In their recent Journal article, Palmer et al.1 showed that autism prevalence among schoolchildren is inversely related to the percentage of Hispanics in Texas school districts. The authors speculated that this may be a result of under-diagnosis of autism among Hispanic children, genetic vulnerability in non-Hispanic White children, or heightened exposure to unknown environmental factors in non-Hispanic White children. They identified the parameter of urban versus rural as having the highest risk ratio for autism. Importantly, in this study an urban versus rural designation was not correlated with the diagnoses of intellectual disability and learning disability, which did not include autism or autistic spectrum disorders. In two previous studies in Texas, the authors found the same parameter of urban versus rural as having the greatest risk for autism.2,3 However, in those studies the focus was on exposure to environmental mercury.
Increased risk for autism with increasing degree of urbanization has been identified as a significant factor in multiple geographically and ethnically diverse areas including Japan,4 Denmark,5 the United States,6,7 and in a meta-analysis of 40 population-based autism prevalence studies.7 This may suggest that potentially increased autism risk as a result of urbanicity is not ethnically specific but may be more directly related to urbanicity.
Hughtrain. Source: GapingVoid.com. Printed with permission.
There are an increasing number of reports of immune, autoimmune, and inflammatory aspects in the etiology of autism,9 with the suggestion that neonatal environmental microbial exposures, as they relate to urban versus rural demographics, may be important in a role for urbanization in the risk for autism.10 Increased disease risk with increased urbanization is a prominent feature in determining risk for asthma, allergic, inflammatory, and autoimmune disorders and is thought to be related to lowered microbial exposure in pregnancy and neonatal life.11 Immune sensitization in early development, rather than ethnic background, may be related to urban versus rural disease distribution and may play a role in autism disease risk.
Acknowledgments
This research was supported entirely by the Intramural Research Program of the National Institute on Aging.
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