Pesticides, Parasites Spell Double
Trouble for Frogs
By Cat Lazaroff
Environmental News Service
STATE COLLEGE, Pennsylvania, July 9, 2002 (ENS)
- Scientists at Penn State may have resolved the conflicting
evidence regarding why so many frogs and other amphibians
are developing deformities. Their research shows that
a combination of infection by parasites and exposure to
pesticides, rather than either problem alone, leads to
the most deformities.
A decade of scientific research has resulted in two prominent
hypotheses about the causes of frog deformities: one is
that they are caused by human induced contamination of the
frogs' environment with chemicals like pesticides, and the
other is that they are caused by a common naturally occurring
parasite, the trematode worm.
Scientists had found evidence to support each hypothesis,
but the research results were not conclusive enough to
resolve the controversy.
Now, a study by Penn State researchers combining both
field and laboratory studies has linked deformities in
Pennsylvania wood frogs to the combination of their infection
by parasites and a weakening of their immune system caused
by exposure to pesticides.
"The field experiments showed that only the tadpoles
that were infected with trematodes developed limb deformities
and that these deformities occurred with more frequency
in the groups of tadpoles that also were exposed to pesticides,"
said Joseph Kiesecker, an assistant professor of biology
at Penn State and the leader of the research team. "The
lab experiments show that pesticides can weaken the immune
response of the tadpoles, which can result in more infections,
making these tadpoles more likely to develop limb deformities."
"More rigorously designed experiments were required
to determine which factor is more important and how these
two factors work together," explained Kiesecker.
The research, which appears in today's issue of the "Proceedings
of the National Academy of Sciences," includes the first
experimental studies of amphibian deformities conducted
in ponds where the animals live.
The discoveries, which show the effect of environmental
stress on disease outbreaks, may help to explain how disease
affects the distribution, growth, development, and survival
of frogs, which have been disappearing during recent years
at alarming rates worldwide.
"It is not uncommon now for 20 to 30 percent of the
frogs at many locations to have limb deformities," said
Since the early 1990s, when school children and amateur
naturalists first began finding frogs with deformed legs
in U. S. wetlands, scientists have been trying to determine
the reason for the problem's escalating occurrence. These
deformities in frogs bear a chilling resemblance to the
deformities in human caused decades ago by the drug Thalidomide.
"Both the general public and scientists suspect that
whatever is causing these problems in frogs may also cause
harm to humans," Kiesecker said.
Kiesecker's breakthrough was in designing a study that
combined field experiments at the ponds where the frogs
live with experiments in the laboratory.
"The kind of field experiments that we did in this study
have never been done before," Kiesecker said.
The Kiesecker team collected tadpoles from ponds in
Centre County, Pennsylvania, then used some of those tadpoles
in a series of laboratory experiments and some in a series
of field experiments conducted in six ponds within the
same region. Half of the ponds receive runoff from agricultural
fields and contain detectable levels of pesticides, and
half are free of pesticides and agricultural runoff.
The Kiesecker team designed the experiments to test
four key hypotheses regarding the relationship between
pesticides, trematode parasites, and limb deformities
The first hypothesis was that limb deformities occur
in frogs that are infected with the trematode parasite.
Trematode parasites inhabit a series of host species during
their life cycle, including pond snails. When they leave
the snail, in the form of trematode larvae called cercariae,
they swim around in the pond in search of a tadpole, which
is the next host they need to invade in order to survive.
The researchers placed groups of their tadpoles in the
six ponds within two kinds of enclosures located side
by side - one with a fine screen that prevented the trematode
larvae from entering the enclosure, and the other with
a larger mesh screen that allowed the trematode larvae
to infect the tadpoles.
"These same trematode larvae cause 'swimmer's itch,' which
is a common problem among people who swim in lakes in this
part of the country," Kiesecker said, explaining that the
swimmer's immune system eventually kicks the larvae out,
leaving just an annoying rash. "A more serious problem for
people occurs in tropical climates, where trematodes cause
an infection known as Schistosomiasis that kills millions
of people every year."
The only tadpoles that developed limb deformities in
the first experiment were from cages that were exposed
to the trematode larvae, while tadpoles in cages that
protected them from the larvae did not get any limb deformities.
"We learned from the first field experiment that tadpoles
have to be exposed to trematode infection for limb deformities
to develop," Kiesecker explained.
The second hypothesis the team tested is that limb deformities
in trematode infected tadpoles are affected by pesticides.
When they analyzed the rates of limb deformities among
their research animals, they found much higher rates of
deformities in trematode infected tadpoles at the three
ponds that receive agricultural runoff and contain pesticides
than in the ponds that do not.
The team then moved into the lab to test their third
hypothesis, which is that pesticide exposure - not some
other factor - influenced the increased rates of deformities
developed by the trematode infected tadpoles in the field
study. These laboratory experiments involved three groups
of tadpoles that the researchers exposed to three different
pesticides, plus one group that they did not expose to
The pesticides were atrazine - the most commonly used
pesticide in North America; malathion - a common household
pesticide that also is used to control insect pests in
agricultural fields; and esfenvalerate - a synthetic pyrethroid
"Synthetic pyrethroids have become increasingly popular
during the last couple of years because they are not very
toxic to birds and mammals," Kiesecker noted. "However,
they are highly toxic to many other kinds of organisms."
The researchers also took a blood sample from each tadpole,
and then exposed the four groups of tadpoles to trematode
larvae under conditions that assured the tadpoles would
be invaded by the parasites. Trematode infections can
cause limb deformities if the larvae are able to evade
the defenses of the tadpole's immune system long enough
to transform themselves into hard cysts.
If the location of the cyst is on cells that are supposed
to develop into legs, the cyst will cause growth disruptions
that lead to missing limbs, split limbs, or multiple limbs.
The researchers counted the number of cysts that developed
in each infected tadpole and found a higher number of
cysts in the animals that were exposed to pesticides.
"From this experiment, we learned that a trematode infected
tadpole that is exposed to pesticides is more likely to
develop limb deforming cysts than is an infected tadpole
that is not exposed to pesticides," Kiesecker said.
Kiesecker and his team designed their experiments to
learn how pesticide exposure affects the immune response
of the animals and their ability to fight off trematode
parasites. They studied the blood of all the trematode
infected tadpoles - both those that were and were not
exposed to pesticides during the laboratory experiments
- to determine the prevalence of a type of white blood
cell that fights parasites like trematode larvae.
Kiesecker's team then compared this measure of immune
system strength with the number of trematode cysts that
had formed in each animal.
"The tadpoles that we exposed to pesticides had fewer of
this particular kind of white blood cell compared to the
tadpoles that we did not expose to pesticides, suggesting
that pesticides make these animals more susceptible to parasitic
infections," Kiesecker said. "The kicker is that the concentrations
that caused deformities were incredibly low for esfenvalerate
and atrazine - low enough for humans to drink, based on
Environmental Protection Agency standards."
Kiesecker said the experiments offer an early warning
that human health problems may be just over the horizon.
"We can learn a lot from experiments with amphibians
because they are particularly sensitive to environmental
changes that appear to be associated with the recent emergence
of new diseases and resurgence of old diseases that infect
humans," Kiesecker said. "Frogs may be a sentinel species
that is warning us about the interplay between human caused
environmental change and disease susceptibility. Hopefully,
people will listen."