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Pesticides were tied to Parkinson's disease in the Rocky Mountain and Great Plains region of the U.S., the MAP-PD study showed.

Of 14 pesticides strongly associated with Parkinson's in the area, simazine, atrazine, and lindane had the strongest relationship, reported Brittany Krzyzanowski, PhD, of Barrow Neurological Institute in Phoenix, and co-authors, in an abstract released in advance of the American Academy of Neurology annual meeting.

The region included parts of Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, Utah, and Wyoming.

Counties with the highest decile of simazine application had a 36% (95% CI 32%-41%) higher relative risk of Parkinson's disease compared with counties that had the lowest decile of exposure. Relative risks were 31% (95% CI 26%-35%) greater for atrazine and 25% (95% CI 21%-29%) greater for lindane in counties with the highest versus lowest decile of application.

A modest dose-response relationship between Parkinson's risk and all three pesticides emerged, the researchers said. Results remained similar when findings were adjusted for potential confounders, including air pollution exposure.

"We have increasing evidence that environmental toxicants in our food, water, and air are contributing to the rise of Parkinson's disease," said E. Ray Dorsey, MD, of the University of Rochester in New York, who wasn't involved with the study.

Pesticides, especially paraquat, have long been tied to Parkinson's, Dorsey noted. This study "extends the evidence linking fat-soluble organochlorine pesticides lindane and the weed killers atrazine and simazine to Parkinson's disease," he told MedPage Today.

"This latest research -- combined with epidemiological studies finding a near perfect correlation between use of pesticides in rural areas and prevalence of Parkinson's disease, and animal studies demonstrating dopaminergic neuron loss -- lead us to the conclusion that much of Parkinson's may be preventable," he said. "We have that information in hand. We should seek to create a world where Parkinson's is increasingly rare, not common."

Krzyzanowski and colleagues conducted a nationwide study of 21,549,400 Medicare beneficiaries who were 67 or older in 2009 and 465 pesticides identified in the . The outcome variable was county-level standardized relative risk; independent variables were estimates of average annual pesticide application from 1992 to 2008.

The researchers mapped the nationwide association for 65 pesticides and used linear regression to quantify the nationwide relationships between Parkinson's risk and pesticides. They included county-level air pollution, rural or urban residence, and median income as potential confounding factors in their analysis.

"Our methods enabled us to identify parts of the nation where there was a relationship between most pesticides and Parkinson's disease and subsequently pinpoint where the relationship was strongest so we could explore specific pesticides in that region," Krzyzanowski said in a statement.

"It's concerning that previous studies have identified other pesticides and herbicides as potential risk factors for Parkinson's, and there are hundreds of pesticides that have not yet been studied for any relationship to the disease," she noted. "Much more research is needed to determine these relationships and hopefully to inspire others to take steps to lower the risk of disease by reducing the levels of these pesticides."

A study limitation was that the analysis used county-level estimates about pesticide exposure as individual exposure data were not available, the researchers acknowledged.

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