Legionella pneumophila is a common bacterium found in rivers or lakes but usually in low numbers. In these settings it does not present a health risk.
However, the bug can survive over a wide range of temperatures, from 6C to 60C, and will proliferate when the water temperature is between 20C and 45C. Stagnant water will favour the build up of larger numbers of the organism, as will the presence of other bacteria.
The commonest site where such conditions prevail is in cooling tower systems, for example in air-conditioning and ventilation units. These are located outside the building and can distribute small droplets of contaminated water over a wide area. Such towers are a common source of outbreaks of legionnaires’ disease.
Identifying the outbreak’s source can be difficult. The bacterium takes some time to culture, between three to ten days.
In addition, simply finding the bacterium in a water-cooling system does not prove that it is the cause of the outbreak. Since it is common in the environment, further tests must be employed to match a sample with the bacterium that has been found in patients.
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The recent warm weather in Scotland may have encouraged excessive growth of the bacteria in water-cooling systems, and high winds may have facilitated distribution of the bacterium over a wide area.
Currently, disinfection of potential sources of the outbreak in Edinburgh is being carried out.
In the meantime, heightened awareness of legionnaires’ disease in the city will ensure doctors treating patients for pneumonia will begin therapy with antibiotics which target this infection.
The number of cases may well rise in the next few days and since the incubation period can be up to 14 days after exposure, new cases may continue ot present even if the source has been removed.
Tom Evans is Professor of Molecular Microbiology at the University of Glasgow