Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 32, Number 6;   June, 2005

Deformed Frogs Are Less of a Mystery

by Sara Shipley

St. Louis, MO--Scientists have puzzled over the growing plague of abnormal amphibians ever since Minnesota schoolchildren found a pond full of deformed frogs in 1995. What caused the frogs' missing legs, multiple limbs and grotesque formations? Was it pollution? The thinning ozone layer? Some new disease?

New research from a Washington University ecologist and his colleague points to a wily parasite that thrives in ponds and wetlands laden with fertilizer, farm runoff and sewage. High-nutrient ponds are more likely to host deformed frogs, the researchers found. That's because the nutrients feed algae, which, in turn, feeds snails that host a parasitic worm linked to frog deformities, the team concluded.

"While the parasite occurs naturally, high levels of deformity occur more frequently in polluted, unnatural environments, like cow ponds, artificial wetlands and rainwater retention pits," said Pieter Johnson, an ecologist at the University of Wisconsin in Madison. "This parasite is part of the food web," said Jonathan Chase, an ecologist at Washington University who collaborated on the project. "The problem is the (human) change. We've kicked the system out of whack."

Reports of deformed frogs date back to the 1700s, but this was something different, Johnson said. Instead of having a mis-shapen frog here or there, some ponds were swarming with them. Instead of a missing digit, frogs lacked whole legs or had two or three extra ones. At one of Johnson's current research sites, deformities occur in a third of leopard frogs and half of American toads. "From time to time, we find these poor guys with no hind legs at all, just kind of dog paddling in the water," Johnson said.

Initial research that focused on pesticides, heavy metals and other chemicals didn't find a direct link, Johnson said. Johnson was one of the first to discover the parasite factor. He noticed that ponds with high levels of deformed frogs also had plenty of ramshorn snails. The snail turned out to be a host for a flatworm with a complex life cycle. For a creature with no brain, this parasite is quite smart about ensuring its survival. It first infects a snail, where it divides into free-swimming larvae. The larvae burrow into a tadpole's soft skin in the spot where limbs will develop, forcing legs to grow irregularly.

The hobbled frog becomes easy prey for a bird, the parasite's final host. The parasite lays a multitude of eggs in the bird's feces. The eggs hatch in the water, and the parasite finds another snail to continue the pattern. "It's amazing to watch," said Johnson, who found up to 960 parasites in a single deformed frog. In laboratory experiments, he infected tadpoles with parasites and got the same kind of deformities he saw in the field.

Johnson published his findings in the journal Science in 1999 but he couldn't explain why parasites were so plentiful until he heard Chase speak about the same snail at a conference that year in Spokane, Wash. Chase studied the food web, looking at how changing one part of the ecosystem affects the rest. He found that ponds hopped up on phosphorus and nitrogen grew plentiful algae blooms. The more algae there was, the more ramshorn snails flourished. The snails feed on the algae, crowding out competitors.

The two scientists had a meeting of the minds over lunch, and they've been working together ever since. "What we wanted to do was add the parasite angle to Jon's work with the snails," Johnson said. "The collaboration was really natural."

Chase says the pair's findings stop short of being a "smoking gun" that perfectly explains the seeming explosion in frog deformities. It's more like a "warm gun," he said. "My work was, A leads to B. His work was, B leads to C. So we made the link," Chase said.

In a study of 27 Michigan ponds, high levels of phosphorus correlated with large snail populations. In a multistate pond survey, large snail populations correlated with a high level of parasitic infection.