SPECIAL TO THE WASHINGTON POST
Friday, April 30, 1999 ; Page A03
Correction: Correction published 5/1/99 follows:
PHOTOGRAPHS OF A TADPOLE AND A FROG ACCOMPANYING AN ARTICLE ON FROG DEFORMITIES YESTERDAY
SHOULD HAVE BEEN CREDITED TO STEVEN HOLT/STOCKPIX.COM.
A common parasite can cause limb deformities in frogs, making it a prime suspect for
the malformed amphibians that have been discovered around the country in recent years,
researchers reported yesterday.
Field surveys and lab experiments indicate that small parasitic flatworms known as
Ribeiroia trematodes can infect tadpoles, causing their legs to grow abnormally, according
to a report in today's issue of the journal Science.
Frogs with missing, deformed and extra legs have been reported throughout the United
States and in parts of Canada, raising concerns that they may be a harbinger of a serious
environmental problem. The cause has been a mystery, but researchers have been
investigating a variety of possibilities, including pollutants or exposure to ultraviolet
light because of ozone loss. But scientists have always suspected that some natural cause
must account for at least some deformities because abnormal limbs have been observed in
amphibians for hundreds of years.
While studying at Stanford University, Pieter Johnson and several fellow students
identified two trematodes that appeared to be associated with high rates of limb
abnormalities. When they exposed Pacific tree frog tadpoles in the lab to the same
parasites, one of the trematodes formed cysts at the bases of developing legs that caused
the same kinds of deformities.
Johnson's experiment confirms a link between parasites and amphibian abnormalities
first proposed almost 10 years ago by Stan Sessions, a biologist who also studied deformed
Pacific tree frogs in Northern California.
Johnson said it is unclear whether parasites interfere mechanically with developing
limbs or in some way influence the chemical signaling that normally regulates limb
development. Sessions, now at Hartwick College in New York, argues for mechanical
disruption in a companion article in today's issue of Science, but this view is not widely
shared among other scientists.
Sessions also examined extra legs in frogs from several West Coast sites -- most of
them animals he collected in the late 1980s -- and determined that their structures were
not consistent with experimental evidence suggesting that a class of chemicals called
retinoids might cause such defects.
Johnson called the trematodes a "cosmopolitan" parasite commonly found across
North America. "It certainly merits further study in other parts of the
country," he said.
Mike Lannoo, U.S. coordinator of the Declining Amphibian Populations Task Force, agreed
that parasites need careful evaluation wherever deformities are being reported but
emphasized that little is known about the distribution of trematode parasites or the
vulnerability of amphibians to infection. He said his own comparisons of frogs from
Minnesota with frogs from Johnson's sites using high-resolution radiographs showed
different bone malformations that suggest different causes for the deformities in the two
Other researchers echoed praise for Johnson's work while saying the cause of
deformities in different species at other locations is still an open question.
David Hoppe, a herpetologist investigating malformed frogs in Minnesota, reevaluated
frogs from one of his sites where the deformity rate is so high that the whole population
appears to be in jeopardy. He failed to find any correlation between trematode cysts and
deformities but said nothing could be ruled out at this point.
"I seriously doubt that any one cause will explain all that is going on,"
said Hoppe. "I haven't seen anything yet that explains even one Minnesota site."
Jim Burkhart, a biochemist with the National Institute of Environmental Health
Sciences, which is working with the Minnesota researchers, said his group has narrowed its
investigation to a handful of chemical contaminants that have been shown to cause limb
defects in the lab. Burkhart said those results had been achieved using doses comparable
to what they found in the environment.
Cutline: A tadpole, above, that has been experimentally exposed to the parasitic
flatworm Ribeiroia grows two extra limbs.
Leg deformities such as the ones exibited in this Pacific tree frog have led to new
concerns about pollution.