Lessons From a Tertiary-Care Hospital Legionella Outbreak: Integrating Water System Design, Surveillance, and Quality Improvement

Legionnaires’ disease, caused by Legionella pneumophila, is a severe form of pneumonia that poses a considerable risk to hospitalized patients, particularly those who are immunocompromised. Hospital-acquired infections are a major public health concern associated with substantial morbidity and mortality. This article integrates a tertiary-care hospital outbreak investigation with a narrative review of the literature and a quality improvement framework to examine sources of L. pneumophila contamination and effective prevention strategies in healthcare water systems. An outbreak investigation was conducted at Ramathibodi Hospital using standard epidemiological methods supported by root cause analysis and the Plan–Do–Check–Act cycle, alongside environmental sampling and system assessment. Between 2019 and 2025, 4 confirmed hospital-acquired cases of L. pneumophila infection were identified, including 2 fatalities, all in immunocompromised patients. Environmental investigations implicated the hospital water supply, hot-water systems, and cooling towers, with contributory factors including suboptimal free residual chlorine levels and inadequate temperature control. Following implementation of corrective measures — including adjustment of chlorine levels, system maintenance, and enhanced monitoring — no further hospital-acquired cases were detected. This integrated review and institutional experience highlights the importance of proactive water system management, multidisciplinary collaboration, and continuous quality improvement in preventing healthcare-associated Legionnaires’ disease, particularly in high-risk patient populations.