Schreinemachers, DM. 2003. Birth Malformations and Other
Adverse Perinatal Outcomes in Four U.S. Wheat - Producing States.
Health Perspectives 111:1259-1264.
a study comparing the rates of birth defects counties where wheat
is grown abundantly to places where it is not, Schreinemachers concludes
that herbicides used on wheat may be causing birth defects.
Babies born in wheat growing areas of the US west are more likely
to have several types of birth defects than babies born in the same
region but where wheat is less common.
the study falls short of proving causation, it is consistent with
earlier research on animals and with people indicating that a family
of herbicides commonly used on wheat, the chlorophenoxy herbicides
such as 2,4-D, disrupt fetal development.
did she do? Schreinemachers analyzed the incidence of birth
defects in 147 rural counties in 4
comparing rural regions of high wheat production with rural regions
of low wheat growing. Two chlorophenoxy herbicides, 2-4,D and MCPA,
are used heavily on wheat, much less so on crops like soy that dominate
the counties where wheat is less abundant. Hence by comparing high
wheat and low wheat counties, Schreinemachers sets up a comparison
of high chlorophenoxy exposure vs. low.
study focused only on Caucasian singleton (not twin) children born
to mothers at least 18 yrs old. Data for the comparisons were obtained
from health records maintained in the National Center for Health
Statistics. In all, records for 43,634 births in the counties of
interest from 1995 to 1997 were analyzed.
did she find? Birth outcomes differed significantly in
defects of the respiratory and circulatory systems were greater
in high wheat areas, as were defects of the musculo-skeletal system
like clubfoot, fused digits and extra digits. These birth defects
were 60% to 90% more likely in high wheat counties. The odds-ratios
for these increases were all significant.
chances of birth defects rose for babies conceived in the spring,
when herbicide spraying was most intense. Boys born in April-May
in high wheat growing counties were almost 5x more likely to have
a birth defect than boys born in low wheat counties at other times
of the year.
death due to congenital abnormalities was higher in boys born
in wheat growing counties compared to boys in low wheat counties.
Most of the infant boys' deaths were caused by heart and musculoskeletal
birth defects. No comparable elevation in risk of infant death
was seen for girls.
does it mean? Epidemiologists in general agree that 'ecologic'
studies like Schreinemachers--examining geographic patterns to highlight
statistical associations--stop short of demonstrating causation.
Instead they are one piece of evidence that can be considered along
with parallel studies using other techniques, all approaching the
question of human impact from different directions. Indirect work
like this is necessary because purposeful experimentation on humans
to determine, for example, whether an herbicide causes birth defects,
would be unethical.
work neatly sidesteps a common weakness of ecologic studies of the
effects of agricultural pesticides by focusing all of her analysis
on rural communities. Comparisons are often made between rural and
urban residents; these studies unavoidably add a raft of confounding
variables due to the differences in urban vs rural environments,
health care, etc.
by comparing people living in different rural counties, varying
in the intensity of wheat growing, Schreinmacher doesn't avoid all
confounding variables. Two stand out: first, some wheat is grown
in the low wheat counties, and thus some, albeit reduced, exposure
to chlorophenoxy herbicides will take place. Second, other types
of herbicides can be used heavily on the crops grown instead of
wheat, for example, atrazine on corn.
of these considerations would bias Schreinemachers' analysis toward
showing a link between chlorophenoxy herbicides and birth defects.
If anything, they would have decreased her chances of finding a
positive result, by increasing the numbers of birth defects in low
wheat counties. The ideal comparison would have been rural counties
with no agriculture vs. high wheat, but rural counties with no agriculture
within the region are rare at best.
the fact that Schreinemachers found heightened risk of birth defects
for several comparisons needs to be taken seriously, even more so
because the weight of evidence from other studies strongly links
chlorophenoxy herbicides with birth defects. Notable amidst that
earlier work are studies showing higher birth
defect rates in Minnesota, New Zealand
and Norway associated with chlorophenoxy exposures,
and experimental studies in showing that chlorophenoxy herbicides
cause birth defects in rodents.
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