Prenatal Exposure to Pesticide DDT Linked to Autism

A study of 778 children born with autism and their matched case-control pairs found that prenatal exposure to the pesticide DDT (dichlorodiphenyltrichloroethane) increased the risk of autism.

For mothers with elevated exposure to the pesticide DDT (75^th percentile or greater), there was double the likelihood of their child developing autism with intellectual disability (odds ratio = 2.21, 95% CI = 1.32, 3.69).

The study examined levels of the pesticide DDT along with its metabolite, DDE (p,p'-dichlorodiphenyl dichloroethylene), and PCBs (polychlorinated biphenyls) in samples from women early in their pregnancies. While DDT and PCBs were banned in many countries over 30 years ago, including in Finland, where the study took place, and the US, they continue to affect populations because their breakdown can take as long as several decades.

"We think of these chemicals in the past tense, relegated to a long-gone era of dangerous 20th century toxins," says lead author Alan S. Brown, MD, MPH, professor of Epidemiology at Columbia University's Mailman School of Public Health and of Psychiatry at Columbia University Medical Center. "Unfortunately, they are still present in the environment and are in our blood and tissues. In pregnant women, they are passed along to the developing fetus.”

The study drew from the Finnish Maternity Cohort, which represents 98% of pregnant women in Finland. The 778 children with autism born between 1987 to 2005 were matched with mother-child pairs without childhood autism. Maternal blood samples from early pregnancy were examined for the DDT metabolite DDE, as well as the environmental pollutants PCBs.

The study found that odds for autism in offspring were significantly higher for cases where maternal DDE exposure levels in the highest 75^th percentile, even adjusting for maternal age, parity, and history of psychiatric disorders (odds ratio = 1.32, 95% CI = 1.02, 1.71). There was no connection found between PCB exposure and autism in the offspring.

“Along with genetic and other environmental factors, our findings suggest that prenatal exposure to the DDT toxin may be a trigger for autism," concluded Brown. However, the authors noted that these findings indicate a correlation, but do not prove causation, though they did control for a number of factors.

While maternal DDE exposure was related to autism, the study did not find a link between PCB exposure and autism. Investigators offered 2 possible explanations for this. First, DDE is related to low birthweight, which is considered a risk factors for autism, while PCB is not. Second, DDE inhibits androgen receptor binding, an important neurodevelopmental process, while PCBs androgen receptor transcription.

The investigators concluded that their results "provide the first biomarker-based evidence that maternal exposure to insecticides is associated with autism among offspring."

The study, “Association of Maternal Insecticide Levels With Autism in Offspring From a National Birth Cohort,” was published in the American Journal of Psychiatry on August 16, 2018.