Parkinson's disease among diphenyl-exposed paper mill workers

Introduction
The cause of Parkinson’s disease (PD) is still unknown. A combination of individual susceptibility to environmental neurotoxins, and progressive loss of dopamine neurons in the substantia nigra depending on age has been suggested. Several epidemiological studies have shown a two-fold increased risk of developing PD after exposure to pesticides. We have earlier reported a cluster of five cases of PD among paper mill workers with occupational exposure to a fungicide (diphenyl).

Methods
A cohort of 255 diphenyl-exposed workers was identified. The number of expected cases in the exposed group was calculated from the age- and sex-standardized prevalence of PD in the general population. Exposed workers with PD or suspected cases of PD were referred to a neurologist at the University Hospital. In addition, the cohort was scrutinized in the National Hospital Discharge Register with respect to the diagnosis of PD.

Results
Five cases of PD were found in the group of 255 diphenyl-exposed workers. The number of expected cases was estimated to be 0.9, thus resulting in a relative risk of 5.6 (95% CI 1.8 to 13). After completion of the primary study, more cases of PD among diphenyl-exposed paper mill workers have been investigated at our clinic.

Discussion
Previous epidemiological studies have indicated an elevated risk of developing PD after exposure to pesticides. Diphenyl is a pesticide (fungicide), and has some structural similarities to the wellknown dopaminergic neurotoxin MPTP. Exposure to diphenyl might have contributed to this PD cluster. Further investigations of other diphenyl-exposed workers are warranted, as well as animal studies on diphenyl.