Over the past decade, scientists have been tracking an alarming increase in frog deformities. One identified cause is a parasitic infection. Parasites cause a wide range of health problems for humans and wildlife. But what caused the surge in parasites? A new study suggeests nutrient pollution.
Nutrient pollution of lakes and ponds comes from a variety of sources, including agricultural fertilizers and sewage. Known as eutrophication, it can cause substantial changes to freshwater systems.
Until now, little research had been done into how eutrophication might affect waterborne parasites, which cause health problems ranging from skin rashes to river blindness.
Ecologist Pieter Johnson of the University of Colorado tested the effect of nutrients on one particular parasite, a flatworm that infects frogs. Johnson says and his colleagues were interested in how eutrophication influences patterns of infectious disease. They also wanted to understand the specific factors that might be controlling the abundance of parasites that cause deformities in frogs.
Frog deformities first gained widespread attention in 1995, when a group of school children in the Midwestern United States came across a pond where more than half of the frogs were abnormal. "Since then reports have really poured in from all over the place," says Johnson. "There's been a lot of debate and controversy over what's causing this."
Research has shown that ultraviolet radiation, exposure to pesticides and other chemicals, and parasitic infections all can cause deformities in amphibians. In the case of the parasite, the target is the base of the tadpole's tail, where the hind legs develop. Infected tadpoles grow into frogs with too many legs, abnormal legs, or sometimes no legs at all.
According to Johnson, "the animals with these deformities don't jump very well, they don't swim very well, and they're essentially fairly debilitated." The deformed frogs make an easy target for birds and other predators, and generally do not survive. "As a result," Johnson explains, "there's concern that deformities are contributing to large-scale patterns of decline in amphibians worldwide."
The frogs are just one of three hosts that this deformity-causing parasite infects as part of its complex life cycle. The parasites also infect freshwater snails, and birds. The parasites multiply inside the snails, then emerge to infect the frogs, some of which are eaten by birds.
In an effort to better understand how eutrophication might affect the parasite's life cycle, Johnson and his colleagues added nutrients to a set of small artificial ponds. Some ponds got more nutrients, and others less.
"Our goal," saysJohnson, "was to watch how adding nutrients cascaded through the lifecycle of this parasite, and through the food web inside these small ponds, to ultimately affect the levels of parasitism in the frogs."
The nutrients acted as fertilizer for the algae, which meant more food for the snails. More snails, in turn, meant more hosts for the parasite. In fact, in high-nutrient ponds, the rate of parasitic infections in tadpoles was about 2 to 5 times higher than in low-nutrient treatments.
Johnson says the next step is to try to understand how the parasites are affecting frog populations: "We know that deformed animals do not survive to maturity, and we know that parasites can often kill large numbers of frogs." But, he says, what the long-term consequences of that are for amphibian population in the future has yet to be determined.
His results could have implications for other parasitic infections, as well. Johnson believes that eutrophication could be affecting other parasite-transmitted diseases, in both wildlife and humans.
More information about this study is available in the September 24, online edition of the Proceedings of the National Academy of Sciences.