Environmental Surveillance of Legionella spp. in an Italian University Hospital: results of 7 years of analysis.

Introduction: Nosocomial structures pose a high risk of Legionella spp. contamination due to complex water systems with challenging disinfection; moreover, the risk of severe legionellosis as a consequence of nosocomial exposure is very high in settings characterized by vulnerable patient conditions.

Methods: In the present work, we described the results of 7 years of environmental surveillance in a reference hospital in Liguria, in which a specific water safety plan (WSP) has been implemented in 2017, including data collected during the COVID-19 pandemic.

Results: During the study period, 1190 water samples were collected, of which 277 (23.3%) tested positive for Legionella spp. Positive samples with concentration values above 1,000 CFU/l were 184 (66.4%). Based on the new structure categorization contained in the WSP, hospital buildings classified as at "very high" risk resulted the most affected structures over the entire study period; however, the absolute number of positive samples greatly decreased over time, from 61 contaminated water samples in 2017 to only 9 in 2023.

Conclusions: Our findings prompted the reinforcement of control and prevention measures, affirming the appropriateness of risk-category classification. Indeed, the majority of contamination cases were associated with the water networks of buildings classified as "very high" risk.