BOSTON, June 26 — Exposure to pesticides increases the risk of Parkinson's disease by about 70%, researchers here reported.
Among more than 140,000 men and women followed through 2001 as part of a cancer prevention study, those who reported being exposed to pesticides or herbicides before 1982 had a 70% higher rate of Parkinson's disease 10 to 20 years after the initial exposure, reported Alberto Ascherio, M.D., Dr.Ph., of the Harvard School of Public Health, and colleagues.
The risk was highest among people with occupational exposure to pesticides, such as farmers and ranchers. There were no significant associations between Parkinson's disease and other occupational exposures, such as asbestos, acids and solvents, coal, stone dust, or other materials, the investigators reported in the July issue of the Annals of Neurology.
The findings echo a similar but smaller recent study by Mayo Clinic researchers. In that study, the Mayo investigators found that men who have been exposed to pesticides are more than twice as likely to develop Parkinson's disease as men who have managed to avoid contact with the toxic chemicals, reported Walter A. Rocca, M.D., M.P.H. and colleagues in the online edition of Movement Disorders.
In both the Mayo and Harvard studies, the Parkinson's risk from reported pesticide exposure was equally high among farmers and non-farmers, and in both studies there was no elevated risk associated with exposure to any other categories of occupational exposures or household or industrial chemicals.
The Harvard investigators prospectively evaluated U.S. men and women who were part of the Cancer Prevention Study II Nutrition cohort started in 1992 by the American Cancer Society. Of this group, 143,325 who returned study surveys in 2001 and who did not have a diagnosis of Parkinson's or parkinsonian symptoms at baseline were included.
A total of 7,864 participants (5.7%) reported exposure to pesticides, including 5,203 men and 2,661 women. In all, 1,956 participants fell into the category of farmers, ranchers, or fishermen, and people in this group had a 14-fold greater likelihood of pesticide exposure compared with those in other occupations. Blue collar workers were twice as likely as others to be exposed to pesticides.
The authors found that in an analysis adjusted for age, gender, and smoking status (current, past, or never), the relative risk for pesticide exposure and Parkinson's was 1.7 (95% confidence interval, 1.2-2.3, P=0.002).
Among the 2,308 participants who reported duration of pesticide exposure, the risks were not significantly different between those who reported exposure for more than a decade (relative risk 2.3, 95% CI, 1.1-4.9) or for less than 10 years (2,1, 95% CI, 0.7-6.5).
The authors noted that chemicals commonly used as pesticides in the U.S. have been shown to cause degeneration of dopaminergic neurons in the substantia nigra and to produce neurologic abnormalities when administered to experimental animals at high doses.
In addition, "in postmortem studies, higher levels of organochlorine insecticides have been found in the substantia nigra or striatum of individuals with Parkinson's disease," the authors wrote. "This finding reflects that organochlorine insecticides, unlike organophosphates and most other pesticides, persist in tissues many years after cessation of exposure."
In autopsy studies to which the authors refer, significantly higher levels of organochlorines were found in the brains of people with Parkinson's disease than in those with Alzheimer's disease, Lewy-body dementia, or in nondemented patients.
Organochlorine pesticides include DDT (banned for use in the United States, but still in use in some developing nations), chlordane, lindane and toxaphene. This chemical group also includes polychlorinated biphenyls (PCBs) which are not pesticides but are classified as hazardous industrial pollutants.
The authors acknowledged that the study is limited by a lack of specifics on the duration, frequency, and intensity of pesticide exposure of information about the specific chemicals that participants were exposed to.
Nonetheless, the large sample size, prospective design and availability of information on potential confounding factors provide a strong foundation for their conclusions, they wrote.
They also pointed out that while pesticides could be a marker of other aspects of rural living, farmers not exposed to pesticides were not at increased risk, further evidence of a direct effect.
The study was supported by a grant from the Michael J. Fox Foundation and Kinetic Foundation.