Are Pesticides Causing Parkinson's Disease?

This story originally appeared in OnEarth Magazine.

Jackie Christensen was 32 when her body began to betray her. She had just returned to work after the birth of her second son and when she tried to type, two fingers on her left hand refused to cooperate. "They wouldn't go where I would want them to on the keyboard," says Christensen, who at the time -- it was 1997 -- was co-director of the food and health program at the Institute for Agriculture and Trade Policy, a Minneapolis think tank. "I also had what they frequently call frozen shoulder, with a very low range of motion in my left arm."

The first neurologist Christensen went to responded flippantly to her suggestion that she might have multiple sclerosis, which she had self-diagnosed because of her relatively young age and the fact that she was female. "If you want me to write that down, I will," she remembers him saying, refusing to pursue the matter further. A second neurologist thought it was all in Christensen's mind and referred her to a psychiatrist. Over the next several months, her symptoms got progressively worse, and she finally consulted neurologist number three. His startling diagnosis: Parkinson's disease.

"I thought, 'I can't have Parkinson's because I'm not old,'" Christensen recalls. But a trial of the standard treatment, a drug called L-dopa, seemed to work. Based on that clinical observation, the diagnosis was confirmed. This was in 1998, when Christensen was not quite 35, and she has been on L-dopa, with varying degrees of success, ever since.

Why did a disease that usually affects people in their sixties and seventies, and that affects men more often than women, strike this vibrant young mother? Christensen, a lifelong environmental activist, suspected an environmental cause -- not only because she was politically inclined to, but because she knew that accumulating scientific information was pointing in that direction. In the past few years, Christensen has been part of a movement exploring a possible connection between exposure to environmental toxins -- in particular, the organophosphate pesticides -- and Parkinson's disease, through her work with the Collaborative on Health and the Environment, a national network of advocacy and scientific organizations. She is co-founder of CHE's working group on Parkinson's Disease and the Environment.

A cause-and-effect relationship between environmental neurotoxins and Parkinson's is difficult to prove. As with many other scientific efforts to establish disease causation through population studies, there will probably never be a smoking gun that settles things once and for all. Population studies can detect associations between certain suspected agents and diseases such as cancer, but it's hard to draw conclusions about what causes a disease from studies that can register only correlations. In the case of Parkinson's and the environment, however, there has been a steadily mounting consensus about such a connection, and the pace has quickened in the past year or so.

A January 2009 consensus statement from CHE, 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.