UC Toxics News: Spring/Summer 2009 Table of Contents   Next Article From Silent Spring to Silent Night: Effects of Atrazine on Amphibians by Mika Pringle Tolson "The death of amphibians and the role that pesticides have played in global amphibian declines is important for us to start thinking about our own health and implications." -Professor Tyrone Hayes of UC Berkeley "Until recently, atrazine was the number one herbicide sold in the world," began Tyrone Hayes, a professor of Integrative Biology at UC Berkeley. Atrazine is used with monocot crops such as corn in more than 80 countries, but it is now outlawed in all of Europe. Hayes has studied the role of environmental factors on growth and development in amphibians since his days as a graduate student. His focus on atrazine began when he conducted research for the manufacturing company Novartis in 1997. His laboratory found atrazine to be a potent endocrine disruptor in frogs. "The animals exposed to atrazine become hermaphrodites. There are no frogs that are functionally or morphologically hermaphroditic." He implored the audience to think about the implications for all vertebrates, including humans, of the effects of atrazine. "The hormone in pregnancy in humans and the hormone that stimulates egg development in frogs is so similar that they produce the same effect," said Hayes. When frogs were exposed to atrazine in the lab, blood testosterone levels were significantly reduced. Hayes' group looked at what happens to the males at metamorphosis - mating activity was significantly reduced in exposed males. Only two of the males in all their studies have been able to successfully mate with females. And there's a severe reduction in the males' ability to fertilize eggs. The group also looked at feminizing effects. The 10% of the exposed males not demasculinized ended up malformed with multiple testes, multiple gonads, or a mix of testes and ovaries. These males preferred to mate with other males and they also laid viable eggs. "For three generations, we've had brothers mating," said Hayes.

When African Clawed Frogs were exposed to atrazine, successful mating was significantly reduced.

But are these effects specific to the African Clawed Frog they studied, or for amphibians in general? Hayes research group looked at a different species, the North American Leopard Frog and found the same effects. The work was published in Nature.

They also wanted to know if the effects could be found in the wild. A typical farmer applies atrazine at 290 million times the levels Hayes' group used in their lab studies. "There's enough atrazine in rainwater to make hermaphroditic frogs," said Hayes. The USGS can measure atrazine in the rainwater in Minnesota from applications on Kansas farms.

The group sampled sites along several rivers in the north mid-western US. They found testicular tubules and eggs throughout the testes in these samples. There were hermaphrodites with 100% correlation for atrazine contamination. In their first study of the North Platte River, 92% were hermaphrodites. They measured the same river 3 years later and found no atrazine and 0% hermaphrodites. "If this were a naturally occurring phenomenon, there would be hermaphrodites all the time, not only when there's atrazine contamination," Hayes said. This work was also published in Nature.

How are the other herbicides, fungicides, and insecticides in ponds contributing to amphibian decline? "No individual compound delays metamorphosis relative to controls, but when you mix them, they retard development," said Hayes. Tadpoles must metamorphose before the pond dries out, but as the water leaves, it concentrates the pesticides that retard metamorphosis. "This is an important interaction that people haven't thought about," said Hayes.

Frogs are also immunosuppressed when they are exposed to mixtures. Hayes looked at disease rates in animals cultured in waters collected from agricultural sites and found worms in the kidneys. The majority of parasites attack the liver and kidneys. When the internal pesticide load increases in the animals, disease levels increase.

"There are at least a dozen papers showing that atrazine increases susceptibility to diseases," said Hayes. "You don't need a fancy virulent disease to wipe out amphibian populations. Even the hearty bull frog can't survive a yeast infection when it's exposed to agriculture runoff."

"In much the same way Rachel Carson (in Silent Spring) felt the death of birds was a warning to us about our own health, I feel the death of amphibians and the role that pesticides have played in global amphibian declines is important for us to start thinking about our own health and implications."

Tyrone Hayes concluded with a wrap-up rap. "Your son or daughter will develop in water just like tadpoles do."

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