WEDNESDAY, April 17, 2002 (HealthDayNews) -- In findings that scientists fear may have implications for humans, researchers say exposing frog larvae to a common weed killer leads to reproductive abnormalities.
These abnormalities may impair sexual behavior, and be a factor in the current decline in the worldwide frog population.
The study, the first to look at levels of the herbicide atrazine -- which is thought to be safe in drinking water or in limited exposure -- appears in the current issue of the Proceedings of the National Academy of Sciences.
The frogs in the study were exposed to levels of atrazine between 0.1 and 220 parts per billion, levels considered safe for humans.
"The findings were a surprise because there was not a lot of identification in the literature that atrazine at these doses would be active," says Tyrone Hayes, co-author of the study and associate professor of integrative biology at the University of California, Berkeley. "It's unbelievably low. There are very few places in the country or the world where you can find less than [0].1 parts per billion. The published literature shows it as high as 40 parts per billion in rainwater in Iowa, and that's not even getting into what's coming off of agricultural lands."
Atrazine is the most commonly used herbicide in North America, and possibly the world. According to the U.S. Department of Agriculture, more than 30,000 tons are used every year in the United States. The chemical supposedly has few, if any, effects on humans.
However, when researchers at Berkeley exposed African clawed frog larvae to these extraordinarily low doses of atrazine, the resulting juvenile frogs had multiple sex organs or had both ovaries and testes. Male frogs who had been exposed also had much smaller voice boxes and dramatically lower testosterone levels -- about on a par with those found in female frogs.
Hayes and his team hypothesize the abnormalities are the result of changes in a chemical called aromatase, which converts testosterone to estrogen.
Unfortunately, the Hayes study adds to a growing body of literature that suggests aromatase can cause reproductive abnormalities in various species. Similar results were found in alligators and in human adrenal cell experiments.
"Here you have a situation where you started with alligators, have now used human cells in culture, and are getting very similar results. What that says is that there's a red flag. We should start looking at this in far more detail than we have in the past," says Louis Guillette, a professor of zoology at the University of Florida. His lab conducted the alligator studies. "The data that's being presented here from frogs is one more story that's actually becoming a very large book that tells us that there are other ways that chemical pollutants influence living organisms."
The endocrine systems of frogs and humans also have tremendous similarities, Guillette adds.
This study and others like it may also point up the need for new ways to study the effects of chemicals on humans. The changes observed in frogs are, on one level, very subtle: They can't be seen from the outside, and sometimes they can't even be seen on the inside. The same could be true in humans.
Guillette argues that frog and other animal studies need to be taken seriously now.
"We will never have causal data in humans. That's immoral," he says. "But when you have frogs and alligators and fish and human cells that we are ethically able to study and we see a similar phenomenon taking place, then it suggests we should be really concerned as humans."
What To Do: To see existing information and guidelines on atrazine, visit the U.S. Environmental Protection Agency. You can also try the University of Southampton to learn more about agricultural chemicals.
