Researchers have discovered 10 autism clusters in California, mainly in areas where parents have higher-than-average education levels.
The findings, released this week by UC Davis, provide another important clue for solving the autism mystery, but leave many questions unanswered.
The clusters have autism rates nearly double those of surrounding areas. Three are in the Bay Area:
Researchers identified no clusters in the East Bay. Others are in Southern California or the Central Valley. They also identified two potential clusters, one of which is in Modesto.
A San Diego-area cluster has autism rates of 61.2 per 10,000 births, compared with 27.1 per 10,000 births in the surrounding region.
Parents in the cluster neighborhoods are more likely to be white, have higher education levels, and be slightly older, said Irva Hertz-Picciotto, a study author and leading autism researcher at the UC Davis MIND Institute.
These are all factors that other studies have shown put children at higher risk of being diagnosed with autism.
Because the clusters have similar demographics, "what we can say is that it's unlikely that these clusters are due to some environmental contamination in these neighborhoods," Hertz-Picciotto said.
It is probably not a nearby toxic waste site or polluting manufacturer that is to blame, she said.
Instead, it may be a more widespread environmental factor, such as household products that are used by families with higher education levels, or something else that's a part of their lifestyles.
Or it may be that such families have better access to specialists and know how to navigate the system to get their child diagnosed with autism, Hertz-Picciotto said. If that is the case, surrounding areas could have more undiagnosed cases of autism.
Several of the clusters are near major autism treatment centers. A Kaiser researcher who was not involved with the study called the findings interesting and said this should help shape future research. She noted that several studies have now shown a link with higher education levels and older parents.
"Is it a biologic factor, or certain exposures that they may have more of?" asked Lisa Croen, director of the autism research program at the Kaiser Permanente Division of Research in Oakland. "That's the big unknown."
Autism is a neurodevelopmental disorder often marked by an inability to communicate with others and repetitive behaviors such as hand-flapping.
It is considered a lifelong condition that typically develops by the time a child is 3.
UC Davis researchers examined the 2.5 million births in California from 1996 to 2000. Nearly 10,000 of these children were later diagnosed with an autism spectrum disorder.
Researchers mapped the children based on where their mothers lived at the time of their births, and used sophisticated techniques to check for clustering.
The study included only those autistic children who are signed up to receive services through regional centers associated with the state Department of Developmental Services.
The findings were published online this week in the journal Autism Research.
Many researchers say that some children are genetically susceptible to autism, then encounter one or more environmental exposures that trigger the disorder.
Both Hertz-Picciotto and Croen are involved in studies to pinpoint what may be contributing to a rising number of diagnosed cases throughout the United States.
Researchers are collecting specimens from children to measure their environmental exposures and to check for genetic factors.
They are also following women who have one autistic child and become pregnant with a second child. Such women are more likely to have a second child diagnosed with the disorder. Scientists hope to find clues by following the second child's development from before birth and during the first few years.
Hertz-Picciotto said the MIND Institute is analyzing dust samples from the homes of autistic children to check for the presence of pesticides, flame retardants and other substances.
Though it is unlikely that a single, neighborhood pollutant will be to blame, Hertz-Picciotto said she believes that when the autism mystery is solved, it will involve more than genetics.
"We will be finding environmental factors," she said, "but they're not necessarily related to where you live."
Contact Sandy Kleffman at 925-943-8249.