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X-Rays Shed Light on Frog Deformities
by Mary Losure
March 29, 2000
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A study of X-rays taken of deformed frogs lends new support to the idea that there are many different causes for the frog deformities found in Minnesota and many other states. It's the most extensive and detailed study to date of bone abnormalities found in deformed frogs.

To see photographs of the USGS research, visit the USGS amphibian research site.
Photo: U.S. Geological Survey
SCIENTISTS STILL DON'T KNOW what's causing the frog deformities that first made headlines in 1995, when Minnesota schoolchildren discovered an outbreak in a farm pond in southern Minnesota.

One of the major and most publicized theories is parasites. It's been shown in California that cysts formed by immature flatworms - known as metacercariae - can lodge in the limbs of developing frogs, and disrupt their development. But many researchers believe the parasite theory still doesn't explain the wide range of frog deformities. This latest study supports that belief.

Researchers at the National Wildlife Health Center in Madison Wisconsin took X-rays of more than 150 deformed frogs, and made detailed descriptions of their bone abnormalities. Wildlife pathologist Carol Meteyer headed the study. She classified hundreds of different deformities in the frog's toes alone. She was struck by how precise and specific many of problems were.

"There were two frogs that were missing one bone in each toe of both feet," Meteyer said. "Now that type of precision doesn't lend itself to a large metacercarial cyst or an immature worm messing up the limb field. It's a very precise type of malformation."

Frogs for the study were collected from three states where outbreaks of deformities have been severe - Minnesota, Vermont, and Maine. The researchers found certain types of deformities seemed to be specific to certain sites. That evidence also suggests there is more than one cause responsible for the deformities.

"The sites that we looked at in Vermont, had limbs that were truncated or limbs that stopped short of complete development," Meteyer said. "None of the frogs from Vermont had any multiple toes, had any multiple limbs, or any of the multiple malformations that were so prominent at some of the other sites. When we looked at Maine, they almost solely were multiple malformations."

Researchers also found very high rates of multiple deformities at two sites in Minnesota. The causes scientists have advanced to explain frog deformities also include water born contamination and higher than normal levels of ultra-violet light coming through the earth's thinning ozone layer.

Meteyer says this latest study will provide a detailed data useful for researchers investigating the various theories. "I think it leaves a lot to still be investigated, and I think one of the errors that we try to make to simplify our life and how we interpret our environment is try to say, "OK, we've shown this to be a cause, so let's try to show it as a cause across the board,'" she said.

As scientists struggle to find a cause, the problem of frog deformities has gotten more acute. In the years since schoolchildren discovered malformed frogs in Minnesota, they've been documented in 44 states, in a wide range of frog and toad species. Estimates of the deformity rates run as high as 60 percent in some local populations.