Legionellosis is a collective term for diseases caused by legionella bacteria including the most serious legionnaires’ disease, as well as the similar but less serious conditions of Pontiac fever and Lochgoilhead fever. Legionnaires’ disease is a potentially fatal form of pneumonia and everyone is susceptible to infection.
The bacterium Legionella pneumophila and related bacteria are common in natural water sources such as rivers, lakes and reservoirs, but usually in low numbers. They may also be found in purpose-built water systems, such as cooling towers, evaporative condensers, hot and cold water systems and spa pools. If conditions are favourable, the bacteria may multiply, increasing the risks of legionnaires’ disease, and it is therefore important to control the risks by introducing appropriate measures.
Legionella bacteria are widespread in natural water systems, eg rivers and ponds. However, the conditions are rarely conducive for people to catch the disease from these sources. Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where water is maintained at a temperature high enough to encourage growth.
Legionnaires’ disease is normally contracted by inhaling small droplets of water (aerosols), suspended in the air, containing the bacteria. Certain conditions increase the risk from legionella if: (a) the water temperature in all or some parts of the system may be between 20–45 °C, which is suitable for growth; (b) it is possible for water droplets to be produced and if so, they can be dispersed; (c) water is stored and/or re-circulated; (d) there are deposits that can support bacterial growth, such as rust, sludge, scale, organic matter and biofilms.