A second study shows that animals exposed to paraquat have a build-up of a protein called alpha-synuclein in their brains. This protein has been linked to Parkinson's in the past.

A third piece of the puzzle shows that this buildup of protein kills the same brain cells affected in Parkinson's.

"All of these pieces really look like they are coming together now," Dr. William Langston, founder of the non-profit Parkinson's Institute, told Reuters.

Langston and colleagues said they were energized by research presented at the Parkinson's Disease Environmental Research meeting in Monterey, California, earlier this month.

Parkinson's disease, which affects more than 1 million patients in the United States, is marked by the death of brain cells that produce dopamine.

Dopamine is a neurotransmitter, or message-carrying chemical, associated with movement. Drugs can delay symptoms for a while but there is no good treatment and no cure.

Farm workers are at especially high risk but links to pesticides have been difficult to document because years usually pass between a person's exposure to pesticides and the development of the disease.

Dr. Beate Ritz of the University of California at Los Angeles and Dr. Caroline Tanner of the Parkinson's Institute looked at 80,000 people in Iowa and North Carolina and found farm workers exposed to paraquat had twice the expected risk of Parkinson's over their lifetimes.

Exposure to another pesticide called dieldrin also raised the risk, the study, funded by the National Institute of Environmental Health Sciences, found.

A second study found similar effects in farm workers in central California.

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What made the studies especially important was that pesticide exposure could be carefully documented through records of pesticide purchase, Langston said. Details will be published in a scientific journal later.

Dr. Donato Di Monte of the Parkinson's Institute gave paraquat to laboratory animals and found it caused a buildup of alpha-synuclein in the brain that killed the same neurons affected by people with Parkinson's disease.

"This increase in alpha-synuclein in the brain could be the missing link between the exposure to this agent and how this agent causes the disease," Di Monte said in a telephone interview.

"Maybe being exposed to paraquat may not be enough to cause the disease but increases the probability the disease may develop," Di Monte said.

Langston and Di Monte said inflammation also could be a factor.

"Give an animal a compound that creates a marked inflammation response in the body ... and months later the animal loses cells in same area of the brain that is associated with Parkinson's," Langston said.

"This suggests that systemic inflammation may somehow sensitize the brain."

Multiple concussions, which can cause inflammation in the brain, raise the risk of Parkinson's, Langston said.

Two other groups of people that have a higher-than-average risk of Parkinson's are health workers and teachers.

"At first glance that doesn't make sense," Langston said.

But both do have something in common -- frequent exposure to viruses.