By Tom Mullaney
In the largest environmental study of its kind, scientists from the University of Chicago have found a geographic link between the incidence of autism spectrum disorder (ASD) and intellectual disability (ID).
The seven-member research team, led by Andrey Rzhetsky, MD, at University of Chicago Medicine, analyzed more than 100 million U.S. medical records and discovered a link between the incidence of genital malformations in newborn males relative to environmental exposure to harmful environmental factors.
They found that autism rates jump by 283 percent for every one percent rise in the incidence of malformations at the county level. Rates for intellectual disabilities rose by 94 percent. Autism and ID rates decrease significantly in states with stronger regulations on harmful environmental factors. The study appeared online last month on the PLOS Computational Biology website.
ASDs are a collection of chronic, complex neuropsychiatric diseases with limited effective treatment options. The disorder’s etiology is a scientific puzzle. Since it is highly inheritable and clustered within families, a great deal of earlier scientific attention was focused on finding the predisposing genetic factors.
In the last decade, researchers have turned increasingly to studying many environmental influences such as prenatal exposure to pesticides, toxic chemicals, maternal obesity and diabetes. Up to now, it has been impossible to study such undocumented connections across states and counties.
The Chicago medical team had access to Truven Health’s insurance-claims’ database covering nearly one-third of the U.S. population spread over all 50 states and 3,111 counties. They supplemented this source with census data to allow for county-level comparisons of malformation rates based on gender, ethnicity, socioeconomic and geopolitical factors.
Researchers conducted a mathematical, bivariate analysis in which they tested 50 free parameters against the resulting increase in autism and intellectual disability rates. They found a dramatic 283 percent in autism rates for every 1 percent increase in the incidence of male reproductive malformations at the county level.
Researchers also showed strong geospatial clustering of ASD in parts of California, Texas, North Carolina and Utah. This clustering could indicate localized risk factors such as pesticides, environmental toxins (Southern California, Texas and Utah are major oil exploration and mining states) use of sex hormones, alcohol, and parents with obesity and diabetes.
Studies to date have focused on in-state socioeconomic predictors such as levels of parental education and controversial state financial incentives for special-education services.
Among the survey’s other findings: A 1 percent increase in urbanization in a county resulted in a 3 percent increase in ASD and ID incidence, and counties with high ASD rates also appear to have high ID rates. Also, ASD and ID incidence is much higher in male babies than females.
The study also found a link in increased infant birth defects associated with certain maternal occupations such as maid, janitor and landscaper, while most other occupations, outside of pre-school teachers, exhibited a decreased incidence.
Rzhetsky believes that his team’s comprehensive study can play an important role in unraveling the environmental causes contributing to the steep rise in ASD diagnoses. He calls the study “the canary in the coal mine.”
Published on April 1, 2014