Ever since Florida biologists discovered altered hormone activity in alligator hatchlings taken from a pesticide-polluted lake in 1997, chemical manufacturers have challenged the evidence linking environmental contaminants to endocrine disruption and abnormal sexual development. But an international team of researchers reported last week that exposure to atrazine, one of the most common pesticide contaminants in ground, surface, and drinking water, can alter the reproductive traits of an alarming range of animals, from fish and frogs to rats and turtles.
To evaluate the strength of the evidence linking the weed-killer atrazine to reproductive defects in amphibians, fish, reptiles, and mammals, the researchers — a team of 22 scientists with extensive expertise in reproductive, cellular, and ecological toxicity — reviewed more than 140 studies. They found compelling evidence that atrazine acts as an endocrine disruptor, interfering with the normal function of hormones to wreak havoc with the male reproductive system.
"More and more evidence is piling up on the same side of the equation," said amphibian expert and ecotoxicologist Val Beasley, a co-author of the study who is professor emeritus of comparative biosciences at the University of Illinois. "This class of chemicals, and particularly atrazine, continually seems to feminize males and sometimes cause sex reversal."
Tyrone Hayes, the lead author of the study, has likened the herbicide's effects to chemical castration because the testes of exposed males typically contain few sperm. Hayes, an expert in amphibian endocrine development at UC Berkeley, first reported atrazine's gender-bending effects in 2002. He was traveling in India this week and unavailable for comment on the new report.
Atrazine is one of the most commonly used pesticides in the world. Although the European Union removed the herbicide from its list of approved pesticides in 2003, citing unacceptable levels of groundwater contamination, it remains in widespread use elsewhere around the globe. Some 77 million pounds douse corn, sugarcane, and other crops each year in the United States alone. Applications are heaviest in the Midwest, where in 1998 researchers first linked atrazine to hermaphrodite cricket frogs, a once-common species that nearly disappeared in northeastern Illinois following a dramatic increase in reproductive problems among the population.
Since then, accumulating evidence suggests that atrazine delivers a one-two punch to the breeding prospects of exposed animals, targeting both the sex-specific traits animals use to attract mates and the anatomical equipment they need to reproduce. In laboratory experiments atrazine not only messed with one of the bullfrog's most important secondary sex characteristics — shrinking the laryngeal muscles that produce its famous deep-throated mating call — but deformed, emasculated, and "feminized" its gonads, producing eggs where sperm should be.
In other studies, atrazine left male minnows, snapping turtles, and leopard frogs with ovaries and egg-studded testicles, caused frogs that should be males based on their genetic profile to act — and reproduce — like females, and reduced sperm abundance and motility in frogs, alligators, and rats.
Syngenta, the creator and primary producer of atrazine, maintains that its product is safe. The Switzerland-based agrochemical giant, which saw revenues for its crop protection division increase 14 percent in the first half of 2011 over the previous year to $7.7 billion, disputes the review team's findings. "The [research] group conveniently ignores sound science conducted or reviewed by independent organizations such as the EPA, World Health Organization, the Australian Pesticides, and Veterinary Medicines Authority, and others that consistently support the safety of atrazine," Syngenta spokeswoman Ann Bryan wrote in an email. "Instead, they recycle old claims that have already been disproven or discredited."Beasley called Syngenta's characterization of the review "ridiculous." "People have tried to demonize investigators and demean research while the data keep piling up," he said.
The team's report confirmed the growing body of evidence that atrazine is emasculating and feminizing males. And it's doing this, Beasley noted, to amphibians and other species that are already declining in response to severe stress from habitat loss, infectious diseases, and toxic chemicals in their environment.
Syngenta also maintains there is no evidence that atrazine poses a risk to humans. "Atrazine cannot, does not, and will not cause adverse effects at levels to which people would ever be exposed in the real world," Bryan stated.
Yet as mounting evidence attests to atrazine's ability to cause reproductive mayhem in laboratory and wildlife studies, researchers are finding signs that it may compromise fertility in humans. One study examined by the review team looked at the effect of eight pesticides on semen quality and found that men living in agricultural areas with heavy atrazine use were more likely to have lower fertility, reduced sperm count, and poorer semen quality than men living in urban areas. Although the researchers of the study, published in Environmental Health Perspectives in 2003, acknowledge that it has limitations, including a small sample size, they warned that if further research confirms their results, the ubiquity of the pesticides studied means that "implications for public health and agricultural practice could be considerable."
Just last week, a study published in Environmental Research found irregular menstrual cycles and altered hormone levels in women exposed to atrazine-tainted drinking water at levels below the EPA's maximum allowable level.
For Beasley, the evidence supports a call for global regulation to remove problem chemicals from the market. "We're in the sixth extinction," he said. "When we see a chemical out there that's feminizing [animals], altering their breeding behavior, reducing their fertility, we ought to do something about it. We ought to find a way to grow our food without messing up reproduction of wildlife."