In tropical climates trematodes cause schistosomiasis, a disease that kills millions of people. The World Health Organization estimates that 120 million people worldwide suffer from schistosomiasis.
Kiesecker's team took tadpoles from Centre County, Pennsylvania, and placed them in six local pondsthree affected by pesticide runoff and three pesticide-free. In each pond, the tadpoles were separated into two groups. One group was placed inside a fine mesh that kept out cercariae.
Only the tadpoles that were exposed to cercariae developed deformities. "We learned from the first field experiment that tadpoles have to be exposed to trematode infection for limb deformities to develop," Kiesecker said.
Kiesecker's team then compared the rate of infection between trematode-exposed tadpoles in the different ponds. The team discovered that rates of infection were much higher in the ponds that received pesticide runoff.
This result parallels the finding of a study in 2000 in which frogs from the same pondthat is, those experiencing the same environmental conditionswere found to have similar deformities.
To examine the effects of pesticides on cercaria development, the team conducted lab experiments on four groups of tadpolesthree groups exposed to three common pesticides and a control group. The pesticides were Atrazine, the most commonly used pesticide in North America; Malathion, a common household pesticide that also is used to control insect pests in agricultural fields; and Esfenvalerate, a synthetic pyrethroid pesticide. The tadpoles were all exposed to cercariae.
When they counted the number of cysts that formed in the tadpoles, the researchers found much higher levels in the tadpoles exposed to pesticides. The team also took blood samples before and after the experiments to determine whether the tadpoles' white blood cell counta measure of immune system healthwas affected.
"The tadpoles that we exposed to pesticides had fewer of this particular kind of white blood cell compared to the tadpoles that we did not expose to pesticides, suggesting that pesticides make these animals more susceptible to parasitic infections," Kiesecker said.
Pesticides have been found to have additional harmful effects on frogs. A study published in April in the Proceedings of the National Academy of Sciences found that Atrazine interfered with the sexual development of male frogs in the Midwest, reducing their levels of testosterone to below the levels found in female frogs.
"Atrazine-exposed frogs don't have normal reproductive systems," said Tyrone Hayes, the leader of a team from the University of California at Berkeley. "The males have ovaries in their testes and much smaller vocal organs."
Kiesecker said society can learn a lot from the experiments because "amphibians are particularly sensitive to environmental changes that appear to be associated with the recent emergence of new diseases and resurgence of old diseases that infect humans."
Especially disturbing, he added, is that the concentrations of two of the pesticides that caused the deformities in frogs, Esfenvalerate and Atrazine, were low enough for the water to be considered safe for human consumption under Environmental Protection Agency standards.
"Frogs may be a sentinel species that is warning us about the interplay between human-caused environmental change and disease susceptibility," he said, adding: "Hopefully, people will listen."
In other recent research on this problem, a study published in the July 1 issue of Environmental Science & Technology indicates that frog deformities may also occur as a result of exposure to ultraviolet (UV) radiation. At levels close to 60 percent of normal sunlight, frogs experienced deformities.
A survey of ponds in the Duluth, Minnesota, area showed that frogs in only three of 26 ponds were at risk of UV-induced deformities, because wetlands absorb a significant portion of the radiation. However, Steve Diamond of the Environmental Protection Agency's Duluth office and leader of the UV study said there may be cause for concern if human activities cause UV levels to rise.
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