UCLA researchers first found a link between Parkinson’s and the fungicide maneb and the herbicide paraquat – two chemicals commonly sprayed on crops to deter pests – in April 2009. They didn’t examine the farmers who worked with the pesticides, but rather those who lived near the fields. These residents had a 75 percent increase in Parkinson’s disease, researchers concluded.
This follow-up study, again conducted in California’s Central Valley, implicates a third pesticide, ziram, in the pathology of Parkinson’s disease and studied those who worked near the fields. The researchers found that the combined exposure to ziram, maneb and paraquat near any workplace increased the risk of Parkinson’s disease threefold, while combined exposure to ziram and paraquat alone was associated with an 80 percent increase in risk.
“Our estimates of risk for ambient exposure in the workplaces were actually greater than for exposure at residences,” said Dr. Beate Ritz, senior author and a professor of epidemiology at the UCLA School of Public Health. “And, of course, people who both live and work near these fields experience the greatest [Parkinson’s disease] risk. These workplace results give us independent confirmation of our earlier work that focused only on residences, and of the damage these chemicals are doing.”
Ritz noted that is the first study that provides strong evidence in humans that the combination of the three chemicals confers a greater risk of Parkinson’s than exposure to the individual chemicals alone. Because these pesticides affect different mechanisms leading to cell death, they may act together to increase the risk of developing the disorder.
Scientists knew that in animal models and cell cultures, such pesticides trigger a neurodegenerative process that leads to Parkinson’s, a degenerative disorder of the central nervous system that often impairs motor skills, speech and other functions and for which there is no cure. The disease has been reported to occur at high rates among farmers and in rural populations, contributing to the hypothesis that agricultural pesticides may be partially responsible.
From 1998 to 2007, the researchers enrolled 362 people with Parkinson’s and 341 controls living in the Central Valley and obtained participants’ historical occupational and residential addresses. Researchers employed their geographic information system-based tool, which Ritz and colleagues developed to estimate human exposure to pesticides applied to agricultural crops according to the distance from fields on which pesticides are sprayed. They estimated ambient exposures to the pesticides ziram, maneb and paraquat, both at work and home, from 1974 to 1999.
The results reaffirmed what their previous research had suggested, that the data “suggests that the critical window of exposure to toxicants may have occurred years before the onset of motor symptoms, when a diagnosis of Parkinson’s is made.”
The study results appear in the current online edition of the European Journal of Epidemiology.