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Stanford Report, May 10, 2000

Residential pesticide exposure associated with risk of Parkinson's Disease  


A study of almost 500 people newly diagnosed with Parkinson's disease has found that home exposure to pesticides is associated with an increased risk of developing the disease. "This study is the largest yet of newly diagnosed individuals with Parkinson's disease and it is the first study to show a significant association between home pesticide use and the risk of developing Parkinson's disease," said neuroepidemiologist Lorene Nelson, PhD, associate professor of health research and policy. Nelson conducted the study with her colleagues Stephen Van Den Eeden, PhD, from the Kaiser Permanente Medical Care Program of Northern California and Caroline Tanner, MD, PhD, from the Parkinson's Institute in Sunnyvale.

The researchers questioned 496 people who had first been diagnosed with Parkinson's disease in 1994 or 1995 about past use of pesticides. They wanted to know if each patient had handled or applied insecticides in the home or garden, herbicides or weed killers in the garden, or fungicides to control mold or mildew in the home or garden. The researchers asked detailed questions about past pesticide use including when patients first were exposed and how frequently they came into contact with it. The researchers also queried the respondents on lifestyle factors such as cigarette, alcohol and coffee consumption. They then asked the same questions of 541 people without Parkinson's disease who served as age- and gender-matched control subjects.

When the researchers compared the lifetime histories of the patients and the control subjects they found that people who had handled or applied pesticides in the home or garden were more likely to develop Parkinson's disease than those who had not received such exposure. People exposed to in-home insecticides were 70 percent more likely to develop the disease than those who had not been exposed. The average amount of time that people reported being exposed to products in this category was 77 days. Exposure to garden insecticides carried a 50 percent increased risk of disease. Among herbicide users, risk of developing Parkinson's disease increased as the number of days that people were in contact with herbicides accumulated. Respondents who reported handling or applying those products for up to 30 days were 40 percent more likely to develop disease whereas respondents that reported higher levels of exposure (an average of 160 days) had a 70 percent increased risk of developing Parkinson's disease. Exposure to fungicides was not found to be a risk factor.

Nelson's study is believed to be the first study to specifically examine the link between Parkinson's disease and domestic exposure to pesticides. The researchers announced their findings May 5 at a presentation at the annual meeting of the American Academy of Neurology, in San Diego.

According to Nelson, the preliminary results from this study mirror what is already known about the increased risk of Parkinson's disease associated with occupational exposure to both insecticides and herbicides. Similarly, a previous study by other researchers found increased risk of the disease among hobby gardeners, although pesticide exposure as a possible cause of the increased risk was not investigated. But many more studies are needed before any conclusive statements can be made about the causes of Parkinson's disease, including any possible genetic influences on a person's probability of developing the disease, Nelson cautioned.

Damage to nerve cells in a part of the brain called the substantia nigra and subsequent deficiency in the neurotransmitter dopamine leads to the balance and movement difficulties characteristic of Parkinson's disease. Therefore, people exposed to chemicals that have a particular affinity for this region of the brain may be at particular risk for developing the disease, said Nelson. "Pesticides that contain chemicals that target dopamine-producing neurons in the substantia nigra may cause selective death of those neurons," she said. "If we could understand why these neurons are being killed in certain circumstances we can then try to prevent or treat the disease," she said.

But Nelson also stressed that the results of the study must be interpreted with caution. "No specific guidelines regarding avoidance of pesticides can be given at this time. However, this is an area of public health importance that needs to be pursued with additional studies," said Nelson. The study results also may serve as a reminder that people should read manufacturers' labels and heed the warnings noted, she added.

Funding for the study was provided by the National Institutes of Health. SR