Pesticides destroy frog immunity, fertility

Frogs exposed to low doses of nine agricultural pesticides developed more slowly, were unable to fight off disease, and males produced eggs instead of sperm. A recent study by University of California professor Tyrone Hayes shows, yet again, that exposure to combinations of pesticides even at extremely low levels can be significantly more harmful than exposures to single pesticides.

Hayes studied frogs exposed to a mixture of pesticides commonly found in agricultural run-off. The levels of each pesticides were 10 to 100 times lower than the acceptable levels set by the Environmental Protection Agency. The results were startling.

Frogs developing in water containing the agricultural pesticides were 10-12% smaller than frogs in a control group. Seventy percent of the exposed frogs were unable to fight off a common pathogen, which did not sicken frogs in the control group.

The exposed frogs developed holes in their thymus, an organ crucial for suppressing disease. Exposed frogs had high levels of corticosterone, associated in humans with stress, decreased growth and retarded development.

Frogs exposed to minute doses of the chemicals, less than one tenth of a part per billion, became sterile. The urine of a farm worker contains, on average 2,400 parts per billion of a number of these pesticides.

Hayes suspects that not all of the nine pesticides in the mixture tested affect frogs, but that, when combined, some enhance or trigger the harmful effects of others. “Metalachor [a common herbicide] doesn’t do anything on its own,” Hayes stated.” But mix it with something else and it becomes bad somehow. You add them all up and you get significant effects.”
“Estimating the ecological risk and the impact of pesticides on amphibians using studies that examine single pesticides at high concentrations only may lead to gross underestimations of the role of pesticides in amphibian declines,” he noted.

The study was published in Environmental Health Perspectives, February 2006.
In Canada and the US, governments conduct safety tests on pesticides and other chemicals in isolation from other chemicals. In the real world, people and animals are exposed to multiple chemicals in combination. Hayes’ study underlines a major flaw in present chemical testing methods.

In April, the US EPA announced a first step in discussing an alternative approach for evaluating health risks of multiple chemicals exposures. This discussion has the potential to lead to a long awaited change in how chemicals are evaluated in North America.