Legionnaires Disease is a potentially fatal form of pneumonia which can affect anybody but which principally affects those who are susceptible because of age, illness, immunosuppression, smoking etc. It is caused by the bacterium Legionella pneumophila and related bacteria. Legionella bacteria can also cause less serious illnesses which are not fatal or permanently debilitating. The collective term used to cover the group of diseases caused by legionella bacteria is legionellosis.
Legionnaires Disease was first identified following a large outbreak of pneumonia among people who attended an American Legion Convention in Philadelphia in 1976. A previously unrecognised bacterium was isolated from lung tissue samples and was subsequently named Legionella pneumophila.
It is normally contracted by inhaling legionella bacteria, either in tiny droplets of water (aerosols), or in droplet nuclei (the particles left after water has evaporated) contaminated with legionella, deep into the lungs. There is evidence that the disease may also be contracted by inhaling legionella bacteria following ingestion of contaminated water by susceptible individuals. Person to person spread of the disease has not been documented.
Initial symptoms of the disease include high fever, chills, headache and muscle pain. Patients may develop a dry cough and most suffer with difficulty in breathing. About one third of patients infected also develop diarrhoea or vomiting and about half become confused or delirious. Legionnaires disease can be treated effectively with suitable antibiotics.
The incubation is between 2 – 10 days, usually 3-6 days. Not everyone exposed will develop symptoms of the disease and those that do not develop the ‘full-blown’ disease may only present with a mild flu-like infection.
Legionella bacteria are common and can be found naturally in environmental water sources such as rivers, lakes and reservoirs, usually in low numbers. Legionella bacteria can survive under a wide variety of conditions and have been found in water at temperatures of between 6 oC and 60 oC. Water in the temperature range 20 oC to 45 oC seem to favour growth. The organisms do not appear to multiply below 20 oC and will not survive above 60 oC. They may, however, remain dormant in cool water and only multiply when temperatures reach a suitable level. Temperatures may also influence virulence. Legionella held at 37 oC have greater virulence than the same legionella bacteria kept at less than 25 oC.
Legionella bacteria also require a source of nutrients to multiply. Sources can include commonly encountered organisms within the water itself such as algae, amoebae and other bacteria. The presence of sediment, sludge, scale and other material within the system, together with bio-films, are also thought to play an important role in harbouring and providing favourable conditions in which the legionella bacteria may grow.
A bio-film is a thin layer of micro-organisms which may form a slime on the surfaces in contact with water. Such bio-films, sludge and scale can protect legionella bacteria from temperatures and concentrations of biocide that would kill or otherwise inhibit these organisms if they were freely suspended in the water.
As legionella bacteria are commonly encountered in environmental sources they may eventually colonise manufactured water systems and be found in cooling towers, hot and cold water systems and other plant used to store or distribute water services. To reduce the possibility of creating conditions in which the risk from exposure to legionella is increased, it is important to control the risk by introducing measures which:
a) do not allow proliferation of the organism in the water system, and
b) reduce, so far as is reasonably practicable, exposure to water droplets and aerosol.