MERCED, Calif. — A thousand acres stretched before him as Gary Rieke walked briskly behind a harvester, the parched, yellow stalks of rice sweeping against his knees. Stopping to adjust a bolt on the machine, Rieke struggled to maneuver a wrench with his trembling fingers.
It was 1988, and Rieke was in his mid-40s, too young and too fit to feel his body betraying him. For two decades, he had farmed in the heart of the San Joaquin Valley, and he knew what he wanted his hand to do. But for some frustrating reason, it refused to obey.
Unbeknownst to Rieke, by the time he noticed the slightest tremor, some 400,000 of his brain cells had been wiped out. Like an estimated other 1 million Americans, most over 55, he had Parkinson's disease, and his thoughts could no longer control his movements. In time, he would struggle to walk and talk.
Rieke, who was exposed to weedkillers and other toxic compounds all his life, has long suspected that they were somehow responsible for his disease.
Now many experts are increasingly confident that Rieke's hunch is correct. Scientists have amassed a growing body of evidence that long-term exposure to toxic compounds, particularly pesticides, can destroy neurons and trigger Parkinson's in some people.
So far, they have implicated several pesticides that cause Parkinson's symptoms in animals. But hundreds of agricultural and industrial chemicals probably play a role, they believe.
Researchers don't use the word "cause" when linking environmental exposures to a disease. Instead, epidemiologists look for clusters and patterns in people, and neurobiologists test theories in animals. If their findings are repeatedly consistent, that is as close to proving cause and effect as they get.
Now, with Parkinson's, this medical detective work has edged closer to proving the case than with almost any other human ailment. In most patients, scientists say, Parkinson's is a disease with environmental origins.
Scientists are "definitely there, beyond a doubt, in showing that environmental toxicants have to be involved" in some cases of Parkinson's disease, said Freya Kamel, an epidemiologist with the National Institute of Environmental Health Sciences who has documented a high rate of neurological problems in farmers who use pesticides.
"It's not one nasty thing that is causing this disease. I think it's exposure to a combination of many environmental chemicals over a lifetime. We just don't know what those chemicals are yet, but we certainly have our suspicions."
