Medically Reviewed by Jacqueline Brooks, MBBCH, MRCPsych From the WebMD Archives
Nov. 5, 2000 — The real causes of Parkinson’s disease remain a mystery, but scientists are slowly unravelling their secrets. Researchers from Emory University in Atlanta announced that they have been able to show that environmental factors are likely to play a major role in the development of the illness.
The disease, which affects about 1% of all people over the age of 65 and strikes an estimated 50,000 Americans each year, is characterized by rigid movement, reduced movement, and tremors. It is caused by a progressive decay of nerve cells in a section of the brain responsible for controlling motion. These cells contain a brain chemical called dopamine, which transports messages from one part of the brain to another in order to control muscle activity.
The scientists reported in the journal Nature Neuroscience thatthey were able to duplicate symptoms of the illness by injecting rats with a common organic pesticide, rotenone. The rodents also developed several Parkinson’s characteristics that previous studies had never shown: tremors that resemble Parkinson’s, gradual decay of dopamine nerve cells, and abnormalities in the brain.
Lots of studies “have looked at risk factors for Parkinson’s disease, and these kept turning up pesticides as a possibility. But we couldn’t find a smoking gun,” says Timothy Greenamyre, MD, PhD, an Emory University professor of neurology and senior author of the study.
The results of the research present scientists with the first evidence giving credence to the theory that persistent exposure to environmental pesticides may contribute to the illness in people.
Greenamyre tells WebMD that he began looking into this connection for several reasons. One: There is some evidence that those living in rural areas who are exposed more to pesticides than the rest of the population may have higher incidence of Parkinson’s.
Another reason has to do with structures in our cells called mitochondria, which are responsible for producing energy. Since the early 1980s, researchers have believed that mitochondria may not be functioning properly in Parkinson’s patients, and pesticides like rotenone interfere with mitochondria, so Greenamyre wanted to find out if it could actually cause Parkinson’s disease symptoms.
He believes the results of his team’s research will have a significant impact on further Parkinson’s research, especially in investigating other pesticides and potential treatments.
Greenamyre says he’d be willing to bet that other pesticides also influence the activity of the mitochondria and contribute to Parkinson’s. He also believes that it’s possible that some people may develop Parkinson’s from their diet.
Greenamyre bases this on the fact that rotenone comes from the roots, seeds, and leaves of plants in the pea family. Similar compounds are found in paw-paw, rhubarb, and the molds that are found in many foods.
Interestingly, the Environmental Protection Agency recently placed rotenone on its “Restricted Use Product” list. According to agency spokesman David Barry, this means it can no longer be sold over the counter. It can, however, be sold to people with a license to use it. Most state agricultural departments issue such licenses.
Barry tells WebMD that when a product is placed on the restricted use list, it means there has been proof “for whatever reason” that long-term use of the product causes harm to people or the environment. An agency official in Washington had not yet responded to an inquiry from WebMD about rotenone’s exact findings at the time of publication.
According to a 1996 report from a panel of toxicologists from five universities, rotenone causes skin and eye problems, sore throat, and vomiting, and breathing problems and seizures if inhaled at high doses. The pesticide is used to destroy fish for waterway management, for garden insect control, and for lice and tick control on pets.
Richard Dewey, MD, says that insecticides have been at the “top of the list” for a long time, but no previous study has been able to prove a link with Parkinson’s disease. Dewey is assistant professor of neurology at the University of Texas Southwestern Medical Center in Dallas.
“This could be a very important study because the Parkinson’s research community has long hypothesized that something in the environment was involved,” Dewey says. However, he warns that the similarities between effects of rotenone on rats and in humans depends on a couple of factors: Whether the rodents showed similar gradual death of the brain’s nerve cells involved in Parkinson’s, and whether the dosage given the animals was comparable to the amount to which a person would be exposed.
“Most research is pointing to Parkinson’s being a combination of genetics and environmental causes,” Dewey says. “Some people probably have a gene that makes them more susceptible to something to which they are exposed.
“That’s what has made it so hard to ferret out the cause. Each study on the agriculture risk factor has shown different results. It won’t be a simple toxic exposure disease,” Dewey says. “If it was, [scientists] would long ago have figured out what it is.”
