Are Pesticides Causing Parkinson's Disease?

By Robin Marantz Henig, OnEarth Magazine. Posted June 19, 2009.

In 1982, six young people showed up in emergency rooms in northern California unable to move, speak, or eat on their own. This time the detective work was accomplished much more rapidly. It took only a few weeks for William Langston, then a neurologist at the Santa Clara Valley Medical Center in San Jose, to put the story together. The patients were all heroin users, and they had all used a batch of garage-concocted heroin that was contaminated with the chemical compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, or MPTP. "At the molecular level, very little separated a toxic chemical from a harmless one," Langston and John Palfreman wrote in their book, The Case of the Frozen Addicts. But that small chemical change was enough to turn the designer heroin into one of the most potent known neurotoxins, virtually wiping out all the cells of the substantia nigra, which produces dopamine. MPTP has a molecular structure very much like the herbicide Paraquat. So the "frozen addicts" were taken as further evidence that both pesticide exposure and MPTP could be related to the same kind of dramatic brain damage.

The tragedy of the addicts (who recovered some function with L-dopa treatments) had a silver lining. MPTP turned out to be an excellent way to create parkinsonian symptoms in experimental animals -- a necessary first step in the search for a cure.

Scientists also observed these symptoms in groups of people exposed to unrelated compounds, such as heavy metals. One in particular, manganese, was implicated in a 2006 study of residents of the steelmaking town of Hamilton, Ontario, who had a higher-than-expected rate of Parkinson's disease. Investigators attributed this to the manganese content of particulate air pollution from factory emissions. It turns out that manganese is an ingredient in the widely used fungicide Maneb.

But pesticides remain the clearest culprit. One study found that in the brains of people who had died of Parkinson's disease, the substantia nigra had higher levels of Dieldrin (an organochlorine pesticide no longer approved for use in the United States) and of lindane (an insecticide occasionally still used to treat scabies and lice) than did the brains of people who had died of other causes. Laboratory studies have also provided important clues to the connection between pesticides and brain damage. When human brain cells are grown in culture and exposed to a variety of chemicals, several widely used pesticides -- in particular, Paraquat and Rotenone, a natural pesticide approved for use in organic foods -- have been shown to cause increased levels of alpha-synuclein, a protein in the substantia nigra, similar to the levels that are seen in people with Parkinson's disease.

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Robin Marantz Henig is a contributing writer to the New York Times Magazine. She is the recipient of a 2009 Guggenheim Foundation fellowship, and is the author of eight books on science, including The Monk in the Garden (Houghton Mifflin), a finalist for the National Book Critics Circle Award.

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