A new study published on August 16 in the American Journal of Psychiatry has found that pregnant women exposed to the insecticide dichlorodiphenyltrichloroethane, commonly known as DDT, are at a higher risk of giving birth to a child that will develop severe autism. The international collaboration, led by a team of researchers at Columbia University’s Mailman School of Public Health and the Department of Psychiatry, analysed more than one million pregnancies in Finland and showed that elevated levels of DDE, a metabolite of DDT, in the blood of pregnant women is linked to an increased risk of autism in their offspring.
Under the Stockholm Convention, the use of DDT as a pesticide was globally banned in 2001 after the discovery that it is dangerous to wildlife and the environment. Since DDT breaks down extremely slowly ― it may take several decades ― the chemical still persists in the environment today and can be found in the food chain, despite having been banned in many countries including the US and Finland more than 30 years ago. This means populations still face continued exposure. Furthermore, the toxic insecticide is even now sometimes used in Africa to control mosquitos.
Led by Dr Alan Brown, a psychiatrist and epidemiologist at Columbia University in New York City, the analysis compared health records of children with and without autism against blood serum samples of their mother’s during pregnancy, for those that gave birth between 1987 and 2005. Prenatal exposure to DDT and polychlorinated biphenyls (PCBs), common in building materials and electronics, were examined. Both contaminants may still be lingering in the food chain: DDT can remain in soil and water for decades, and often amasses in plants and animals and PPBs are found in high concentrations in some fish.
The study found that mothers with high concentrations of the DDE metabolite in their blood were 32% more likely than women with lower levels to bear a child that develop autism. Moreover, the probability of a child with autism accompanied by intellectual disability doubled in mothers who were exposed to high levels of DDT. However, no association between PCBs and autism was found.
Findings of this study do not prove that autism is caused by DDT exposure, however, a strong correlation is now known. Another study, led by Jonathan Chevrier, an epidemiologist at McGill University in Montreal, Canada, is underway in South Africa ― following 700 children ― to determine whether DDT exposure is linked to intellectual disabilities and may provide some insights into how DDT affects the brain.
Even so, the risk of having a child with autism is relatively low, even for mother’s that have been exposed to DDT. According to the World Health Organisation, around one in 160 children are estimated to have autism. The cause is suspected to be due to a number of contributing factors including genetics as well as environmental influences. Further studies are still needed to determine the mechanism by which chemical exposure could trigger autism.
(1) Brown, A.S. et al. Association of Maternal Insecticide Levels With Autism in Offspring From a National Birth Cohort. American Journal of Psychiatry (2018) DOI: 10.1176/appi.ajp.2018.17101129