The research, supported by a grant from the Ecology of Infectious Diseases Program, and jointly administered by the National Science Foundation (NSF) and the National Institutes of Health (NIH), includes the first experimental studies of amphibian deformities conducted in ponds where the animals live. The discoveries, which show the effect of environmental stress on disease outbreaks, may help to explain how disease affects the distribution, growth, development, and survival of frogs.
"Frog, toad, and salamander populations have been declining on a worldwide basis for the past several decades," says Sam Scheiner, program director in NSF's division of environmental biology. "These declines may be a warning of an overall decline in the health of the environment. By understanding the link between pesticides and parasite infection, we can better manage the environment and improve human health."
It is not uncommon now for 20 to 30 percent of the frogs at many locations to have limb deformities, says Joseph Kiesecker, a biologist at Penn State and the leader of the research team. Since the early 1990s, when school children and amateur naturalists first began finding frogs with deformed legs in U. S. wetlands, scientists have been trying to determine the reason for the problem's escalation. These deformities in frogs resemble the deformities in human caused decades ago by the drug thalidomide. "Both the general public and scientists suspect that whatever is causing these problems in frogs may also cause harm to humans," Kiesecker says.
The Kiesecker team designed experiments to test hypotheses regarding the relationship between pesticides, trematode parasites, and limb deformities in frogs. The first hypothesis was that limb deformities occur in frogs infected with the trematode parasite. Trematode parasites inhabit a series of host species, including pond snails, during their life cycle. When they leave the snail, in the form of trematode larvae called cercariae, they swim around in the pond in search of a tadpole, which is the next host they need to invade in order to survive. The researchers placed groups of their tadpoles in the 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.
The only tadpoles that developed limb deformities in the first experiment were those exposed to the trematode larvae, while protected tadpoles were not deformed. "We learned from the first field experiment that tadpoles have to be exposed to trematode infection for limb deformities to develop," Kiesecker explains.
The second hypothesis the team tested is that limb deformities in trematode-infected tadpoles are affected by pesticides. When they analyzed the rates of limb deformities among their research animals, they found much higher rates of deformities in trematode-infected tadpoles at the three ponds that receive agricultural runoff and contain pesticides than in the ponds that do not.
The team then moved into the lab to test their third hypothesis, which is that pesticide exposure--not some other factor--influenced the increased rates of deformities developed by the trematode-infected tadpoles in the field study. These laboratory experiments involved three groups of tadpoles that the researchers exposed to three different pesticides, plus one group that they did not expose to pesticides. 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. "Synthetic pyrethroids have become increasingly popular during the last couple of years because they are not very toxic to birds and mammals; however, they are highly toxic to many other kinds of organisms," Kiesecker says.
In summary, Kiesecker says, "The field experiments showed that only the tadpoles that were 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. The kicker is that the concentrations that caused deformities were incredibly low for Esfenvalerate and Atrazine-low enough for humans to drink, based on Environmental Protection Agency standards."
PHOTOS: High-resolution images for publication are available to reporters from a link at http://www.science.psu.edu/alert/Kiesecker7-2002.htm
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