Environmental relevance of cooling towers - a systematic review of legionellosis outbreaks

Abstract

Background Bioaerosols from cooling towers are often suspected to cause community-acquired legionellosis outbreaks. Uncertainty exists about the release and distribution of Legionella into the air, the occurrence of the respirable virulent form and the level of the infective dose. Aims We aimed to evaluate studies on Legionella outbreaks attributed to cooling towers. A systematic review, including data on morbidity and mortality, risk groups, diagnostic tools, meteorological conditions, maintenance actions, and comparative analysis of clinical and environmental isolates, was performed. Methods The literature search was conducted in Medline including publications from January 2001 to August 2012. Relevant findings were synthesized in form of a narrative summary. Results 19 outbreaks were identified affecting 12 countries. Recurring events were observed in Spain and Great Britain, 1609 confirmed cases of legionellosis and 102 cases of death (case-fatality rate 0-29%). Median time between onset of symptoms of first and last case was 36.5 days. The most frequent risk factors were smoking and underlying diseases in about half of the studies. The meteorological conditions varied strongly. Several studies reported a temporal association of outbreaks with technical interventions. A match of clinical and environmental isolates by serotyping and/or molecular subtyping was confirmed in 16 (84%) outbreaks. Conclusion Legionella-contaminated cooling towers as environmental trigger can cause severe health problems and even death of susceptible individuals. Consequently, public health authorities should enforce prevention and rapid risk assessment of outbreaks. Therefore, the registration of cooling towers is important to accelerate source identification and to shorten outbreak duration. To prevent and control Legionella contamination of cooling towers, maintenance actions should aim at low-emission procedures.