Comparative Risk Assessment of Legionella spp. Colonization in Water Distribution Systems Across Hotels, Passenger Ships, and Healthcare Facilities During the COVID-19 Era

Colonization of Legionella spp. in engineered water systems constitutes a major public health threat. In this study, a six-year environmental surveillance (2020–2025) of Legionella colonization in five different types of facilities in Crete, Greece, is presented, including hotels, cruise ships, and healthcare facilities with primary healthcare facilities, public hospitals, and private clinics. A total of 1,081 water samples were collected and analyzed, with the overall positivity calculated using culture-based methods. Only 16.46% of the samples exceeded the regulatory limit (>10³ CFU/L) in the total sample, with the overall Legionella positivity being 44.59%. Colonization by facility category showed higher rates in primary healthcare facilities with 85.96%, followed by public hospitals with 46.36%, passenger ships with 36.93%, and hotels with 38.08%, and finally private clinics. The association of environmental risk factors with Legionella positivity revealed a strong effect at hot water temperatures < 50°C (RR = 2.05) and free chlorine residuals < 0.2 mg/L (RR = 2.22) (p < 0.0001). Serotyping analysis revealed primarily an overall dominance of serogroups 2-15 of L. pneumophila; nevertheless, serogroup 1 was particularly prevalent in hospitals, ships, and hotels. Based on these findings, the requirement for continuous environmental monitoring, as well as risk management plans tailored to each facility with preventive thermochemical controls, is highlighted. Finally, operational disruptions such as those experienced during the COVID-19 pandemic, especially in primary care facilities and marine systems, require special attention.