Environment linked to autism rise once again

HOW much can environmental factors explain the apparent rise in autism spectrum disorders? Roughly 1 per cent of children in the US population is affected by autism spectrum disorder (ASD). Rates in many countries, including the US, have risen sharply in recent years  but no one is sure why. It is still not clear whether this is prompted by something in the environment,  a result of people having children later, or increased awareness of the condition and changes in diagnoses. In the largest study of its kind, Andrey Rzhetsky and colleagues at the University of Chicago analysed health insurance claims in the US covering over 100 million patient records – a third of the population. They used rates of genital malformations in newborn boys  as a proxy for parents’ exposure to environmental risk factors. This is based on research linking a proportion of these birth defects to toxins in the environment, including pesticides, lead and medicines. The team compared the rates of these defects to rates of ASD county by county. After adjusting for gender, income, ethnicity and socio-economic status, they found that a 1 per cent increase in birth defects was associated with an average increase of 283 per cent in cases of ASD.  Rates of autism were several times greater in some counties than others, which the researchers suggest is  due to  environmental toxins (PLoS Computational Biology, doi.org/rxd). But Dorothy Bishop at the University of Oxford points out that genetic factors can also affect rates of malformation. David Skuse at University College London says geographical variation will also be affected by the counties’ differing healthcare provision and relevant expertise. What’s more,  he adds, using insurance claims  data means the sample isn’t random: “There are all sorts of potential biases.”  Simon Makin  ■