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Home Topics Infectious Diseases Infections A-Z Legionnaires' disease

Legionnaires' disease

Legionella pneumophila - Photo: CDC/Dr. Barry Fields

Legionnaires' disease is an uncommon form of pneumonia caused by the legionella bacterium. The majority of cases are reported as single (isolated) cases but outbreaks can occur. All ages can be affected but the disease mainly affects people over 50 years of age, and generally men more than women. Smokers and the immunocompromised are at a higher risk.

The early symptoms of Legionnaires' disease include a 'flu-like' illness with muscle aches, tiredness, headaches, dry cough and fever. Sometimes diarrhoea occurs and confusion may develop. Deaths occur in 10-15% of the general population and may be higher in some groups of patients. The incubation period can range from 2 to 19 days with a median of 6 to 7 days after exposure.

People become infected when they inhale legionella bacteria which have been released into the air in aerosolised form from a contaminated source. Once in the lungs the bacteria multiply and cause either pneumonia or a less serious flu like illness (Pontiac fever ).

The bacteria are widely distributed in the environment. They can live in all types of water including both natural sources such as rivers and streams, and artificial water sources such as water towers associated with cooling systems, hot and cold water systems and spa pools. They only become a risk to health when the temperature allows the legionellae to grow rapidly, such as in water systems which are not properly designed, installed and/or maintained.

Control and prevention of the disease is through treatment of the source of the infection, i.e. by treating the contaminated water systems, and good design and maintenance to prevent growth in the first place.

What's new

  • 25 November 2013: The control of Legionella bacteria in water systems (L8 fourth edition) - The Health and Safety Executive (HSE), have published the revised fourth edition of the 'Approved code of practice and guidance' for the control of Legionella bacteria in water systems. Copies of the revised document are available from the HSE website: Legionnaires' disease: The control of legionella bacteria in water systems [external link]

  • 31 July 2013: Update on copper ionisation systems - Following the consultation period on the European Commission’s (EC) decision to ban the marketing and use of elemental copper as a biocide, on 11 July 2013 the EC agreed to grant an ‘essential use derogation’ for the continued use of copper for the control of Legionella in UK water systems.  The UK has until the end of 2014 to submit a formal application, until then the Health and Safety Executive (HSE) will continue to follow a sensible and proportionate approach to enforcement, focusing on effective control measures to minimize risk of exposure to the organism:  View a draft of the proposed derogation [external link]

  • 27 February 2013: Copper ionisation systems - Important information on changes to the use of elemental copper in water treatment systems for the control of Legionella:  HSE's information 29/11/2012, amended 20/02/2013 [external link].