By Karin Zeitvogel (AFP) – Mar 1, 2010
WASHINGTON — One of the most common weed-killers in the world, atrazine, causes chemical castration in frogs and could be killing off amphibian populations worldwide, a study published showed.
Researchers compared 40 male control frogs with 40 male frogs reared from the moment they hatched from eggs until full sexual maturity in atrazine concentrations in the range that animals experience year-round in areas where the chemical herbicide is found.
Ninety percent of the male frogs exposed to atrazine had low testosterone levels, decreased breeding gland size, feminized laryngeal development, suppressed mating behavior, reduced sperm production and decreased fertility, while the control group showed features typically found in male frogs.
And what happened to the remaining 10 percent of atrazine-exposed frogs was deemed "the most dramatic finding" of the study by the researchers, led by Tyrone Hayes of the University of California at Berkeley: they developed into females that copulated with males and produced eggs.
The larvae from those eggs were all male, the study published in the Proceedings of the National Academy of Sciences (PNAS) found.
"Many studies have focused on death from disease and its role in global amphibian declines and sudden disappearances of populations, but virtually no attention has been paid to the slow, gradual loss of amphibian populations due to failed recruitment," the study said.
"The production of sex-reversed all male-producing animals could drive populations to extinction," the authors of the study warned.
Earlier studies have found that atrazine also caused feminization in zebra fish and leopard frogs and caused a significant decline in sperm production in male salmon and caiman lizards.
Atrazine is widely used by farmers in 60 countries around the world as a weed- and grass-killer, particularly in fields of corn, grain sorghum and sugar cane. It is also used on golf courses.
Apart from the United States, Australia and China are among the biggest markets for atrazine, according to Tim Pastoor, principal scientist for Syngenta, the US company that makes the herbicide.
Atrazine is banned in the European Union, but farmers there use an almost identical chemical called terbuthylazine, Pastoor said.
Environmental activist organizations in the United States have called for atrazine to be taken off the market in the United States, but the Environmental Protection Agency (EPA) four years ago refused to ban the chemical.
"The EPA have already considered the question of whether or not atrazine will affect frogs' sexual development, and the answer to that is clearly no," Pastoor told AFP.
"But the main reason Syngenta stands behind atrazine is because farmers have come to us and said, 'Please keep this product on the market.'
"The reason they ask us is because it's a product that works, it increases crop yields, and it's inexpensive, and something that's inexpensive and works is magic to a farmer," said Pastoor.
Around 80 million pounds of atrazine are applied annually to crop fields in the United States alone, and half a million pounds of the herbicide fall to earth in rainfall in the United States, some of it hundreds of miles from the farmland where it was originally applied, the study says.
"Atrazine can be transported more than 1,000 kilometers (620 miles) from the point of application via rainfall and, as a result, contaminates otherwise pristine habitats," the study says.
Atrazine has also been associated with low sperm count, poor semen quality and prostate problems in humans.
A small study led in 2003 by University of Rochester professor in environmental medicine Shanna Swan found that men who lived in the Midwest -- the biggest corn-growing region in the United States -- who had high levels of atrazine in their urine also had low sperm counts.
"We haven't been able to follow up and confirm that study, but it did find an association," Swan told AFP.
"But the important message here is: this is a very prevalent exposure. Human beings are definitely exposed, and a lot of them, because it's the most commonly used pesticide," Swan said.
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