How to prevent an outbreak of legionnaires' disease

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Nosocomial legionellosis usually presents as pneumonia (legionnaires’ disease’), much less commonly as extra-pulmonary infection* and sometimes as the non-pneumonic respiratory disease Pontiac fever. Large outbreaks of legionellosis are dramatic and attract the attention of the media. These are usually due to aerosol infection from poorly maintained cooling towers or evaporative condensers. Infection from potable water tends to be confined to single or small numbers of cases and hence may well pass unnoticed-although experience in Wadsworth Medical Center in Los Angeles3 and Pittsburgh Veteran’s Administration Medical Center4 revealed that infected potable water can, on occasion, give rise to large outbreaks of nosocomial legionellosis over a long period of time. It is impossible to predict the size of an outbreak so prevention is important, not only to protect the occasional individual (a concept which could lead to false cost/benefit analysis) but also to protect a potentially much larger number with all the distress to patients, relatives and staff that would ensue. In the UK legionellosis occurs mainly in the community and especially in travellers, and nosocomial infection appears to be relatively uncommon. The exception is the rare large outbreak like the one that occurred at Staffordshire General Hospital in 1985.’ However, there is evidence to suggest that in general not only is legionellosis underdiagnosed but there is well-documented evidence4 that nosocomial infections are commonly missed. Legionellae are part of the ‘normal flora’ of water and only occasionally give rise to disease. If a parallel is taken with normal flora in man it can be