ne compared to controls. This effect
is highly significant statistically. Treated male testosterone
levels were indistinguishable from control females.
Hayes et al.
does it mean?
This research eliminates any doubt about atrazine as an endocrine
disruptor at extraordinarily low levels of exposure. It puts endocrine
disruption high onto the list of plausible factors contributing
to frog population declines. And because of the apparent mechanism
of action, via enhancement of aromatase conversion of testosterone
to estrogen, it raises important concerns about atrazine disrupting
the hormonal control of development in other organisms, including
Hayes et al. summarize in their paper, atrazine contamination
is extremely widespread , including, via atmospheric transport,
in rainwater in regions where atrazine is not used for agricultural
crops. The level of atrazine sufficient to cause reproductive abnormalities
in Xenopushermaphroditismis 1/30th of the level allowed
by the US EPA in drinking water:
recommended application level of atrazine ranges from 2,500,000
29,300,000 ppb, the allowable contaminant level for atrazine
in drinking water is 3 ppb, and short-term exposures of 200
ppb are not considered a health risk. Atrazine can be as high
as 21 ppb in ground water, 42 ppb in surface waters, 102 ppb
in river basins in agricultural areas, up to 224 ppb in Midwestern
streams, and up to 2,300 ppb in tailwater pits in Midwestern
agricultural areas. Atrazine can be found in excess of 1 ppb
in precipitation in localities where it is not used and up to
40 ppb in rainfall in Midwestern agricultural areas."
in many places in the world are undoubtedly exposed to atrazine
at these levels. It remains to be determined whether other frog
species are as sensitive to atrazine as is Xenopus, and whether
the effects are similar. Hayes' work should reinvigorate efforts
to understand the contribution of chemical exposure to frog declines.
has strongly implicated infection by a chytrid fungus as an important
factor driving frog extinctions, other causes, including contaminants
and the introduction of exotic species, remain plausible contributors
to what is most likely a multi-factoral process. And given the ability
of some contaminants to reduce
disease resistance, it is not implausible to hypothesize that
contaminants are involved in the rapid pace at which amphibians
have succumbed to the fungal infection. No research has assessed
the impact of low-level atrazine exposure on the development of
frog immune competency.
added in August 2002: new research
confirms atrazine impairs frog immune systems
research by Hayes et al. also points out the crucial need
to carry out experiments assessing toxicological impacts at environmentally-relevant
levels. Prior work had seemingly dismissed atrazine as an endocrine
disruptor affecting frogs, but the research used exposures that
were 10,000 to 30,000 times higher than the dosage found by Hayes
et al. to produce developmental disruption:
teratogenesis,growth inhibition, and mortality in amphibians
in response to atrazine were not considered environmental concerns
of the high doses required to produce these effects. Effects
in the current study, however, occurred at levels 10,000 times
lower than the dose required to produce effects in amphibians
in these previous studies. Allran
and Karasov (2001) reached the conclusion that atrazine
was probably not a significant factor in amphibian declines
based on their studies of toxicity, deformities, and effects
on feeding and ventilation in leopard frogs that did not produce
noticeable effects below 3 ppm. The current data show that negative
effects on sex differentiation occur at doses 30,000 times lower
than effective doses reported by Allran and Karasov."