The research team, led by Janie F. Shelton of the Department of Public Health Sciences at UC-Davis, recently published their findings in the journal Environmental Health Perspectives.
Autism, also referred to as autism spectrum disorder (ASD), is a group of disorders characterized by problems with brain development. Individuals with autism tend to have difficulties with social interaction, adopt repetitive behaviors and have problems with verbal and nonverbal communication.
According to a recent report from the US Centers for Disease Control and Prevention (CDC), the number of children in the US with autism has increased by 30% in the past 2 years, from 1 in 88 in 2012 to 1 in 68 in 2014.
Past research has indicated that environmental factors - such as exposure to pesticides - during pregnancy may influence whether a child experiences developmental delay disorders.
As such, the UC-Davis team set out to investigate whether expectant mothers' residential proximity to agricultural pesticides influences the risk of autism or other developmental delay disorders among offspring.
To reach their findings, the researchers analyzed data from the Childhood Risk of Autism from Genetics and the Environment (CHARGE) study, which involved families with children ages 2-5 years who had been diagnosed with autism or developmental delay, or who had experienced typical development.
During preconception and pregnancy periods, participants completed a questionnaire that disclosed their area of residency. The researchers note that the majority of families lived in the Sacramento Valley, Central Valley and San Francisco areas of California.
The team then used data from the California Pesticide Use Report to determine the levels of commercial pesticide application in these areas.
They found that the most commonly used pesticides were organophosphates - such as chlorpyrifos, acephate and diazinon - while the second most commonly applied pesticides were pyrethroids, including esfenvalerate, lambda-cyhalothrin permethrin, cypermethrin and tau-fluvalinate. Carbamates, such as methomyl and carbaryl, were also identified.
Results of the study revealed that around a third of study participants lived in close proximity (1.25-1.75 kilometers) to sites where commercial pesticides had been applied.
The team found that mothers who lived in close proximity to such sites during pregnancy were two thirds more likely to have children with autism or other developmental delays than mothers who lived far away from these sites.
Autism risk was higher for children whose mothers had been exposed to organophosphates during pregnancy, particularly for those who had been exposed to chlorpyrifos in their second trimester.
Pyrethroids significantly increased autism risk for children whose mothers were exposed to the chemicals prior to conception and in the third trimester, while exposure to carbamates increased the risk of developmental delay if a mother was exposed to the pesticides during pregnancy.
The researchers say that pesticides are neurotoxic, meaning they interfere with the brain's neurotransmitters that are responsible for mood, learning, social interactions and behavior.
They stress that fetal exposure to agricultural pesticides is of particular concern, as the developing brain may be more vulnerable to these neurotoxic chemicals than the brains of adults.
"In that early developmental gestational period, the brain is developing synapses - the spaces between neurons - where electrical impulses are turned into neurotransmitting chemicals that leap from one neuron to another to pass messages along," explains principal investigator Irva Hertz-Picciotto, a researcher at the MIND Institute and professor and vice chair of the Department of Public Health Sciences at UC-Davis. "The formation of these junctions is really important and may well be where these pesticides are operating and affecting neurotransmission."
The team says their findings emphasize the value of maternal nutrition - particularly the use of prenatal vitamins - in reducing offspring autism risk, and the importance of finding ways to reduce agricultural chemical exposure during pregnancy.
"While we still must investigate whether certain sub-groups 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."
The findings are subject to some limitations, the researchers say. For example, they note that other sources of pesticide exposure - such as that from schools and other institutions - were not taken into consideration, which may have affected the results.
Earlier this year, Medical News Today reported on a study by researchers from the University of Chicago, which found that rates of autism and intellectual disability in the US correlated with rates of genital malformation in newborn males at county level - an indicator of fetus exposure to pesticides and other environmental toxins.
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