The evidence that environmental factors play a role in Parkinson's disease is growing. The largest-ever epidemiological study of the ailment, published online in the journal Neuroepidemiology and reported yesterday by Yale Environment 360, shows that the incidence of the illness is extremely high in many parts of the Northeast and Midwest.

"These are the two regions of the country most involved in metal processing and agriculture," says Dr. Allison Wright Willis, the paper's lead author and an assistant professor of neurology at Washington University School of Medicine in St. Louis, "and chemicals used in these fields are the strongest potential environmental risk factors for Parkinson's disease that we've identified so far."

The study was based on data from 36 million Medicare patients aged 65 and older and found numerous areas in the Northeast and Midwest where 14 percent or more of the population suffers from the neurodegenerative condition. Parkinson's causes tremors, stiffness, and mood and behavioral changes.

Many regions of the West, as well as Alaska, had extremely low rates of the disease, the researchers found.

The study is yet more evidence of the link between Parkinson's and pesticides, which was reported by science writer Robin Marantz Henig in OnEarth magazine last summer. Henig acknowledges that it's tough to prove a cause-and-effect relationship between neurotoxins and the disease ("there will probably never be a smoking gun," she writes) but cites a wealth of population studies and other scientific evidence that have produced a steadily mounting consensus about such a connection.

A January 2009 consensus statement from CHE (the Collaborative on Health and the Environment), in collaboration with the Parkinson's Action Network, a patient advocacy group, found that there was "limited suggestive evidence of an association" between pesticides and Parkinson's, and between farming or agricultural work and Parkinson's. This followed by just a few months the publication of Environmental Threats to Healthy Aging, a report co-authored by the Science and Environmental Health Network, a consortium of advocacy groups based in Ames, Iowa; it included a summary of 31 population studies that have looked at the possible connection between pesticide exposure and Parkinson's. Twenty-four of those studies, according to the report, found a positive association, and in 12 cases the association was statistically significant. In some studies, the group found, there was as much as a sevenfold greater risk of Parkinson's in people exposed to pesticides. In addition, in April 2009, scientists at the University of California, Los Angeles (UCLA), published a provocative study connecting the disease not only to occupational pesticide exposure but also to living in homes or going to schools that were close to a pesticide-treated field.

"Taken together," Henig writes, "30-plus years of research add up to an increasingly persuasive conclusion: exposure to pesticides and other toxins increases the risk of Parkinson's disease, and we are only now beginning to wrestle with the true scope of the damage."

Now the Washington University School of Medicine study can be added to that pile of evidence. According to Willis, its lead author, genetic factors explain only a small percent of Parkinson's cases. Environmental factors -- including prolonged exposure to herbicides and insecticides used in farming, as well as metals such as copper, manganese, and lead -- appear to be more common contributors to developing the disease.

Read more about the evidence for a link between Parkinson's and pesticides here.

Map: The largest U.S. study of the epidemiology of Parkinson's disease shows the highest prevelance (13,800 cases or more per 100,000 residents ages 65 and older) in red. Lower prevalence rates are progressively indicated by orange, yellow, light green and green. Neuroepidemiology/S. Karger AG