Atrazine is receiving lots of attention for turning male frogs into girls, but that's not all the common herbicide can do. It also weakens amphibian immune systems, leaving the fragile creatures vulnerable to disease.

Though less obvious than gender bending, immunosuppression could also play a part in the worldwide decline of amphibians, which have porous skin and easily absorb chemicals from rain, groundwater and even water vapor.

"Numerous studies have documented the effects of environmental pollutants on the amphibian immune system. Nearly all of these studies suggest that amphibians are particularly sensitive," wrote Tyrone Hayes, a University of California, Berkeley biologist, in a paper published in the March 15 Journal of Experimental Biology. "In particular, the widespread herbicide atrazine impairs immune function and increases disease rates."

Hayes is also an author of a March 1 Proceedings of the National Academy of Sciences study on the developmental changes wrought in male frogs by groundwater atrazine concentrations regularly found in the United States, where approximately 80 million pounds of the herbicide are used every year. The frogs had low levels of sperm and testosterone; some even produced estrogen, developed female reproductive organs and were ultimately impregnated by their former gender mates.

The findings are disturbing, but atrazine's effects don't end there. As described in the JEB paper, a large body of scientific literature describes how atrazine drives down white cell counts and turns off immune system-regulating genes. Atrazine also suppresses immune function in snails, which often carry amphibian-infesting parasites, and feeds the algae on which snails live.

It's a perfect storm of infection: atrazine makes amphibians more vulnerable to disease, and carriers of disease more common.

The best-known amphibian killer, the chytrid fungus, has killed 95 percent of all frogs in Colombia and Panama, and driven 30 species in the Atelopus genus to extinction — and it was only identified in 1993. However, according to Hayes, chytrid is only the most apparent amphibian affliction. Others may go unnoticed because they don't kill the fragile creatures, but damage their health just enough to prevent populations from sustaining themselves. About 70 percent of amphibian populations around the world are now in decline.

Atrazine, which is scheduled for review by the Environmental Protection Agency after being declared safe by the Bush-era EPA, isn't the only potential chemical culprit. Many common pesticides and herbicides have also been linked to amphibian immune malfunction. But as bad as other chemicals may be, Hayes considers atrazine — which can travel up to 600 miles in groundwater — the worst.

"It's so common, so mobile and persistent, and so active at low levels," he said. "But there may be chemicals out there just as bad, only we don't have data on them."

*Image: Rainforest Harley/Flickr *

See Also:

Citations: "The cause of global amphibian declines: a developmental endocrinologist’s perspective." By T. B. Hayes, P. Falso, S. Gallipeau and M. Stice. Journal of Experimental Biology, Vol. 213 No. 5, March 15, 2010.

"Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis)." By Tyrone B. Hayes, Vicky Khourya, Anne Narayana, Mariam Nazira, Andrew Parka, Travis Brown, Lillian Adame, Elton Chan, Daniel Buchholz, Theresa Stuevea, Sherrie Gallipeau. Proceedings of the National Academy of Sciences, Vol. 107 No. 9, March 1, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.