UCLA researchers have provided strong new evidence linking at least some cases of Parkinson's disease to exposure to pesticides. Researchers have suspected for some time that pesticides may cause the neurodegenerative disorder, and experiments in animals have shown that the chemicals, particularly the fungicide maneb and the herbicide paraquat, can cause Parkinson-like symptoms in animals. But proving it in humans has been difficult because of problems in assessing exposure to the agents.
Parkinson's is a disorder of the central nervous system that often impairs the sufferer's motor skills, speech and other functions. It is not fatal of itself, but complications often are. The disease has been recognized since the Middle Ages but became more prevalent in the 20th century. As many as 180 of every 100,000 Americans develop it.
To explore a potential connection to pesticides, epidemiologist Beate Ritz of UCLA and her graduate student Sadie Costello, now at UC Berkeley, studied public records of pesticide applications in California's Central Valley from 1974 to 1999. Every application of pesticides to crops must be registered with the state. Working with Myles Cockburn of USC, they developed a tool to estimate pesticide exposure in areas immediately adjacent to the fields.
They then identified 368 longtime residents who lived within 500 yards of fields where the chemicals had been sprayed and compared them to 341 carefully matched controls who did not live near the fields.
They reported in the current issue of the American Journal of Epidemiology that people who lived next to fields where maneb or paraquat had been sprayed were, on average, about 75% more likely to develop the disease. But those who developed the early-onset form of the disease -- contracting it before the age of 60 -- had double the risk of contracting it if they were exposed to either maneb or paraquat alone and four times the risk if they were exposed to both. In most cases, the exposure occurred years before the onset of the disease. Exposure to other pesticides did not appreciably alter the risk.
"The results confirmed two previous observations from animal studies," Ritz said. "One, that exposure to multiple chemicals may increase the effect of each chemical. That's important, since humans are often exposed to more than one pesticide in the environment. And second, that the timing of the exposure is also important."
-- Thomas H. Maugh II