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What is Legionella and how dangerous is it?

Since first being identified, following the outbreak at the convention of the American Legion in Philadelphia, in 1976, much work has gone into identifying what conditions favour the growth and proliferation of the Legionella bacteria in building water systems.

Legionella Pneumophila (its full name) and other related bacteria are naturally occurring organisms, found in all water systems, both in nature and in buildings. The diseases they cause (several of them, collectively termed Legionellosis), one of which is Legionnaires’ disease, are wholly preventable, through good management, maintenance, and design. Management procedures should identify and assess the risk of Legionellosis, and record all actions, and engineering and maintenance solutions to remove, and reduce those risks.

Legionella pneumophila is only one member of a genus of bacteria that can cause disease and illness. To date, over 50 species in this genus have been identified, with more than 24 associated with human infections. In addition to Legionnaires’ disease, other conditions that can occur in humans are called, for example, Pontiac fever, and Lochgoilhead fever, (named after the places in which the first outbreaks were discovered).

Whilst Pontiac and Lochgoilhead fevers produce mild respiratory illnesses, Legionnaires’ disease causes acute pneumonia, resulting in a mortality of between 5% and 50%. Healthy individuals can usually fight off infection from the bacteria. However, older people, especially males, and those with underlying health issues, or a suppressed immune system, are particularly at risk.

The incidence of infection (morbidity rate) of those exposed to Legionella Pneumophila is typically 5%, although this is greatly dependent upon the susceptibility of the exposed population. Although they are much less severe diseases, the morbidity rates for Pontiac and Lochgoilhead Fevers are much higher, at 70% to 90%. It means that this proportion of people exposed to the bacteria go on to develop the disease.

The bacteria responsible for these infections naturally occur at low levels in mud, soil, and watercourses, and at these levels they pose very little threat of human infection. However, the conditions found in human-constructed environments, such as cooling towers, spa pools, and hot and cold water systems, can favour a rapid rise in the number of bacteria, to levels at which dissemination and infection becomes possible.

Legionella Risk Assessments are required, to identify risks, and conditions in which Legionella can multiply to dangerous levels.

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