A study that followed more than a million pregnancies in Finland found that elevated levels of a DDT (dichloro-diphenyl-trichloroethane) metabolite in the blood of pregnant women are associated with an increased risk for autism in newborns.
The team also looked at maternal levels of polychlorinated biphenyls (PCBs) and found no link to suggest any association with autism.
The finding sheds light on another potential cause of autism, a neurodevelopmental condition that affects communication, behavior and the ability to interact with others.
The researchers identified 778 cases of children diagnosed with autism who were born from 1987 to 2005, then matched those individuals with controls - that is, children who were born during that same period but did not have an autism diagnosis. There was no connection found between PCB exposure and autism in the offspring.
One major weakness of the study, which the authors point out, is that since they didn't examine children with intellectual disability but without autism, they can't rule out the possibility that intellectual disability was not a contributing factor to autism in the children who had both. Specifically, women whose children were autistic had much higher levels of DDE in their bodies than their peers whose children didn't have autism. Furthermore, the odds of autism with intellectual disability were a staggering 121 percent higher if the mother had DDE levels above this threshold.
The finding held even when the researchers controlled for several factors, including the age of the mother, the mother's socioeconomic status and whether the parents had a history of psychiatric disorders.
DDE is a breakdown product of the pesticide DDT (dichlorodiphenyltrichloroethane), which, though banned in many countries decades ago, can still be found in the food chain. Most of these studies, however, have assumed exposure to these chemicals based on the participants' proximity to a contaminated site; they did not directly measure levels of the chemical in pregnant women's blood during pregnancy. "What happened was DDT was sprayed in the air as an insecticide to kill mosquitoes, to kill bugs on fruits and vegetables", Brown said.
Virtually everybody has some level of DDT and PCBs in their body.
The level of DDT in the mother's system was quantified by a laboratory test for DDE, the chemical that DDT is metabolized into. These chemicals are known to transfer across the placenta, meaning that they could potentially go to a fetus' bloodstream. "Even though they're not produced any more in the Western world, nearly everyone is exposed to some of them", said study author Dr. Alan Brown.
As mentioned, DDE may just be one piece of the puzzle for explaining what causes autism. First, maternal DDE is associated with low birthweight, a well-replicated risk factor for autism.
Brown said researchers don't know how DDT exposure might lead to autism, though they suspect the chemical may alter the function of certain genes.