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Tuesday, 8 June, 1999, 12:05 GMT 13:05 UK
Parasites blamed for frog deformities
Scientists think they have finally identified one of the main reasons why so many deformed frogs have been turning up in the US over recent years.

Two new pieces of research strongly suggest that parasites are to blame. The issue has been a major concern since a group of schoolchildren in Minnesota discovered a strikingly high rate of limb deformities in local frogs in the mid-80s.

Many suspected pesticide pollution was the cause. In particular, environmental campaigners pointed to a certain class of powerful chemicals known as retinoids. These are capable of interfering with the growth of limbs in amphibians.

But a study by Stanley Sessions and colleagues at Hartwick College in Oneonta, New York, shows the type of deformities being seen in the wild are more characteristic of the damage done by small parasitic flatworms called Riberoria trematodes.

Rearranged bud cells

These creatures burrow into the hindquarters of tadpoles where they physically rearrange the limb bud cells and thereby interfere with limb development.

"It's about as close to using an egg beater on the limb bud cells as you can get," says Sessions. His team studied five species of frogs from 12 different locations in California, Oregon, Arizona, and New York.

In a separate study by Pieter Johnson and his colleagues at Stanford University in California, tadpoles were exposed to trematodes under carefully controlled laboratory conditions.

The tadpoles grew into frogs with deformed hind legs that closely matched those found in the wild. In contrast, tadpoles raised without any exposure to parasites developed free of deformities.

"It worked much better than I had expected," says Johnson. "We found a high frequency of deformities even [under conditions with] low parasite density."

Other deformities

Both teams stress this is not the end of the story. Other kinds of frog deformities are being reported, including missing or malformed limbs and eyes, which may be caused by something other than parasites.

"We've chipped off the most mysterious-looking piece of the deformed limb puzzle," says Sessions. "That's how we have to do it. First we have to figure out what's causing these deformities. Then we can roll up our sleeves and look at what's happening over time.

"We need to ask whether the rate of deformities has been increasing, and if so, why?"

And both men suggest chemicals may still have an influence, even if only indirectly. It is possible, they say, that chemical leaching from fertiliser-runoff is spiking water courses with excess nutrients.

This would boost the numbers of aquatic snails which play host to trematodes at an earlier stage in parasites' development.

The research is published in the journal Science.

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