http://viartis.net/parkinsons.disease/toxic.causes.htm

Rotenone is an insecticide that has the potential to cause Parkinson's disease. Insecticides are also known to affect well water. Rotenone is commonly used in powdered form to treat parasitic mites on chickens and other fowl, and so can be found in poultry. Rotenone is produced by extraction from the roots, seeds, and leaves of certain tropical legumes. Rotenone inhibits tyrosine hydroxylation, which is essential for the formation of dopamine. So rotenone could cause Parkinson's disease by lowering dopamine levels. When given intravenously to mice, rotenone has been demonstrated to cause a model of Parkinson's disease. Rotenone toxicity is also caused by complex I inhibition, depletion of cellular and oxidative damage. These processes cause loss of midbrain dopaminergic neurons, leading to depletion of dopamine in the brain.