2 Frog Studies Link Parasite to Deformities
Large numbers of mysterious frog-leg deformities that have caused scientists to fear the impact of an undetermined chemical pollutant may in fact be the result of a simple parasite found in nature, two new studies conclude.
The research, published today in the journal Science, could play a significant role in solving one of the great environmental riddles of the 1990s: Why deformed frogs are being found with increasing frequency from Minnesota to Vermont to Oregon and California.
Although no one contends that the new research solves that riddle, the findings do suggest that the intense environmental alarms over frog deformities may have been premature.
The discovery of mangled frogs--some missing limbs, some sprouting too many--has baffled experts. Many of those experts have warned that deformities in frogs could be an early warning of much more widespread malformations to come in other species, perhaps even including humans, wrought by some unknown environmental threat.
But the research appearing today suggests that many multi-limb or missing limb deformities are caused by a tiny parasitic worm that bores into tadpoles, disturbing the cells responsible for leg growth long before they turn into frogs.
“The cells just get moved around to the wrong places,” said Stanley Sessions, chief author of one study. When that occurs, “they try to repair that problem by making new cells, lots of new cells. That results in new legs.”
“This is very good news, because the alternative is just a catastrophe,” said Sessions, an associate professor of biology at Hartwick College in Oneonta, N.Y.
Some scientists remain unconvinced by the new findings. Even the lead author of the second study notes that although the parasites appear to be a major culprit in the multi-leg deformities in the Western United States, that does not necessarily mean that environmental pollution is blameless.
“We should be cautious” about concluding that parasites are causing all the deformities or “that parasites are the sole cause,” said Pieter T.J. Johnson, the 22-year-old Claremont native who began his frog work as his undergraduate honors thesis at Stanford University.
“Much of our research points to causes of deformities in the Western states, and something different could be going on in the Midwest and Canada,” he said.
A top federal scientist added Thursday that riddles still remain, such as why some frog populations are declining precipitously.
“I don’t think there’s any single answer,” said William Y. Brown, science advisor to Interior Secretary Bruce Babbitt and chairman of a federal task force examining amphibian decline and abnormalities.
“It looks like [parasites] are important in the deformations in the ponds in Santa Clara County,” Brown said. “But I wouldn’t take that to mean that it is a single cause of deformations.”
The mysterious frog deformities first received widespread attention in 1995 when Minnesota middle-school students on a field trip discovered that many northern leopard frogs in a farm pond had extra or missing limbs.
More malformed frogs were found in other Midwestern and Northeastern states, the South and on the West Coast. The discoveries, coupled with fast-shrinking populations of many amphibians, alarmed scientists. Frogs’ permeable skin makes them more sensitive to environmental changes than other animals, scientists say, a fact that makes frogs a good early indicator of problems in the environment.
The puzzle has spawned numerous theories. Some experts, including UC Irvine researcher David Gardiner, suspect chemicals called retinoids--hormone-like substances found in nature and in pesticides--are to blame. Others are studying whether damage to the ozone layer in the atmosphere could be increasing the number of mutations in frog embryos.
Johnson, who is now a research scientist at Claremont McKenna College, has been searching for an answer since late 1996, when he was an undergraduate biological sciences major at Stanford. He and his colleagues found more than 1,000 deformed frogs in four ponds in Santa Clara County. They dissected some of those frogs, finding evidence that a parasite was responsible for the deformities, Johnson said.
The Stanford team then moved to the laboratory, exposing tadpoles to the flatworm parasites, called Ribeiroia trematodes. That produced frog deformities similar to those found in the Santa Clara County ponds. Tadpoles not exposed to the parasites developed normally.
Johnson describes visiting the laboratory daily, waiting to see if deformities developed.
“There was one day when there was no doubt,” he said. He had placed a tadpole under a microscope and found two limbs where there should have been one. As the days passed, he and his colleagues found more multiple limbs, missing or swollen limbs.
“It’s fascinating to see something in the wild and to bring it in the laboratory and be able to reproduce it,” he said. “Nature has given you a piece of information.”
Those findings dovetail with the other Science report, from a team led by Sessions, that frogs found in several states suffered the type of leg deformity typically caused by parasites, not by retinoids.
“What these two papers mean is that we now have an explanation for one of the major deformities we’ve been seeing,” Sessions said.
The findings are provoking mixed reviews among scientists working on the frog problem--many of whom have disagreed sharply over what could be causing the deformities.
“I think it’s a major breakthrough. I think it’s definitely one of the best series of papers on the multi-legged phenomenon,” said Oregon State University professor and ecologist Andrew Blaustein. He called Johnson’s work “a beautiful experimental study.”
At UC Irvine, Gardiner called Johnson’s experiment “fascinating, and of considerable importance.”
But he and others suggested that different forces could be deforming frogs in different parts of the country.
“There’s good evidence for retinoid chemicals,” said Gardiner, who plans to continue studying environmental chemicals. “That’s what everyone worries about. I don’t think anyone is losing sleep over trematode parasites.”
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Cycle of Deformity
New research suggests a parasite, rather than an unknown environmental factor, is the cause of some deformities in frogs.
1. Parasite infects snail.
2. Parasite reproduces, creating hundreds of swimming forms of itself called cercariae.
3. Cercariae leave snail in search of tadpole.
4. Cercariae burrow into tadpole, take a dormant form inside a cyst.
5. Cercariae interfere with limb development, causing tadpole to grow into deformed frog.
6. Inside predator (e.g. heron), parasite emergees from cyst, develops into adult. It lays eggs that the predator excretes back into water.
Source: Pieter Johnson
