Here we go again - Autism DDT Link?

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Dr. Alan S. Brown, MPH, a psychiatrist and epidemiologist at Columbia University, has spent much of his career researching the risk factors for autism as well as schizophrenia and bipolar disorder.

Brown and his international team looked at autism’s possible link to the insecticide DDT.

DDT (dichlorodiphenyltrichloroethane) was once widely used in the United States but was banned in 1972 by the Environmental Protection Agency (EPA) under then-President Richard Nixon because it was thought to be harmful to the environment, animals, and perhaps humans.

So why would Brown spend time studying a bug spray that was outlawed in the United States nearly five decades ago?

Because DDT persists in the food chain, he said. It can take as long as several decades for it to break down, resulting in its continued contact with humans, including expectant mothers.

Brown and his international team’s study of more than 1 million pregnancies in Finland showed a link between elevated levels of a metabolite of DDT in the blood of pregnant women and increased risk for autism in their children.

The results of the study, led by Brown and other investigators at Columbia University’s Mailman School of Public Health and the Department of Psychiatry, were published today in the American Journal of Psychiatry.

Conducted in collaboration with investigators at the University of Turku and the National Institute of Health and Welfare in Finland, the study is the first to connect an insecticide with risk for autism using maternal biomarkers of exposure.

The study also examined mothers’ exposure to PCB’s (polychlorinated biphenyls), another class of environmental pollutants, and concluded there was no association between these substances and autism.

Brown said his team identified 778 cases of childhood autism among children born from 1987 to 2005 to women enrolled in the Finnish Maternity Cohort, representing 98 percent of pregnant women in Finland.

They matched these mother-child pairs with a control group of offspring of mothers as well as offspring without autism.

Maternal blood taken during early pregnancy was analyzed for DDE, a metabolite of DDT, and PCBs.

Investigators said they found the odds of autism with intellectual disability in offspring were increased by greater than twofold for the mother with DDE levels in the top quartile.

For the overall sample of autism cases, the odds were nearly one-third higher among offspring exposed to elevated maternal DDE levels.

The findings persisted after adjusting for several factors such as maternal age and psychiatric history. There was no association between maternal PCBs and autism, Brown said.

“This study provides us with a new risk factor that is prevalent in the environment and could account for minority of cases but not a small minority in terms of risk,” Brown told Healthline.

Unfortunately, Brown said, these chemicals are still present in the environment and are in our blood and tissues.

“In pregnant women, they are passed along to the developing fetus,” he said. “Along with genetic and other environmental factors, our findings suggest that prenatal exposure to the DDT toxin may be a trigger for autism.”

Brown’s team offered two reasons for their observation that maternal exposure to DDE was related to autism while maternal PCB exposure was not.

PCBs, or polychlorinated biphenyls, are industrial products or chemicals that were banned in the United States in 1979.

First, Brown’s team explained, maternal DDE is associated with low birthweight, a well-replicated risk factor for autism. In contrast, maternal PCB exposure has not been related to low birthweight.

Second, Brown’s team points to androgen receptor binding, a process key to neurodevelopment.

A study in rats found that DDE inhibits androgen receptor binding, an outcome also seen in a rat model of autism.

In contrast, PCBs increase androgen receptor transcription.

As with most research involving autism, this study brings some respectful disagreement among the experts.

Tracey Woodruff, PhD, MPH, who studies reproductive health and the environment at University of California, San Francisco, told the journal Nature today that the study “is really amazing.”

She said she is impressed by the number and quality of the samples in the Finnish database, and finds the association between DDT and autism striking.

“This just confirms that banning [DDT] was a good idea,” she said

But Thomas Frazier, PhD, chief science officer of Autism Speaks, was slightly less enthusiastic about the study.

He called it important but not groundbreaking.

“It suggests another potential environmental risk factor, DDT, but also did not replicate a previously identified risk factor, PCBs,” he told Healthline. “This highlights the need for large sample replication, particularly for autism risk factors.”

Frazier said the mechanism by which DDT may increase autism “is not known, and it may not be worth speculating until the finding is replicated. It is possible that DDT as a toxin influences gene expression in the developing brain.”

“The other important caveat in this study,” Frazier added, “is that association does not mean causation. While the authors did a good job of identifying similar cases and controls and adjusting for relevant factors, it is not possible to rule out other explanations.”

“Bottom line: This study is not groundbreaking, but it is well done and suggests the need for replication and a careful look at DDT in the future,” Frazier said.

Brown said he agrees with much of what Frazier said, but not all of it.

“I agree there is need for replication, but whether or not the study is groundbreaking, it is the first biomarker-based study, and that is worth noting,” Brown said.

Brown said the study argues for further studies that look at other mechanisms and other chemicals, including other insecticides.

Brown said this study should not alarm women who are expecting.

He said the vast majority of women even with elevated levels of the DDT metabolite did not have offspring with autism.

This suggests that in order for autism to develop there would have to be a combination of other risk factors, including possible genetic mutations.

“It could be that you need some kind of genetic predisposition” combined with environmental exposure to get autism, he said.

Brown said this type of research could ultimately lead to treatments by identifying a subclass of people with certain genetic factors.

“The key is to identify a precise target, which would move this toward precision medicine,” said Brown.

DDT, which was first synthesized in 1874, was initially used by the military in World War II to control malaria, typhus, body lice, and bubonic plague

Farmers used DDT on a variety of food crops in the United States and worldwide, and DDT was also used in buildings for pest control.