Preliminary results from a study of thousands of farmers in Iowa and North Carolina suggest that exposure to several crop pesticides may be linked to the development of Parkinson’s disease.
Doctors have observed that the neurodegenerative disease is more common in people who live in farming communities, leading them to speculate that exposure to pesticides increases a person?s risk of developing the disease.
“The idea of some link to environmental toxins is becoming pretty well-accepted, but the exact ones and how much you have to be exposed to be at risk of Parkinson’s disease isn’t clear,” says Dr. Robin Brey, of the University of Texas Health Sciences Center in San Antonio.
In the mid-1990s, a team of researchers led by Dr. Samuel Goldman of the Parkinson?s Institute in Sunnyvale, California surveyed over 20,000 farmers, 55 of whom reported having Parkinson’s disease, about their health history and exposures to specific pesticides.
Dr. Freya Kamel, of the National Institute of Environmental Health, says the study found the use of several crop pesticides was more common among the Parkinson’s group than the non-affected farmers. The pesticides used were dieldrin, paraquat, maneb, rotenone and a class of insecticides known as organochlorines. People with Parkinson’s were 80% more likely to have been exposed to dieldrin. The other pesticides appeared to carry an increased risk ranging from 20% pto 60%.
These pesticides have been shown in test-tube studies or laboratory animal experiments to have toxic effects on certain brain cells, which can lead to Parkinson?s-like symptoms of tremor, rigidity, slowness of movement and imbalance. Dieldrin, an insecticide, is now banned in the United States. Rotenone, a pesticide, has been banned for agricultural use though it is still used to kill unwanted fish in reservoirs. Paraquat, an herbicide, and maneb, a fungicide, are still used by farmers.
The study also found high-exposure methods of application, where the pesticide was likely to get on the skin or be inhaled, also were more commonly reported by the farmers with Parkinson’s.
But researchers do not think that exposure to these pesticides necessarily results in Parkinson’s disease. “We don’t think any one compound causes Parkinson’s disease because then we would expect to see more clusters of cases,” says Goldman. “So it has got to be a whole lifetime of mild insults that set off a degenerative cascade in the brain.”
Most experts think that a complex interaction between a person’s genes and environmental exposures causes Parkinson’s.
Kamel says the data does not prove a relationship because it included only a small number of Parkinson?s patients and relied on self-reports of the disease, which may be inaccurate. “It’s a very dirty study at this point,” she says. “Self-reports have a tremendous potential for bias. Many patients may have tremor and say they have Parkinson’s. The diagnosis clearly needs to be confirmed.”
Kamel says the next step will be to identify more Parkinson’s patients, have neurologists confirm the diagnosis, and test their blood and homes for pesticide levels, in hopes of definitively answering the question of the role of pesticide exposure in Parkinson’s.
Researchers also report a potential link between eating a lot of foods that contain compounds known as isoquinolines (IQs), such as chocolate, cheese, milk and wine, and Parkinson’s disease. IQs have been shown in laboratory animals to inhibit cellular function and lead to Parkinson?s-like symptoms.
Goldman asked 72 pairs of twins in which one had Parkinson’s and the other did not about their dietary habits in the 10 years prior to one of them developing the disease. He found those who ate a lot of chocolate (two to three candy bars per week) had more than a three-fold increased risk of having Parkinson?s than those who ate less. He also found a smaller link with wine and with a measure of total IQ consumption.
“The population is fairly small and the data is extremely retrospective,” he says. “But in basic science research there’s probably a reason to suspect these compounds.”
Brey says dietary histories are often unreliable. She says, “People don’t remember what they’ve eaten years ago and if you had bad information you are going to have erroneous results. I would hate for someone to stop eating chocolate and cheese because of this study.”
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