UC Davis researchers find California autism clusters, but the cause is a bit of a surprise

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UC Davis researchers searching for autism clusters in hopes of finding an environmental cause for the disorder have identified 10 clusters around the state, but the source of the clusters is not exactly what they expected. The clusters, including five in metropolitan Los Angeles and one in San Diego, are centered on regional developmental services centers in areas with highly educated parents, primarily Caucasians, with high incomes. In short, what they found were clusters of increased diagnostic rates for autism. In one respect, the results were not surprising because it has long been known that high-income, highly educated white parents are more likely to have their children diagnosed with autism and more likely to have them diagnosed at an early age.

‘Looking at clustering is often a way to uncover leads about problems in the environment,’ said epidemiologist Irva Hertz-Picciotto, the senior author of the study. ‘Mapping has a long history of being a way to get clues’ about causes of disease. She was, indeed, surprised by the findings -- ‘not that there are clusters with parents with higher education, but that it was so consistent across the board.’ In virtually every cluster they identified, the rate of autism was about twice as high within the cluster as in adjacent regions.


Hertz-Picciotto and her colleagues obtained birth records for 2,453,717 children born in the state between 1996 and 2000. By 2006, the children had all reached at least age 6, the age by which diagnosis of autism is generally accomplished. State records showed that about 9,900 autism cases were in the records of the Department of Developmental Services. The team reported in the journal Autism Research that they identified 10 clusters of autism among the 21 regional offices of the department and two potential clusters. The clusters were primarily in the high-population areas of Southern California and, to a lesser extent, in the San Francisco Bay area.

The clusters were:

-- The Westside Regional Center in Culver City, which serves western Los Angeles County, including Culver City, Inglewood and Santa Monica.

-- The Harbor Regional Center, headquartered in Torrance, which serves southern Los Angeles County.

-- The North Los Angeles County Regional Center, in Van Nuys, which serves the San Fernando and Antelope valleys. Two clusters were in this region.

-- The South Central Regional Center in Los Angeles, which serves Compton and Gardena.

-- The Regional Center of Orange County in Santa Ana.

--The Regional Center of San Diego County, which serves San Diego and Imperial counties.

-- The Golden Gate Regional Center in San Francisco, which serves San Francisco, Marin and San Mateo counties. There are two clusters in this area.

-- The San Andreas Regional Center in Campbell, which serves Santa Clara, Santa Cruz, Monterey and San Benito counties.


Increased incidence was also noted in two other regions, the Central Valley Regional Center in Stockton and the Valley Mountain Regional Center in Fresno. The incidence of autism was not as high in those regions, however.

Because the team analyzed birth locations and not the location of diagnosis, it is highly unlikely that the parents moved into the cluster regions to seek care, Hertz-Picciotto said.

‘In the U.S., the children of older, white and highly educated parents are more likely to receive a diagnosis of autism or autism spectrum disorder,’ said lead author Karla C. Van Meter, who was a graduate student when the data were collected but is now at the Sonoma County Department of Public Health. ‘For this reason, the clusters we found are probably not a result of a common environmental exposure. Instead, the differences in education, age and ethnicity of parents comparing births in the cluster versus those outside the cluster were striking enough to explain the clusters.’

The team is now looking elsewhere for possible causes. Some previous studies have hinted that exposure to pesticides may play a role and a study in Texas showed that exposure to mercury in the environment --but not in vaccines -- could be a causative agent. ‘We are casting a wide net, looking at everything we can--pesticides, medical conditions in the mother, medications, flame retardants, etc.,’ Hertz-Picciotto said. The problem, she conceded, is that, if the exposure is truly widespread, then linking it to autism will be very difficult.

-- Thomas H. Maugh II

Credit: UC Davis M.I.N.D. Institute