Effect of monochloramine disinfection of municipal drinking water on risk of nosocomial Legionnaires' disease

Background Many Legionella infections are acquired through inhalation or as piration of drinking water. Although about 25% of municipalities in the USA use monochloramine for disinfection of drinking water. the effect of monoc hloramine on the occurrence of Legionnaires' disease has never been studied . Methods We used a case-control study to compare disinfection methods for dr inking water supplied to 32 hospitals that had had outbreaks of Legionnaire s' disease with the disinfection method for water supplied to 48 control-ho spitals, with central for selected hospital characteristics and water treat ment factors. Findings Hospitals supplied with drinking water containing free chlorine as a residual disinfectant were more likely to have a reported outbreak of Le gionnaires' disease than those that used water with monochloramine as a res idual disinfectant (odds ratio 10.2 [95% CI 1.4-460]), This result suggests that 90% of outbreaks associated with drinking water might not have occurr ed if monochloramine had been used Instead of free chlorine for residual di sinfection (attributable proportion 0.90 [029-1.00]). Interpretation The protective effect of monochloramine against legionella s hould be confirmed by other studies. Chloramination of drinking water may b e a cost-effective method for control of Legionnaires' disease at the munic ipal level or in individual hospitals, and widespread implementation could prevent thousands of cases.