Pesticide Action Network Updates Service (PANUPS)
Parkinson's disease is a progressive, incurable ailment. It is the second most common neurodegenerative disorder in the United States after Alzheimer's and affects more than a million people, including about 1% of the population over age 60. Despite years of research, neither definitive causes of the disease nor effective long-term treatments have been found.
Parkinson's disease begins when brain cells that produce dopamine -- a chemical that helps control muscles -- start to die. Symptoms become apparent only after 60 to 80% of the cells are dead. The disease is characterized by resting tremor, rigidity, slow movement, postural instability and progressively involuntary writhing movements, paralysis and an inability to talk or swallow.
Only about 10% of Parkinson's cases are genetic, with the remainder resulting from unknown factors such as environmental exposure or some interaction between genetic susceptibility and the environment.
Researchers believe that chemical exposures, particularly to pesticides, play a role in some cases of Parkinson's. Three lines of evidence suggest this finding. First, people who live in farming areas, especially those who drink well water, and have a history of exposure to pesticides are more likely to contract Parkinson's. Second, several studies have shown that those who die of Parkinson's disease have higher levels of organochlorine pesticides in their brains than the general population. Finally, in the early 1980s, a group of young people developed Parkinson's symptoms after taking an illegal drug called MPTP whose structure is similar to meperidine or Demerol. The structure of its metabolite is similar to the herbicide paraquat.
Although previous investigations only established an association between workplace pesticide exposure and Parkinson's, a study conducted last year at Stanford University showed that exposure to pesticides in homes and gardens may also increase the risk of developing the disease. The researchers interviewed 1038 people, including 496 who had recently developed Parkinson's, about their lifestyle habits and whether or not they had used or been exposed to insecticides, herbicides or fungicides in homes or gardens.
The study revealed that individuals who were exposed to pesticides in the home or garden were 70% more likely to develop Parkinson's than those who were not exposed. "In-home insecticide exposure showed the strongest association, but herbicide exposure in the garden was also associated with an increased risk of [Parkinson's disease]," concluded the researchers.
Although the mechanism by which Parkinson's is induced by pesticides -- if they do cause the disease in humans -- is not yet well understood, and the cause-effect relationship between pesticides and Parkinson's is still unclear, many studies on rats and mice have shown links between pesticide exposure and the development of Parkinson's-like symptoms.
One study at the University of California, Santa Cruz, showed that tiny concentrations of the pesticides rotenone, dieldrin, DDT, 2,4-D and paraquat cause reactions in the brains of mice that may accelerate the development of Parkinson's. Mixtures of metals and pesticides may have a synergistic effect in causing these reactions, the studies suggest.
Other studies have shown that rotenone causes the degeneration of nerve cells in the brain in rats. Rotenone is a natural compound derived from the roots of tropical plants. Generally considered relatively harmless to mammals, it is used to kill nuisance fish in lakes, insects in gardens, and fleas and ticks in pets. While some foresters bathe in it after working in the woods, people would most frequently be exposed to rotenone by ingesting residue in food or by handling it.
Another study at the University of Rochester showed that the combined exposure of the herbicide paraquat and the fungicide maneb -- applied to millions of acres of farmland each year -- are know to affect the neuro-transmitter network in mice and produce a pattern of brain disorders that are very similar to those found in humans with Parkinson's.
Lead researcher Deborah Cory-Slechta said that the findings could be a warning signal that current environmental investigations on the health effects of pesticides do not cover enough ground. She added that the findings would hopefully prompt the U.S. Environmental Protection Agency to investigate the potential hazards of exposure to combined pesticides.
Sources: Hileman, Bette, "The Environment and Parkinson's," Chemical & Engineering News, September 17, 2001; Higgins, Margot, "Pesticides linked to Parkinson's disease," Environmental News Network (ENN), January 11, 2001; Associated Press, "Study links pesticides, Parkinson's," November 6, 2000; Chubb, Lucy, "Pesticide exposure linked to Parkinson's disease," May 6, 2000, ENN.