From our Ask the Edmonds Vet columnist: Connection between widely used herbicide, animal reproductive problems

Both of these African clawed frogs are genetically male, but lifelong exposure to the herbicide atrazine transformed the frog on the bottom to female. The female frog reproduced with normal males twice. (Photo by Tyrone Hayes)

By Dr. David Gross

Dr. Val Beasley, professor emeritus at the University of Illinois, Champaign-Urbana, recently collaborated with 22 other scientists who reviewed the evidence linking exposure to atrazine, a herbicide widely used in the U.S. and more than 60 other nations, to reproductive dysfunction in amphibians, fish, reptiles and mammals

Atrazine is the second-most widely used herbicide in the U.S. More than 75 million pounds of it are applied each year to corn and other crops. It is the most commonly detected pesticide contaminant of groundwater, surface water and rain in the U.S.

The review recently appeared in the Journal of Steroid Biochemistry and Molecular Biology. The researchers looked at studies linking atrazine exposure to abnormal androgen (male hormone) levels and found a common association between exposure to the herbicide and the “feminization” of male gonads in many animals. The most robust findings were in amphibians. At least 10 different studies found that exposure to atrazine feminizes male frogs, sometimes to the point of sex reversal.

Beasley’s lab was one of the first to find that male frogs exposed to atrazine in the wild were more likely to have both male and female gonadal tissue than frogs living in an atrazine-free environment. In a 2010 study, Tyrone Hayes, a professor of integrative biology at the University of California at Berkeley, reported in the Proceedings of the National Academy of Sciences that atrazine exposure in frogs was associated with genetic males becoming females, and functioning as females, at concentrations commonly found in the environment.

The review concluded that atrazine exposure can change the expression of genes involved in hormone signaling, interfere with metamorphosis, inhibit key enzymes that control estrogen and androgen production, skew the sex ratio of wild and laboratory animals (toward female) and otherwise disrupt the normal reproductive development and functioning of males and females.

Apparently atrazine works through a number of different mechanisms. It increases production of cortisol (the stress hormone), undermines immune function by increasing cortisol, and inhibits some key enzymes in steroid hormone production while increasing others. It also apparently prevents androgen (male sex hormone) from binding to its receptor.

“Cortisol is a nonspecific response to chronic stress,” Dr. Beasley explains. “But guess what? Wildlife in many of today’s habitats are stressed a great deal of the time. They’re stressed because they’re crowded into little remnant habitats. They’re stressed because there’s not enough oxygen in the water because there are not enough plants in the water (another consequence of herbicide use). They’re stressed because of other contaminants in the water. And the long-term release of cortisol causes them to be immuno-suppressed.”

There also are studies that show no effects, or different effects, in animals exposed to atrazine, Beasley said. “But the studies are not all the same. There are different species, different times of exposure, different stages of development and different strains within a species.” All in all, he said, the evidence that atrazine harms animals, particularly amphibians and other creatures that encounter it in the water, is compelling. Do we want this stuff in our environment? Do we want our children to drink this stuff? I would think the answer would be no.”

Dr. David Gross of Edmonds graduated from Colorado State University’s veterinary school in 1960 and was in private practice for 10 years. He retired in 2006 as Professor and Head of Veterinary Biosciences, at the University of Illinois, Urbana-Champaign. Dr. Gross is the author of “Animals Don’t Blush,” which describes the unique patients and even more unique clients of a veterinary practice in Sidney, Montana in the early 1960s. The book is available at the Edmonds Bookshop.

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