Autism Linked to Higher Precipitation Levels

First Evidence Environmental Factors Play a Role in Autism Etiology

Caroline Cassels

November 11, 2008

November 11, 2008 — High levels of annual precipitation have been linked to a higher prevalence of autism, a finding that suggests there may be an environmental trigger for the condition in genetically vulnerable children.

Investigators at Cornell University, in Ithaca, New York, correlated autism prevalence rates and average rainfall and found a positive association between the disorder and the annual amount of precipitation in a given area.

"This is first study to show an association between autism and precipitation levels, but more broadly, it is the first peer-reviewed paper to find evidence of an environmental factor linked to autism," study investigator Sean Nicholson, PhD, told Medscape Psychiatry.

The study is published in the November issue of Archives of Pediatrics & Adolescent Medicine.

Thirty years ago it was estimated that 1 in 2500 children had autism. The most recent data from the Centers for Disease Control and Prevention (CDC) show that in 2007 the prevalence stands at 1 in 150. While some of this increase is likely attributable to better case ascertainment, the "possibility of a true increase in prevalence cannot be excluded," the authors write.

However, despite the increased prevalence, little is known about the causes of the condition. "Most people believe genetics combined with some sort of environmental factor is causing this increase, but there really has not been any evidence about what that environmental factor might be," said Dr. Nicholson.

Autism Rates Higher in Rainier Counties

Data from a 2003 autism prevalence survey conducted by the US Department of Education show that the 5 states with the lowest autism prevalence for 6- to 10-year-olds were New Mexico, Mississippi, Colorado, Oklahoma, and Tennessee. Those with the highest rates for this age group include Minnesota, Oregon, Indiana, Maine, and Massachusetts.

Similarly, CDC data also show that of 14 states studied, New Jersey, the second-northernmost state included in the study, had the highest autism prevalence rate, while the lowest was in Alabama, the southernmost state included in the study.

Together, these findings prompted investigators to consider weather as a possible environmental autism trigger.

To examine the relationship between precipitation rates and autism, the investigators looked at mean annual precipitation in counties in Oregon, California, and Washington between 1987 and 1999 and correlated them to autism prevalence rates by county. They then examined the amount of precipitation birth cohorts were exposed to when younger than the age of 3 years.

The Cascade Mountains run north to south across Washington and Oregon. Counties west of the Cascades get 4 times as much rain as those in the east. A comparison of autism rates between eastern and western counties showed autism rates were twice as high in the west.

Harm Inside or Benefit Outside?

While this finding was "interesting," said Dr. Nicholson, it was possible that it could be explained by a wide variety of potential differences between the 2 regions. However, he added, what was most compelling was the "fixed-effects analysis," which examined autism rates in successive birth cohorts in a given county over time and correlated them with precipitation levels.

"Essentially, we found that children born in a county in a period of time when there was a lot of rain by their own county standard subsequently had higher autism rates than those born in the same county during a time when precipitation levels were comparatively lower," said Dr. Nicholson.

"This suggests 1 of 2 possibilities — either there's something beneficial outside that helps prevent autism or something harmful inside that triggers it," he added.

The value of these findings, said Dr. Nicholson, is they are hypothesis generating. The researchers hope that the study will prompt other groups to construct narrower studies to help pinpoint an environmental culprit.

In an accompanying editorial, Noel S. Weiss, MD, DrPH, from the University of Washington, in Seattle, said that while the study findings are tentative, the authors have reported their results responsibly.

"They have made it clear that the message the public should take from their data regarding precipitation and autism is the same one suggested by an editorialist commenting on a recently observed modest association between prenatal exposure to cell phone use and behavior in childhood: No call for alarm, stay tuned," Dr. Weiss writes.

The study was supported by Cornell University. The authors report no relevant disclosures. Dr. Weiss reports no relevant disclosures.

Arch Pediatr Adolesc Med. 2008;162:1026-1034 Abstract, 1095-1096. Abstract

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