Parkinson’s Disease

Parkinson’s disease is a chronic and progressive disease for which there is no cure. It has been linked to rural residence, drinking well water and exposure to pesticides. Tests of specific pesticides are ongoing to better understand the disease and suggested risk factors.

Definitions

Named after a doctor, James Parkinson, who reported it in 1817, Parkinson’s disease is a chronic condition of the nervous system marked by progressive muscle tremor. It is often difficult to clearly diagnose because the symptoms develop slowly and its manifestations vary from person to person. For example, some people with Parkinson’s disease can be severely disabled while others may have only minor symptoms. A general diagnosis of “parkinsonism” can be used in which one or more of the clinical symptoms of Parkinson’s disease exist. Sometimes parkinsonism symptoms result from exposure to drugs and toxins but are reversible after the exposure stops.

Tests of specific pesticides are ongoing to better understand the disease and suggested risk factors.

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Parkinson’s Disease in Society

Men, in general, are more likely to be diagnosed with Parkinson’s disease. It is also more common in people 60 years of age and older. The disease is less common among cigarette smokers. Some early associations, such as with caffeine, have not been validated in robust clinical trials. It has been proposed that both environmental and genetic factors may lead to Parkinson’s disease.

There is no scientific consensus on a preventive course to reduce the risk of Parkinson’s disease. Therefore, efforts by advocacy groups focus on medical advances to reduce symptoms, improve quality of life and seek a cure.

Classification

Pesticides must be evaluated to determine their safety to humans, the environment and non-target wildlife species. Animal toxicology testing of pesticides has focused on several areas related to symptoms, neurotoxicity and Parkinson’s disease. Animal models do not provide sufficient evidence that environmental exposure to specific pesticides causes Parkinson’s disease in humans.¹ Work is being done to define the biological stages of the disease so that toxicologists and regulators can better evaluate its possible relationship to pesticides.

Evidence in Human Studies

The epidemiological evidence of Parkinson’s disease and pesticides is suggestive but conflicting. Some studies have indicated that individuals with Parkinson’s disease were more likely to live in rural areas and/or drink well water. There have also been cases of parkinsonism (symptoms) in agricultural workers.¹ The results vary for studies of pesticides per se and Parkinson’s disease.^2,3,4 The association of pesticides, in general, is most consistently observed in job-based settings.^5,6 However, another investigator recommended that agricultural exposures other than pesticides, such as drinking well water, should be more carefully assessed.⁷

The more scientists learn about the progression of the disease, the more they can learn how to prevent it. The role of environmental exposures, including pesticides, and the onset of Parkinson’s disease remains inconclusive.

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A European Food Safety Authority (EFSA) review reported that the interpretation of studies of Parkinson’s disease and pesticides was limited because the studies varied in design and quality.⁸ The EFSA report also observed that the results are “suggestive only” and that further studies need to be conducted to “disentangle the effect of specific pesticide classes or even individual pesticides.” Large ongoing studies of U.S. farmers and French agricultural workers⁹,¹⁰  are examples of efforts to address these questions.

While studies have looked at many pesticides, resulting in occasional “positive” findings, conclusions can only be drawn from consistent findings across multiple studies rather than from individual ones.

Currently, there are extensive research efforts to better treat Parkinson’s disease symptoms and to find a cure. The more scientists learn about the progression of the disease, the more they can learn how to prevent it. The role of environmental exposures, including pesticides, and the onset of Parkinson’s disease remains inconclusive.