Long-term effects of hospital water network disinfection on Legionella and other waterborne bacteria in an Italian university hospital

Objective and design of the study: Legionella control still remains a critical issue in healthcare settings where the preferred approach to health risk assessment and management is to develop a water safety plan (WSP). We report the experience of a University Hospital, where a WSP has been applied since 2002 and the results obtained with the application of different methods for disinfecting hot water distribution systems in order to provide guidance for the management of water risk. Interventions: The disinfection procedures included continuous chlorination with chlorine dioxide (0.4-0.6 mg/L in recirculation loops), reinforced by endpoint filtration in critical areas and a water treatment based on monochloramine (2-3 mg/L). Real-time PCR and a new immunoseparation and ATP-bioluminescence analysis were applied in environmental monitoring. Results: After nine years, the integrated disinfection-filtration strategy reduced significantly the positive sites rate by 55% and the mean count by 78% (p-value<0.05); however the high costs and the occurrence of a chlorine-tolerant clone belonging to L.pneumophila ST269, prompted us to test a new disinfectant. The shift to monochloramine allowed to eliminate planktonic Legionella and did not require additional endpoint filtration, however nontuberculous mycobacteria were isolated more frequently as long as the monochloramine concentration was 2 mg/L; their cultivability was never regained by increasing the concentration up to 3 mg/L. Conclusions: Any disinfection method needs to be finely adjusted/tuned in order to maintain in individual hospitals satisfactory results over time and only a locally adapted evidence-based approach allows to assess the efficacy and disadvantages of the control measures.