Legionella in Water Systems: New Frontiers in Detection, Prevention, and Public Health Strategies

Legionnaires’ disease incidence has risen substantially across industrialised nations since the early 2000s, yet standard control protocols—primarily secondary disinfection verified by culture—rely on an evidence base that warrants further scrutiny. This review offers three interpretations of the available evidence, without claiming they are established conclusions. First, culture-based verification has inherent methodological limitations: treatments that reduce culturability can induce viable-but-non-culturable (VBNC) states undetected by culture alone, creating potential discrepancies between verified culture reduction and complete microbial clearance. This reading rests on evidence that monochloramine at ≥1 mg/L abolishes culturability while genomic units remain essentially unchanged and that starved VBNC Legionella retain the capacity to infect human macrophages after more than 220 days, albeit at roughly 1% the efficiency of culturable cells; whether this translates into material risk in operating buildings remains undetermined. Second, much of the comparative-effectiveness literature evaluating disinfectants relies on observational single-site studies, and early validation frameworks often depended on self-reported survey data; randomised comparison in occupied healthcare buildings is frequently impractical, though recent systematic reviews now supply quantitative synthesis earlier work lacked. Third, regulatory thresholds remain expressed in the units of the method whose deficiencies motivated the alternatives, and the alignment of verification standards with advances in molecular detection remains an ongoing challenge.