A landmark study published this week in the American Journal of Psychiatry describes a correlation between pesticide levels in a mother's blood and autism risk in their infants.
Though the condition varies between people, symptoms commonly include repetitive behavior, difficulties adapting to change, and trouble in social situations.
Exactly what causes autism and how it develops is still up for debate, but it is widely recognised that there is likely to be interplay between environmental and genetic factors.
Much headway has been made in autism research — but, to date, there are few definitive answers, and there is no cure.
Recently, a group of researchers set out to investigate whether there might be links between pesticide exposure and autism risk. In particular, they were interested in dichlorodiphenyltrichloroethane (DDT).
Scientists from Columbia University's Mailman School of Public Health in New York City, NY, joined forces with researchers from the University of Turku and the National Institute of Health and Welfare, both in Finland.
What is DDT?
First synthesized in 1874, DDT kills a wide range of disease vectors and was used during World War II to control typhus and malaria in Europe and the South Pacific. It is such an effective pesticide that typhus was almost eradicated in some parts of Europe. By 1945, it was available for purchase in the U.S. and widely used in both private homes and agricultural businesses.
Though effective, legitimate safety concerns were raised and DDT was eventually banned in many countries. In the U.S., for instance, it was banned in 1972.
However, just stopping DDT's use did not make the chemical go away. DDT is a persistent organic pollutant, meaning that it breaks down slowly over decades, easily entering the food chain. Over time, levels of the chemical build up in an organism, especially in fatty tissue.
Although the exact impact of long-term DDT exposure on human health is still not confirmed, it is widely thought to be an endocrine disruptor and a potential carcinogen.
Because short-term exposure to the pesticide is considered relatively safe, it is still used to control mosquitoes and other pests in areas with particularly high rates of malaria, such as certain parts of sub-Saharan Africa.
Worryingly, DDT can cross the placenta. So, if a woman already carries some of the pesticide, it can affect her unborn child.
To find out whether DDT is related in any way to the development of autism in some people, they used data from the Finnish Prenatal Study of Autism. The scientists analyzed serum samples from more than 750 children with autism and a large number of matched controls without autism.
The DDT–autism link
The team measured levels of p,p'-dichlorodiphenyl dichloroethylene (DDE), a breakdown product of DDT. They found that the risk of a child developing autism was around one third higher if their mother had elevated levels of DDE in her blood.
Similarly, the risk of a child having autism with intellectual disability more than doubled when the mother's DDE levels were in the 75th percentile or above.
The authors write that their findings "provide the first biomarker-based evidence that maternal exposure to insecticides is associated with autism among offspring."
"We think of these chemicals in the past tense, relegated to a long-gone era of dangerous 20th century toxins. Unfortunately, they are still present in the environment and are in our blood and tissues."
Lead study author Dr. Alan S. Brown
"In pregnant women," he notes, "they are passed along to the developing fetus. Along with genetic and other environmental factors; our findings suggest that prenatal exposure to the DDT toxin may be a trigger for autism."
The authors are quick to note that they have uncovered an association, which is not evidence of causation. However, after they controlled for confounding factors such as maternal age and previous psychiatric conditions, the findings remained significant.
They also measured levels of an industrial chemical known as polychlorinated biphenyls (PCBs). Though earlier studies had found links between PCBs and autism risk, this study found no such relationship.
Because autism is seemingly increasing in prevalence, this research is likely to garner attention. If the link is shown to be causal, the next challenge may be working out ways to remove DDT from both the environment and the human body. For now, more studies are needed.