Determinants of Legionella spp. presence in hospital water surveillance: a five-year, three-centre study from Greece.

BACKGROUND: Hospital water systems are persistent reservoirs of Legionella spp. and a recognised threat to immunocompromised inpatients. AIM: To characterise the burden and factors associated with Legionella spp. contamination across three Greek hospitals over five years METHODS: We retrospectively analysed 292 water samples from three acute-care Greek hospitals (March 2020-April 2025), cultured by a single accredited laboratory using the methods defined in ISO 11731:2017. A sample was classified as positive when viable Legionella spp. were recovered at or above the laboratory limit of detection; records reported as zero counts or below the limit of detection were treated as non-detections. FINDINGS: Overall, 20.9% (61/292) of samples were positive, with marked between-hospital differences: 21.2%, 35.0% and 4.4% (χ2=27.1, p<10-6). Hospital 2 carried the heaviest burden (mean 2188 CFU/L; 9.7% >10 000 CFU/L) and produced nine of the ten highest readings, from hot-water taps. After adjustment, between-hospital differences remained the factor most strongly associated with detection (Hospital 2 vs 3: OR=15.0, 95% CI 4.8-46.4; Hospital 1 vs 3: OR=8.6, 95% CI 2.0-38.0; both p<0.01). Hot-water sampling was independently associated with detection (OR=2.76, 95% CI 1.36-5.59, p=0.005). CONCLUSION: Differences between these three hospitals, together with hot-water systems, were the factors most strongly associated with Legionella spp. Persistent contamination in two of three networks supports a shift from outlet-level reactive disinfection towards whole-system, risk-based water-safety planning aligned with World Health Organization (WHO) and European Study Group for Legionella Infections (ESGLI) guidance. As a three-hospital retrospective study, these findings are preliminary and require confirmation in larger datasets.