Deformed Frogs Caused by Parasites and Fertilizer

In the effort to understand puzzling and disgusting deformities that make frogs grow extra limbs, there have been competing scientific camps: One pinning the deformities on parasitic trematode worms that interrupt the normal development of limbs, and one pinning the deformities on some man-made chemical infiltrating streams and ponds from farms.

Now, a new study has the potential to bring together those two hypotheses. It shows fertilizer from farms leads to explosive growth in the populations of trematode worms, and the rate of frog infestation and deformity rises in kind. Farm runoff has already been blamed on the growing dead zones in coastal waters, such as the record-sized area of depleted oxygen choking the Gulf of Mexico at the mouth of the Mississippi.

There, excess nitrogen from fertilizer is concentrated from America's farm heartland, and dumped into the open waters of the Gulf, where it feeds algae growth that sucks oxygen from the water. In a similar ripple of events, scientists at the University of Colorado say they were able to cause excess deformities by adding fertilizer to ponds, which boosted algae, which boosted the population of snails that feed on algae.

The primary host of the trematode worm that causes frog deformities is the snail (the tadpole is only a stopping-off point for the worm as it develops into an adult), so if you have more snails, you have more worms -- and consequently, more frog deformities. According to the scientists, quoted in the Rocky Mountain News, the findings could have implications for a wide range of diseases -- from cholera and malaria to West Nile virus and diseases affecting coral reefs.

It's also an example of conservation medicine -- a growing field that links environmental conditions to the emergence of disease, including wildlife diseases that can "jump the species barrier" and affect humans, like West Nile virus and avian influenza.

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