Exposure to common agricultural pesticides before birth and in the first year of life is associated with a small to moderately increased risk of autism spectrum disorder (ASD), compared with infants of women without such exposure, a study published in the BMJ found.¹
Pesticides found to be linked with an increased risk of autism include glyphosate herbicide, the world’s most commonly used pesticide, which is used on most genetically modified (GM) crops.
Previous laboratory studies in mice have shown that exposure to pesticides harms brain development and function.^2,3 Prenatal exposures to several types of pesticides have been associated with impaired neurodevelopment.⁴ The few studies that have considered ASD suggested that organophosphates⁵ and organochlorines⁶ could increase risk. But knowledge about pesticide exposure in the real world and the risk of ASD is scarce.
So researchers at the University of California used registry records from the California Department of Developmental Services (DDS) to identify 2,961 patients with a diagnosis of ASD – including 445 with ASD with accompanying intellectual disability – and 35,370 healthy (“control”) patients of the same birth year and sex.
Participants were born between 1998 and 2010 in California’s Central Valley, a heavily agricultural region, and 80% of these ASD cases were male (consistent with data showing that the rate of ASD affects boys more than girls – some researchers estimate the ratio as 3:1⁷).
Data from the California state-mandated Pesticide Use Registry were then integrated into a geographic information system tool to assess prenatal (before birth) and infant exposures to 11 commonly used pesticides.
These pesticides were selected because of their high use and evidence indicating toxic effects on brain development.
After adjusting for potentially confounding factors, the researchers found modest increases in ASD risk among offspring exposed to several pesticides (including glyphosate, chlorpyrifos, diazinon, malathion, permethrin, bifenthrin and methyl bromide) before birth and during the first year of life, compared with controls.
Associations were strongest in those with ASD and intellectual disability, which represents the more severe end of the autism spectrum.
This is an observational study. As such it cannot establish cause, and the researchers point to some limitations, such as relying on patient records for details about diagnoses and being unable to examine clinical outcomes.
Nevertheless, they say their study is by far the largest investigating pesticides and autism spectrum disorder to date and their findings back up earlier work in this field.
“Our findings suggest that ASD risk may increase with prenatal and infant exposure to several common ambient pesticides that impacted neurodevelopment in experimental studies,” they write.
The researchers call for further research to explore underlying mechanisms in the development of autism. However, from a public health and preventive medicine perspective, they say their findings “support the need to avoid prenatal and infant exposure to pesticides to protect the developing child’s brain.”
In a linked editorial, Amanda Bakian and James VanDerslice at the University of Utah agree that reducing exposure to pesticides during pregnancy “is sensible public health policy” but state that this “might be close to impossible for some populations.”⁸
However, while this is true from the point of view of living close to sprayed fields, we know that switching to an organic diet markedly reduced children’s exposure to organophosphate pesticides.⁹ Michelle Perro, MD, a California-based pediatrician and director of GMOScience, commented, “Programs are being implemented in Northern California to bring organic food into public schools. These programs are showing early initial success in terms of improving behavior and academic performance and are presently under investigation at the University of California, San Francisco.
“Considering these positive results, the recommendation is to switch from conventional school food programs to those offering organic foods. It should also be taken into consideration that some children are eating anywhere from one to three meals a day at school. The decrease in overall pesticide load will be significant in these children.”
References
1. Ehrenstein OS von, 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
2. Lee I, Eriksson P, Fredriksson A, Buratovic S, Viberg H. Developmental neurotoxic effects of two pesticides: Behavior and neuroprotein studies on endosulfan and cypermethrin. Toxicology. 2015;335:1-10. doi:10.1016/j.tox.2015.06.010
3. De Felice A, Greco A, Calamandrei G, Minghetti L. Prenatal exposure to the organophosphate insecticide chlorpyrifos enhances brain oxidative stress and prostaglandin E2 synthesis in a mouse model of idiopathic autism. J Neuroinflammation. 2016;13(1):149. doi:10.1186/s12974-016-0617-4
4. Gunier RB, Bradman A, Harley KG, Kogut K, Eskenazi B. Prenatal residential proximity to agricultural pesticide use and IQ in 7-year-old children. Environ Health Perspect. 2017;125(5):057002. doi:10.1289/EHP504
5. Shelton JF, Geraghty EM, Tancredi DJ, et al. Neurodevelopmental disorders and prenatal residential proximity to agricultural pesticides: the CHARGE study. Environ Health Perspect. 2014;122(10):1103-1109. doi:10.1289/ehp.1307044
6. Lyall K, Croen LA, Sjödin A, et al. Polychlorinated biphenyl and organochlorine pesticide concentrations in maternal mid-pregnancy serum samples: Association with Autism Spectrum Disorder and intellectual disability. Environ Health Perspect. 2017;125(3):474-480. doi:10.1289/EHP277
7. Loomes R, Hull L, Mandy WPL. What is the male-to-female ratio in Autism Spectrum Disorder? A systematic review and meta-analysis. J Am Acad Child Adolesc Psychiatry. 2017;56(6):466-474. doi:10.1016/j.jaac.2017.03.013
8. Bakian AV, VanDerslice JA. Pesticides and autism. BMJ. 2019;364:l1149. doi:10.1136/bmj.l1149
9. Lu Chensheng, Toepel Kathryn, Irish Rene, Fenske Richard A., Barr Dana B., Bravo Roberto. Organic diets significantly lower children’s dietary exposure to organophosphorus pesticides. Environ Health Perspect. 2006;114(2):260-263. doi:10.1289/ehp.8418