Increasing deformities in frogs attributed to pesticide exposure, which makes them susceptible to parasite infection

Published: Thursday 15 August 2002

-- parasite infection combined with a weakened immune system caused by pesticide exposure are causing deformities in frogs in the us state of Pennsylvania, indicates a recent research. The research, jointly administered by Virginia-based National Science Foundation (nsf) and Maryland-based National Institutes of Health (nih), includes the first experimental studies of amphibian deformities conducted in ponds where the animals live.

Since the 1990s, when frogs with deformed legs were first discovered in us wetlands, scientists had been trying to determine the reason for the problem. Various hypotheses came up. The researchers designed their experiments to test these hypotheses.

The first was that limb deformities occur in frogs infected with the trematode parasite. Trematode parasites inhabit a series of host species, including tadpoles. The researchers placed their tadpoles in six ponds within two kinds of enclosures located side-by-side -- one with a fine screen that prevented the trematode larvae from entering the enclosure, and the other with a larger-mesh screen that allowed the trematode larvae to infect the tadpoles. They found that the tadpoles which developed limb deformities were the ones exposed to the trematode larvae. "We found that tadpoles have to be ex-posed to trematode infection for deformities to develop," said Joseph Kiesecker, the lead researcher.

The second hypothesis tested states that limb deformities in trematode-infected tadpoles are affected by pesticides along with other pollutants. When the researchers analysed the rates of limb deformities among their subjects, they found much higher rates of deformities in trematode-infected tadpoles caught from ponds that received agricultural runoff and contained pesticide residues.

The team then moved into the lab to test the third hypothesis, which is that pesticide exposure alone influences the increased rates of deformities developed by the trematode-infected tadpoles. Their experiments involved three groups of trematode-infected tadpoles that were exposed to three different pesticides, plus one group that was not exposed to pesticides. The pesticides used for the experiments were atrazine, malathion and esfenvalerate. The researchers studied the blood samples of the tadpoles to determine the prevalence of a type of white blood cells (wbcs) that fights parasites like trematode larvae. The team compared this measure of immune system strength with the number of trematode cysts that had formed in each animal. "The tadpoles that were exposed to pesticides had fewer of this kind of wbcs compared to the tadpoles that were not exposed to pesticides, suggesting that pesticides make these animals more susceptible to parasitic infections," said Kiesecker.

In summary, the researchers proved that only tadpoles infected with trematodes developed limb deformities and that these deformities occurred with more frequency in the groups of tadpoles that also were exposed to pesticides.

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