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Study Finds Association Between Maternal Exposure to Agricultural Pesticides and Autism

Exposure During the Second and Third Trimester of Pregnancy Heightened the Risk

Pregnant women who lived near fields and farms where chemical pesticides were applied experienced a two-thirds increased risk of having a child with autism spectrum disorder or other developmental delay, a study by the University of California at Davis found. The associations were stronger when the exposures occurred during the second and third trimesters of pregnancy.

The large, California-based study examined associations between specific classes of pesticides, including organophosphates, pyrethroids, and carbamates, applied during the study participants’ pregnancies and later diagnoses of autism and developmental delay in their offspring. The research by the university’s MIND Institute (Medical Investigation of Neurodevelopmental Disorders) was published online in Environmental Health Perspectives.

“This study validates the results of earlier research that has reported associations between having a child with autism and prenatal exposure to agricultural chemicals in California,” said lead author Janie F. Shelton, a UC Davis graduate student who consults with the United Nations. “While we still must investigate whether certain subgroups are more vulnerable to exposures to these compounds than others, the message is very clear: Women who are pregnant should take special care to avoid contact with agricultural chemicals whenever possible.”

In California, the nation’s top agricultural-producing state, approximately 200 million pounds of active pesticides are applied each year. Certain commonly used pesticides are neurotoxic and may pose threats to brain development during gestation.

The study was conducted by examining commercial pesticide application using the California Pesticide Use Report and linking the data to the residential addresses of approximately 1,000 participants in the Northern California-based Childhood Risk of Autism from Genetics and the Environment (CHARGE) Study. The study includes families with children between the ages of 2 and 5 years diagnosed with autism or developmental delay or with typical development.

Twenty-one chemical compounds were identified in the organophosphate class, including chlorpyrifos, acephate, and diazinon. The second most commonly applied class of pesticides was pyrethroids, one quarter of which was esfenvalerate, followed by lambda-cyhalothrin permethrin, cypermethrin, and tau-fluvalinate. Eighty percent of the carbamates were methomyl and carbaryl.

Researchers used questionnaires to obtain study participants’ residential addresses during the preconception and pregnancy periods. The addresses then were overlaid on maps with the locations of agricultural chemical application sites based on the pesticide-use reports to determine residential proximity. The study also examined which participants were exposed to which agricultural chemicals.

The researchers found that during the study period approximately one-third of CHARGE Study participants lived within 1.25 to 1.75 kilometers of commercial pesticide application sites. Some associations were greater among mothers living closer to application sites and lower as residential proximity to the application sites decreased, the researchers found.

Organophosphates applied over the course of pregnancy were associated with an elevated risk of autism spectrum disorder, particularly for chlorpyrifos applications in the second trimester. Pyrethroids were moderately associated with autism spectrum disorder immediately prior to conception and in the third trimester. Carbamates applied during pregnancy were associated with developmental delay.

Exposures to insecticides for those living near agricultural areas may be problematic, especially during gestation, because the developing fetal brain may be more vulnerable than it is in adults. Because these pesticides are neurotoxic, in utero exposures during early development may distort the complex processes of structural development and neuronal signaling, producing alterations to the excitation and inhibition mechanisms that govern mood, learning, social interactions and behavior.

Source: UC Davis MIND Institute; June 22, 2014.

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