Prenatal exposure to common agricultural pesticides is associated with a small to moderately increased risk of autism spectrum disorder (ASD), and the risk of ASD with comorbid intellectual disability might be further increased by pesticide exposure during infancy, US research* has revealed. The study authors said today in the BMJ that exposure of pregnant women and babies to these pesticides should be avoided in order to protect early brain development, but the authors of a linked editorial** pointed out that reducing maternal exposure to zero for a pesticide such as glyphosate might be close to impossible in some populations.
The researchers explained that currently, knowledge about pesticide exposure in the real world and risk of ASD is scarce, although prenatal exposure to several types of pesticides has been associated with impaired neurodevelopment. Their population-based case-control study set in California’s main agricultural region, Central Valley, examined associations between early developmental exposure to ambient pesticides and ASD.
They identified 2,961 cases with a diagnosis of ASD, including 445 with intellectual disability comorbidity, through records maintained at the California Department of Developmental Services and linked to their birth records; and matched 35,370 controls, derived from birth records, to cases 10:1 by sex and birth year. They estimated both groups’ prenatal and infant exposures to 11 high-use pesticides, chosen according to previous evidence of neurodevelopmental toxicity in vivo or in vitro (measured as mass of pesticides applied per hectare/month within 2000m of maternal residence).
The study authors reported that risk of ASD was significantly associated with prenatal exposure to glyphosate (odds ratio, OR 1.16), chlorpyrifos (OR 1.13), diazinon (1.11), malathion (1.11), avermectin (1.12) and permethrin (1.10). For ASD with intellectual disability, estimated odds ratios were higher (by about 30%) for prenatal exposure to glyphosate (OR 1.33), chlorpyrifos (1.27), diazinon (1.41), permethrin (1.46), methyl bromide (1.33), and myclobutanil (1.32); exposure in the first year of life increased the odds for the disorder with comorbid intellectual disability by up to 50% for some pesticide substances.
They concluded: “[Our findings] suggest that an offspring’s risk of ASD increases following prenatal exposure to ambient pesticides within 2000m of their mother’s residence during pregnancy, compared with offspring of women from the same agricultural region without such exposure. Infant exposure could further increase risks for ASD with comorbid intellectual disability.”
They called for more research to explore underlying mechanisms in the development of autism, but said that from a public health and preventive medicine perspective their findings “support the need to avoid prenatal and infant exposure to pesticides to protect the developing child’s brain”.
The authors of a linked editorial commented: “Reducing maternal exposure to zero for a pesticide such as glyphosate might be close to impossible in some populations. Although reducing maternal exposure to pesticides during pregnancy is sensible public health policy based on this study, future research exploring dose-response and non-linear relations and individual susceptibilities could help to translate these study findings into more refined public health actions for pregnant women residing in areas of high pesticide use.
*von Ehrenstein OS, Ling C, Cui X, et al. Prenatal and infant exposure to ambient pesticides and autism spectrum disorder in children: population based case-control study. BMJ 2019; 364: l962 doi: 10.1136/bmj.l962.
**Bakian AV, VanDerslice JA. Editorial: Pesticides and autism. BMJ 2019; 364: l1149 doi: 10.1136/bmj.l1149