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This Fact Sheet has been issued by the World Health Organisation (WHO). It explains that Legionellosis varies in severity from a mild febrile illness to a serious and sometimes fatal form of pneumonia and is caused by exposure to the Legionella bacteria species found in contaminated water and potting mixes.

Cases of legionellosis are often categorized as being community, travel or hospital acquired based on the type of exposure.

Worldwide, waterborne Legionella pneumophila is the most common cause of cases including outbreaks. Legionella pneumophila and related species are commonly found in lakes, rivers, creeks, hot springs and other bodies of water. Other species including L. longbeachae can be found in potting mixes.

The most common form of transmission of Legionella is inhalation of contaminated aerosols from contaminated water. Sources of aerosols that have been linked with transmission of Legionella include air conditioning cooling towers, hot and cold water systems, humidifiers and whirlpool spas. Infection can also occur by aspiration of contaminated water or ice, particularly in susceptible hospital patients, and by exposure of babies during water births. To date, there has been no reported direct human-to-human transmission.

Key facts

  • The bacterium L. pneumophila was first identified in 1977, as the cause of an outbreak of severe pneumonia in a convention centre in the USA in 1976.

  • The most common form of transmission of Legionella is inhalation of contaminated aerosols produced in conjunction with water sprays, jets or mists of contaminated water sources. Infection can also occur by aspiration of contaminated water or ice, particularly in susceptible hospital patients.

  • Legionnaires’ disease has an incubation period of 2 to 10 days (but up to 16 days has been recorded in some outbreaks).

  • Treatments exist, but there is currently no vaccine available for Legionnaires' disease.

  • Death occurs through progressive pneumonia with respiratory failure and/or shock and multi-organ failure.

  • Untreated Legionnaires’ disease usually worsens during the first week.

  • Of the reported cases, 75–80% are over 50 years and 60–70% are male.

Prevention and control

Prevention of Legionnaires’ disease depends on applying control measures to minimize the growth of Legionella and dissemination of aerosols. These measures include good maintenance of devices, including regular cleaning and disinfection and applying other physical (temperature) or chemical measures (biocide) to minimize growth. Some examples are:

  • The regular maintenance, cleaning and disinfection of cooling towers together with frequent or continuous addition of biocides.

  • Installation of drift eliminators to reduce dissemination of aerosols from cooling towers.

  • Maintaining an adequate level of a biocide such as chlorine in a spa pool along with a complete drain and clean of the whole system at least weekly.

  • Keeping hot and cold water systems clean and either keeping the hot water above 50 °C (which requires water leaving the heating unit to be at or above 60 °C) and the cold below 25 °C and ideally below 20 °C or alternatively treating them with a suitable biocide to limit growth, particularly in hospitals and other health care settings, and aged-care facilities.

  • Reducing stagnation by flushing unused taps in buildings on a weekly basis.

Applying such controls will greatly reduce the risk of Legionella contamination and prevent the occurrence of sporadic cases and outbreaks. Extra precautions may be required for water and ice provided to highly susceptible patients in hospitals including those at risk of aspiration (for example, ice machines can be a source of Legionella and should not be used by highly susceptible patients).

Control and prevention measures must be accompanied by proper vigilance on the part of general practitioners and community health services for the detection of cases.

WHO September 2022

Below for the full document:

Legionella Factsheet WHO
Download PDF • 183KB

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