June 15, 2006 -- Pesticides uprisk -- but only in men, a new study suggests.
Using chemicals to kill bugs or weeds -- either at work or at home -- more than doubled men's risk of getting Parkinson's disease. But these pesticides did not change women's risk of getting the disabling brain disease.
The finding, by Mayo Clinic researchers Roberta Frigerio, MD, Demetrius M. Maraganore, MD, and colleagues, appears in the June issue of the journal Movement Disorders.
"The risk was 2.4-fold: more than a doubling of the risk in unexposed men," Maraganore tells WebMD. "But when we looked at women only, even when we considered only exposed women, they had absolutely no increased risk. There is something biologically different between men and women when they are exposed to pesticides."
30 Years of Parkinson's Data
Prior studies of pesticide exposure and Parkinson's disease came up with conflicting results. One reason is that pesticide exposure doesn't cause Parkinson's disease in most people. The chemicals affect only people with as-yet-unknown genetic factors, says Lisa Opanashuk, PhD, assistant professor of environmental medicine at the University of Rochester in New York.
"We are not looking at these environmental factors as causes of Parkinson's disease, but as risk factors that combine with genetics and the normal process," Opanashuk tells WebMD. "Pesticides don't directly cause Parkinson's disease -- it is a combination of all those factors."
Another reason for the conflicting previous findings is that Parkinson's disease develops over a long period of time.
The strength of the Mayo study is that the Mayo Clinic has health records for people in Olmsted County, Minn., that go back for decades. Maraganore's team was able to identify all county residents who developed Parkinson's disease between 1976 and 1995. This allowed them to interview 149 patients -- or family members of patients -- and 129 matched people without Parkinson's disease.
When the researchers looked only at men with occupational exposures -- such as farmers -- they did not find a link to Parkinson's disease. But when they included men exposed to pesticides via hobbies, such as gardening, the link became apparent.
Why Men, but Not Women?
It might seem that men simply get exposed to pesticides more than women. But that explanation doesn't fit the data, Maraganore says.
The other theory is that the genes that interact with pesticides to causeare on the X chromosome. Men have only one X chromosome; women have two. If an X-chromosome gene is defective in women, there is a 50-50 chance that the normal gene on the other X chromosome will be the active one.
"Women have only half of the exposure to a random X-chromosome gene variant as men," Maraganore suggests. "So women could be protected vs. men."
But women don't get a free ride. Some chemical exposures seem to put women at greater risk of Parkinson's disease than men, Opanashuk says.
"Women are not off the hook," she says. "There could be genetic differences that render women susceptible. PCBs have been linked to risk for Parkinson's disease. [A recent study] found a potential sex linkage, and women seem to be more susceptible."
As scientists learn more about how chemicals affect Parkinson's risk, they get sorely needed clues about the cause of this mysterious disease.
Meanwhile, Maraganore says everyone should closely follow safety precautions when using pesticides.
"We aren't saying people should not use pesticides, but this argues for regulatory bodies to make sure these agents are safe and to make sure the reasons for their use are clearly stated," he says. "And if I am what I eat, am I going to be a healthy person if I eat foods contaminated with possible harmful substances?"