Over the past decade, a number of studies have suggested that pesticide exposure may double a person's risk of developing Parkinson's disease. Now, for the first time, a family study has been published that reinforces that link.
In the March 28 edition of the journal BMC Neurology, researchers from Duke University Medical Center and the University of Miami Miller School of Medicine reported that people with Parkinson's disease were 61 percent more likely to report direct contact with pesticides—particularly insecticides and herbicides—than their unaffected relatives.
The researchers interviewed 319 people with Parkinson's and more than 200 relatives. Then, they combined data about the frequency of pesticide exposure (how many days per year, on average) and the duration (how many years a person was exposed to pesticides) to get a rate of cumulative exposure.
“We found a dosage effect,” says Dana Hancock, Ph.D., currently a research fellow with the National Institute of Environmental Health Sciences. Dr. Hancock co-authored the study when she was a graduate student at Duke University. “Increasing levels of exposure to pesticides were associated with an increasing risk of Parkinson's,” she notes. People who were exposed through direct pesticide application on more than 10 days a year were 2.07 times more likely to have Parkinson's disease than those who were never exposed.
The fact that this study was done in families is important, says Dr. Hancock. “Since you're matching families, cases and controls are well-matched on unmeasured genetic factors, as well as shared environmental factors,” she explains. (“Cases” are the patients in a study who have a particular condition; “controls” are healthy people who are studied in comparison.) “As a child, you share a much more similar environment with your sister as compared to the rest of the population. It's a way to get around introducing bias due to factors that may influence pesticide exposure and Parkinson's that we currently don't know about.”
The study also began to tease out particular categories of pesticides associated with increased rates of Parkinson's, something that most previous studies have not done. “Pesticides are a very complex group,” says Dr. Hancock, suggesting that some people may be vulnerable to developing disease as a result of exposure to one type of pesticide, while others may be vulnerable to a different one. Some of the most common chemicals found in pesticide classes associated with Parkinson's were the agricultural insecticide chlordane, the now-banned insecticide dichloro-diphenyl-trichloroethane (DDT), the home-and-garden insecticide chlorpyrifos, the household insecticide diazinon, and the agricultural insecticide malathion.
Dr. Hancock and other experts cautioned against jumping to conclusions. If you have Parkinson's disease today and sprayed Raid in your kitchen periodically a few years ago, that doesn't mean it caused the disease. “Most of us think that Parkinson's is caused by some combination of lifetime exposures to various things, both natural and man-made, superimposed on your genetic makeup,” says J. Timothy Greenamyre, M.D., Ph.D., professor of neurology and director of the Pittsburgh Institute for Neurodegenerative Diseases at the University of Pittsburgh. He has also researched the Parkinson's-pesticides connection and found that the agricultural pesticide Rotenone induced Parkinson's-like symptoms in rats.
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“Even if you have a strong exposure history, in any individual case, no one's going to be able to tell you with absolute certainty what caused your disease,” Dr. Greenamyre says.
That said, you can limit your exposure to pesticides and herbicides with some simple safety precautions. “Wash your fruits and vegetables—that's probably one of the most important things you can do,” advises Dr. Greenamyre. Don't rely on buying organic: Rotenone is derived from tropical plant extracts and frequently used in organic gardening. “And if you must use pesticides and herbicides, wear protective gear when applying them, such as masks and rubber gloves, and limit their use indoors, where the fumes aren't going to dissipate as well.”
Much more research still needs to be done. “What we don't have are actual biomarkers [a physical sign found in the body, such as changes seen on MRI images or particular proteins in blood or urine] for pesticide exposure, so we have no way of biologically confirming which pesticides people have actually been exposed to, which is very important,” Dr. Hancock says.