limb defects declined.
Judy Helgen, a wetlands biologist for the Minnesota Pollution Control Agency, called the research "a solid step in the direction we need to go." Helgen, who has provided frogs and water samples to researchers across the country, said other scientists are considering parasites, ultraviolet radiation and immune deficiencies as possible causes.
"We're still open to other hypotheses, but it certainly looks from these [scientific] papers like chemicals or some combinations that include chemicals are responsible for different types of malformations," she said.
During the past four years, deformed frogs have also been reported in several other states, including Wisconsin, Vermont, Oregon and California, and in Quebec.
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