Amyotrophic lateral sclerosis after long-term exposure to drinking water with high selenium content.

We examined 9 years' incidence of amyotrophic lateral sclerosis, a disease previously associated with a high-selenium environment, in a cohort of 5,182 residents of Reggio Emilia, Italy. This cohort had accidentally been exposed to drinking water with high selenium content. Four cases were diagnosed during the follow-up. Using the remainder of the municipal population as the reference group, the standardized incidence ratio was 4.22 (95% confidence interval = 1.15-10.80). The standardized incidence ratio was higher after limiting the analysis to the subcohort with the longest ascertainable exposure period. The findings appear to confirm a causal association between overexposure to environmental selenium and amyotrophic lateral sclerosis.