A Pesticide-Parasite Role in Frogs' Deformities?
By William Souder
Since the mid-1990s, scientists have been trying to figure out what has
been causing frogs around the United States and Canada to become deformed.
Little progress has been made in sorting out which deformities result from
infections by a tiny, aquatic parasite and which, if any, are being caused
by wetland contamination from chemicals that might threaten other life
forms, including people.
Now, research suggests that for at least some outbreaks, both factors
could be involved -- and acting in concert.
The findings come from Joseph Kiesecker of Pennsylvania State University,
the same researcher who last year linked amphibian declines in the western
United States to global warming. Kiesecker tested whether agricultural
pesticides -- a prime suspect in malformation -- make developing frogs more
susceptible to infection by a parasite known as a trematode. Previous
experiments have shown that a parasite, which uses frogs as "intermediate
hosts" in a complex life cycle, causes leg deformities in developing
tadpoles by tunneling under their skin and becoming encysted, usually at the
base of their growing hind legs.
Nobody has figured out how parasitic cysts interfere with limb
development, but Kiesecker now thinks pesticides significantly amplify the
process by weakening the frogs' immune systems and making them less able to
resist parasitic infections. That, say scientists who have looked at the
findings, might be a much greater concern than any yet raised in the
investigation.
"If it's true that commonly used pesticides compromise the immune system
of a vertebrate organism, which is what these findings suggest, then we're
looking at a much bigger problem than deformed frogs," said David Gardiner
of the University of California at Irvine.
Kiesecker conducted parallel experiments. In the field, he identified six
ponds in central Pennsylvania where the parasite was present. Three of the
ponds were also contaminated by runoff containing agricultural pesticides.
Tadpoles were then reared at each pond inside screened enclosures of
different mesh sizes that would either allow or prevent the parasite from
entering.
Back in his lab, Kiesecker reared tadpoles in water treated with one of
three common pesticides at various concentrations and then exposed them to
the parasite.
In the field, deformities occurred only where the parasite could get at
the frogs -- confirming the parasite's essential role in the malformations,
Kiesecker reported last week in the Proceedings of the National Academy of
Sciences. But the deformity rate was four to five times higher where
pesticides were also part of the picture, suggesting a relationship between
pesticides and parasitic infection.
In the lab, Kiesecker found much higher rates of parasitic infection in
tadpoles exposed to pesticides, along with a matching reduction in white
blood cell production -- an indication of a weakened immune response. Frogs
exposed to pesticides were also smaller and developed more slowly. Kiesecker
said inhibited growth rates might also contribute to deformities seen in the
field by creating a longer "window" when parasitic infection could affect
limb development.
"The original purpose of these experiments was to study how disease in
aquatic systems impacts growth and development," Kiesecker said. "To be
honest, we were quite surprised to see limb deformities and this strong
pattern of interaction between parasites and pesticides."
All of the pesticide concentrations used in the experiment were below
EPA-recommended levels for safe drinking water. One of the pesticides tested
was atrazine, the most heavily used agricultural herbicide in the United
States, which has also been linked to reproductive deformities in other frog
species.
Other researchers praised Kiesecker's study but cautioned that it tells
only part of the story. Dan Sutherland, a parasitologist at the University
of Wisconsin at La Crosse who has studied frog deformities for several
years, expressed reservations about Kiesecker's parasite identification.
"I cut open a frog the other day that had 200 parasites encysted in its
pelvic region," Sutherland said. "There were five species. Only one of the
cysts turned out to be" the one Kiesecker studied. But Sutherland added that
Kiesecker's multidisciplinary approach is exactly what is long overdue in
the deformities investigation.
Kiesecker's findings dovetail with data from Canada, where a connection
between agriculture and frog deformities has been apparent for years. But
they also seem to contradict a large-scale survey of parasitic infection and
frog deformities recently published by Pieter Johnson of the University of
Wisconsin. Johnson, as a Stanford undergraduate, first identified a parasite
as a cause of frog deformities. His recent fieldwork, encompassing nine
amphibian species at more than 50 locations in the western United States,
found no connection between pesticides and the incidence of deformities.
Johnson and Kiesecker agreed that different factors could be involved in
different ecosystems, and also that much more work needed to be done to
determine how pesticides change other phases of the parasites' life cycle.
Mike Lannoo, U.S. coordinator of the Declining Amphibian Populations Task
Force, said Kiesecker's work is "interesting and thought-provoking."
"But it's not the last word," Lannoo said. In field surveys he conducted
last summer in Minnesota, Lannoo found parasites in some of the deformity
outbreaks but not others.
David Gardiner, who is studying a class of chemicals that might cause
some of the deformities, said Kiesecker's study was intriguing, but he
worried that because a parasite was the necessary factor in the experiment,
people would assume it is a universal cause of frog malformations.
"What will happen is that this study will become part of the literature
and will then be cited in future studies," Gardiner said.
But all of the researchers said they shared Kiesecker's larger concern
about the impact of pesticides on the environment. This study is only one of
a series of experiments Kiesecker is conducting that is designed to discover
how human alterations of ecosystems contribute to emerging and worsening
disease epidemics. Although the parasite Kiesecker studied does not infect
people, other aquatic trematode parasites do -- notably Schistosoma,
which causes a variety of debilitating and often deadly diseases in people
living in the tropics. Schistosomiasis afflicts as many as 200 million
people worldwide and kills at least 20,000 of them a year.
"Amphibians have become an important model system," Kiesecker said. "We
have to consider that factors that influence infection rates in frogs may
also play a role in human diseases."
© 2002 The Washington Post Company
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