Autism Speaks: UC Davis study uncovers several clusters of elevated risk for autism in California

Two recent, separate publications identified regions with higher than expected numbers of autism cases - or clusters - in California. Using data collected by the California Department of Developmental Services (DDS) on 2.5 million births including almost 10,000 autism cases from 1996-2000, investigators at UC Davis uncovered several clusters of elevated risk for autism. Autism Speaks, the nation's largest autism science and advocacy organization, reviewed these studies and found that the majority of these clusters were found to be strongly associated with higher parental education and, to a lesser extent, with parents of older ages. It is thought that parents with higher levels of education may have better access to the regional diagnostic and service centers in California, as the DDS relies on parents actively seeking services. Thus the distribution of cases is likely influenced by proximity to specialty research and service centers.

However, demographic factors, alone, may not explain the increased numbers of cases in these clusters. The authors propose that other factors, including environmental exposures, may play a role but warrant further investigation to understand their contribution to autism etiology.

"Examination of clusters can help us understand the factors that have led to in increase in autism prevalence over time," said Autism Speaks' Chief Science Officer, Geraldine Dawson, Ph.D.

Similar to the UC Davis study, a second study using data collected by the California DDS, which was conducted by investigators from Columbia University identified a significant cluster of increased risk for autism as well as a set of lower risk clusters in and around Northern Los Angeles. However, rather than looking at incidence of cases by DDS Regional Center catchment area, Mazumdar et. al. examined more than 11,500 autism cases among four million births by place of birth. This approach was used to avoid potential bias caused by parents moving to neighborhoods that improve access to specialty autism services.

"Our paper is different," Columbia University co-author Peter Bearman, Ph.D. said. "It identifies a large and stable primary cluster for autism based on residence at birth that is observed over many years and which crosses over regional center boundaries."

The primary cluster accounted for approximately 3 percent of new autism cases in California each year from 1993-2000. While the primary cluster was found to be in an area of higher socioeconomic status than comparison regions, this factor did not fully account for the increased cases of autism in this region.

"Our findings point strongly to the idea that a local process is associated with the increased risk of autism. Such a local process could be either an environmental factor or a social influence factor, or both," noted Bearman.

Both publications add to the growing body of evidence suggesting that the distribution of autism cases differs across different regions. While the exact causes of the clustering in California are unknown, ongoing studies exploring environmental exposures and social factors will be useful in providing answers.