Male leopard frogs in several states are acquiring female sexual attributes as a result of exposure to atrazine, one of the most heavily used weedkillers in America, according to a study published today in the journal Nature.
Syngenta, the largest U.S. producer of atrazine, questioned the finding. However, even if these effects are real, they have no bearing on human health risks, said Tim Pastoor, a toxicologist based in the company's Greensboro, N.C., offices.
The study, the second this year by UC Berkeley endocrinologist Tyrone B. Hayes to scrutinize the feminization of frogs by the farm chemical, comes as the U.S. Environmental Protection Agency nears the end of an eight-year safety review of atrazine.
The review was undertaken in 1994 after decades of heavy atrazine use on corn and sorghum crops caused it to become the most commonly detected pesticide in American ground- and surface water. It was even routinely detected far from agricultural areas, in presumably pristine sites such as national parks.
The reason, said William A. Battaglin, a hydrologist with the U.S. Geological Survey in Lakewood, Colo., is that atrazine is very soluble: "It moves with rain, with air, on dust."
Hayes was one of dozens of hormone specialists approached to study the chemical for the EPA review. "My original thought was [that] nothing was going to happen," Hayes said. But in April, he published a study in the Proceedings of the National Academy of Sciences showing that South African clawed frogs exposed to atrazine in the laboratory suffered an array of sexual deformities. Hayes speculated that the weedkiller was throwing a series of hormonal switches in the frogs, which led to the conversion of testosterone to estrogen.
Syngenta criticized the April study for using foreign animals in artificial conditions. The Nature study looks at the most common U.S. species, the leopard frog, at eight sites chosen in states in a west-east slice from Utah to Iowa. "Now, we've shown it in the wild, and only where there is atrazine," Hayes said.
The finding revives arguments about atrazine in drinking water. Since 1991, a number of European countries have banned or suspended its use. A French ban is expected to take effect next year because atrazine has exceeded the 1-part-per-billion level in drinking water set for any pesticide. By contrast, the EPA set a maximum average level for atrazine of 3 ppb in U.S. drinking water.
Pastoor said the U.S. level was set after studying hundreds of toxicity trials and allowing wide safety buffers: "At this rate, a 150-pound adult could drink 21,000 gallons of water a day for 70 years and still experience no adverse health effects."
Other scientists remain concerned because frogs are regarded as important markers of water purity.
As global frog populations have crashed over the last 10 years, researchers have focused on three potential causes: climate change, urban development and pesticides.
"From biblical times, people have relied on frogs as indicators of water quality," said Battaglin of the Geological Survey. "If you read records of early settlers in the Western U.S., if they got to a water supply and it didn't have frogs in it, they wouldn't drink."
In California, 55,000 pounds of atrazine are used every year, mainly in agriculture. It may be one factor in the disappearance of some of the state's most common frogs, said Carlos Davidson, an ecologist at Cal State Sacramento. Once-common red-legged frogs have disappeared from 70% of their former sites, and mountain yellow-legged frogs from 90% of their territory.
Davidson points to the pressure of urban development in basins and on coasts, but he suspects that pesticide drift is behind the disappearance of mountain frog populations. "Historic accounts of the Sierra Nevada had people not being able to walk around lakes without stepping on frogs," he said. "You go to these lakes now, and there are no frogs."
A preliminary EPA risk assessment for atrazine is expected in January 2003.