Effect of monochloramine disinfection of municipal drinking water on risk of nosocomial Legionnairesʹdisease

Background Many Legionella infections are acquired through inhalation or aspiration of drinking water. Although about 25% of municipalities in the USA use monochloramine for disinfection of drinking water, the effect of monochloramine on the occurrence of Legionnairesʹdisease has never been studied. Methods We used a case-control study to compare disinfection methods for drinking water supplied to 32 hospitals that had had outbreaks of Legionnairesʹdisease with the disinfection method for water supplied to 48 control-hospitals, with control for selected hospital characteristics and water treatment factors. Findings Hospitals supplied with drinking water containing free chlorine as a residual disinfectant were more likely to have a reported outbreak of Legionnairesʹdisease than those that used water with monochloramine as a residual disinfectant (odds ratio 10·2 [95% CI 1·4–460]). This result suggests that 90% of outbreaks associated with drinking water might not have occurred if monochloramine had been used instead of free chlorine for residual disinfection (attributable proportion 0·90 [0·29–1·00]). Interpretation The protective effect of monochloramine against legionella should be confirmed by other studies. Chloramination of drinking water may be a cost-effective method for control of Legionnairesʹdisease at the municipal level or in individual hospitals, and widespread implementation could prevent thousands of cases.