Clustering of Parkinson disease: Shared cause of coincidence?

Background: The spatial and temporal pattern of excessive disease occurrence, termed clustering, may provide clues about the underlying etiology. Objective: To report the occurrence of 3 clusters of Parkinson disease (PD) in Canada. Design and Patients: We determined the population groups containing the clusters, geographical limits, and duration of exposure to the specific environments. We tested whether there was an excessive presence of Parkinson disease by calculating the probability of the observed cases occurring under the null hypothesis that the disease developed independently and at random in cluster subjects. Results of genetic testing for mutations in the α-synuclein, parkin, tau genes, and spinocerebellar ataxia genes (SCA2 and SCA3) were negative. Results: The probabilities of random occurrence (P values) in the 3 clusters were P=7.9 × 10^-7 for cluster 1, P=2.6 × 10^-7 for cluster 2, and P=1.5 × 10^-7 for cluster 3. Conclusions: Our findings indicate an important role for environmental causation in Parkinson disease. A possible role exists for environmental factors such as viral infection and toxins in the light of current evidence.