California researchers who first established a link between two commonly used pesticides and Parkinson's disease have found a third crop-enhancing chemical -- ziram -- that appears to raise the risk of developing the movement disorder. And they have found that people whose workplaces were close to fields sprayed with these chemicals -- not just those who live nearby -- are at higher risk of developing Parkinson's.
In a study published in the European Journal of Epidemiology, a team of researchers led by UCLA neurologist Dr. Beate Ritz found that exposures to the trio of pesticides were actually higher in workplaces located near sprayed fields than they were in residences. And the combination of exposure to all three pesticides, which act in different ways to harm brain cells involved in Parkinson's disease, appears to be cumulative, the team led by Ritz concluded.
The researchers estimated exposures to the three chemicals that 703 study participants would have had between 1974 and 1999 while living and working in California's agriculturally rich Central Valley. Of those, 362 participants already had been diagnosed with Parkinson's, and the remainder had no sign of the disease. Among participants who had worked for long periods near fields in which all three chemicals were used on crops, rates of Parkinson's disease were three times higher than among subjects whose exposure was less intensive.
In animal studies conducted as part of the research on agricultural chemicals and Parkinson's disease, the researchers found that ziram was powerfully destructive to neurons that use the transmitter chemical dopamine to send messages. These brain cells are the ones that die off in regions of the brain that govern motor function, causing the tremors, unsteady gait and difficulty initiating movement that are the hallmarks of Parkinson's.
UCLA neurologist Jeff Bronstein, one of the study's authors, said it "demonstrates that exposure to ziram is associated with a significantly increased risk of developing PD."