Farm Chemicals & Deformed Frogs

Pesticides and flawed frogs
Researchers reveal first signs linking land runoff to deformities

Carl T. Hall, San Francisco Chronicle Science Writer
Tuesday, July 9, 2002


Raising new questions about the environmental risks of some widely used farm
chemicals, scientists are reporting today the first evidence linking
agricultural runoff to grotesque hind-limb deformities in frogs.

Researchers said frogs appear to be made more vulnerable to a common
parasite when exposed to the pesticides atrazine and malathion. The
parasite, a burrowing trematode worm, tends to infect the hindquarters of
developing tadpoles.

Atrazine is part of a family of chemicals that rank among the world's most
widely used weed killers. Malathion is commonly applied to control
mosquitoes and other insects, and pharmaceutical grades are approved for
killing head lice. Both products are controversial but considered safe for
commercial use in the United States.

Now, effects of these and other chemicals on the environment are coming
under new scrutiny. Research is driven partly by keen public and scientific
interest in the declining health of amphibian populations, often portrayed
as a sentinel for environmental decline and a possible early warning of
health problems affecting humans.

At last count, wild frogs with missing or extra hind limbs have been
observed in at least 43 states and five Canadian provinces. Earlier studies
clearly implicated the trematode parasite but left open the question of what
might be causing the apparent increase in the problem.

The latest study, by ecologist Joseph Kiesecker at Pennsylvania State
University and edited by UC Berkeley amphibian specialist David Wake, tries
to fit in the key remaining puzzle piece. The study appears in the early
edition of this week's Proceedings of the National Academy of Sciences.

Kiesecker said his observations of the common wood frog Rana sylvatica in
the wild, followed by controlled studies in his laboratory, produced
"compelling" evidence that pesticides can weaken the immune system of
exposed amphibians -- even at very low concentrations -- making the frogs
more vulnerable to parasites.

The field studies showed "considerably higher rates of limb deformities
where there was pesticide exposure," Kiesecker said in an interview. "Then
the lab experiments helped support the mechanism for what we saw in the

He also looked at another pesticide, a synthetic chemical called
esfenvalerate, but did not find the same links to growth anomalies as seen
with malathion and atrazine.

For the latter two chemicals, significant effects were seen even at
concentrations considered safe for drinking water by the Environmental
Protection Agency.

Even these very low levels of exposure could produce "dramatic effects on
the immune response" of the animals. And that, in turn, led to significantly
more growth defects.

Kiesecker stopped short of endorsing any effort to further restrict use of
atrazine and malathion. But he said his results underscored the importance
of studying toxic chemical effects in a context approaching the complexity
found in natural ecosystems.

In this case, he explained, the two farm chemicals "disturbed host-pathogen
interactions" with sometimes devastating effects. But all that would be
missed in traditional studies examining only the chemicals and the frogs in

Some other scientists, backed by the farm-chemical industry, challenged
Kiesecker's results. Although they said the new study was intriguing, they
suggested the details couldn't be trusted until corroborated independently.

E-mail Carl T. Hall at

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