JM 2002. Synergism between trematode infection and pesticide
exposure: A link to amphibian limb deformities in nature?
of the National Academy of Sciences 99: 9900–9904.
presents data from a series of field and laboratory experiments
indicating that frog deformities caused by parasites are
more common in habitats contaminated by pesticides because contaminant-exposed
frogs are less able to resist parasite infection. This
study may neatly knit together what had been viewed as two mutually-exclusive
and competing explanations for why frog deformities occur and why
their incidence has increased so dramatically, contamination vs.
did he do? In his field experiments , Keisecker put wood
frog (Rana sylvatica) tadpoles in ponds that had populations
of snails infected with the trematode parasites known to cause limb
deformities in frogs. These snails are a host for the parasites
during a different stage in their life-cycle.
organized the tadpoles into two separate treatments in these ponds.
One set were placed in containers with sides made of a mesh large
enough to allow the larval stage of the parasite to pass into the
container and infect the tadpoles, while the other set were in containers
whose mesh was so small it kept the larval stages out.
ponds themselves differed with respect to pesticide levels. Some
were in close proximity to agricutural fields whereas others were
distant. Keisecker engaged a chemical laboratory to measure the
levels of a range of organochlorine and organophosphate pesticides.
Three ponds were clear of measureable pesticides whereas the other
three had a series of compounds.
combination of treatments—high or low pesticides, accessible
or inaccessible to parasites—allowed Keisecker to examine
separately the effects of pesticides and parasites, and look at
his laboratory experiments, Keisecker tested the impact of three
pesticides on parasite infection rates, while also measuring the
levels of a type of white blood cell (called eosinophils) thought
to be important for resisting parasite infections. The pesticides
tested were atrazine, malathion and a synthetic pyrethroid, esfenvalerate.
Keisecker compared the effect of the pesticides at three different
levels: control (no pesticide), ; low (the EPA maximum permissible
standard for drinking water), and high (ten times the lower level).
The lower level was chosen to ensure that the levels used were biologically
plausible, i.e., that frogs in the real world would encounter them.
did he find? The pond experiments revealed that the parasites
were necessary for limb deformities. None of the tadpoles developed
deformities in the cages made with mesh too small for their passage.
Deformities occurred, however, in the cages with larger mesh sizes,
in all ponds.
were strikingly less common, however, in the ponds that were uncontaminated
results of Keisecker's pond experiments: The 0's indicate
that cages without parasite access never had deformities.
Cages with access had proportions of limb-deformed frogs >0.
Unpolluted ponds with parasites had fewer deformities than
polluted ponds with parasites.
from Keisecker 2002
also noted size differences among treatment groups in the ponds.
Frogs in cages exposed to parasites were smaller than those unexposed.
This size difference was more pronounced in cages in contaminated
ponds than in clean ponds.
lab experiments revealed that even very low levels all three
pesticides (atrazine, malathion and esfenvalerate) altered
the frog's immune systems and increased the rate of parasite infections.
This is reflected in the figure below, summarizing the results for
one of the three pesticides, atrazine. [note that Keisecker used
two different types of trematode in his experiments, Telorchis
and Ribeiroia; the results were similar for both.
graph on the left shows that both levels of atrazine used in the
experiment (3 µg/liter and 30 µg/liter) suppressed the
number of eosinophils compared to the control (dechlorinated tap
water) and a "solvent control" (needed to test for the
effect of the solvent used to dissolve the pesticide into solution).
graph on the right shows that parasite infection rates (the proportion
of trematode larvae that encysted successfully in the exposed frogs)
were greater for both levels of atrazine exposure.
from Keisecker 2002
should also be noted that "high atrazine" was high only
relative to the low level used. Frogs near agricultural fields will
regularly encounter 30 µg/liter atrazine.
does it mean? First, this
paper adds to the growing weight
of evidence that pesticides, particularly atrazine, affect frog
development at levels far beneath those previously thought to be
Keisecker's work provides important new insights into why frog limb
deformities have increased so dramatically. As he notes in his paper,
frog limb deformities have been observed in nature for several hundred
years. Only in the last decade, however, have they been detected
at plague-like proportions. This
paper provides strong evidence that pesticide impacts on frog immune
systems makes them more vulnerable to pathogens they normally would
have been able to resist.
questions remain, nonetheless. A number of authors have noted frog
deformities in ponds where they have not detected parasites (see
Bill Souder's book, "A Plague of Frogs,"
for an excellent, broad review). Are some contaminants capable of
inducing deformities without parasites present? Some patterns of
deformities do not appear associated with parasites at all. What
is causing them? Some evidence
suggests that changes in water quality because of agriculture
made habitat conditions more favorable for the snails, and that
this may be contributing to the increase in deformities. Some
evidence indicates that snails can be exquisitely sensitive
to endocrine-disrupting contaminants. Are there circumstances where
an excess of contaminant would actually suppress snail populations,
deprive the parasites of hosts for another part of their life-cycle,
and actually lead to fewer deformities?
generally, this paper highlights two important issues. More
attention needs to focus on the interactions of developmental contaminants
with other factors affecting health, survival and function.
In this system, adverse effects of natural pathogens are exacerbated
via erosion of immune system function. An analogous example, also
with frogs, was reported by Relyea
and Mills in 2001; they discovered that predator-induced stress
increased the toxicity of a contaminant, carbaryl, to tadpoles.
Animals—and people—live in the real world, with multiple
factors impinging upon health, survival, reproduction, etc., all
at once. Laboratory experiments that work with only one factor at
a time can provide fabulous, reductionist insight into the actions
of single mechanisms by themselves, but totally miss what
may be happening in real life. Our regulatory standards
are based on these reductionist approaches and hence are likely
to be far too lax.
second general point is the impact of contaminants on immune system
development and function. As noted elsewhere,
the way we compile statistics about the causes of death and disease
attributes cause to the disease agent... a virus, bacteria, or parasite...with
no recognition that death or disease might not have resulted from
infection, were the victim's immune system not impaired. A highly
relevant example was published in 1997 on the interactions of PCBs
and Epstein Barr virus in increasing risk for non-Hodgkin's
lymphoma. If this mechanism of contaminant impact is
as widespread as early assessments
suggest, we are grossly underestimating the role of contamination
in undermining human health.