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Legionnaires Disease

Epidemiology of Specific Diseases: Legionnaires Disease

Causal agent
Infection caused by the bacterium Legionella pneumophilia, of which two distinct forms exist; Legionnaires disease (LD)
Pontiac fever (PF)1 

Common clinical features

  • Both characterized initially by anorexia, malaise, myalgia, headache and fever.
  • Legionnaires disease is the more severe form of legionellosis and is characterized by pneumonia, commencing 2-10 days after exposure.
  • Pontiac fever is characterized by the acute onset of flu-like, non-pneumonic illness. Patients recover within 2-5 days without treatment.


  • Approximately 300 cases are reported in the UK each year.
  • Approximately 50% of cases are contracted abroad.
  • LD has been reported to be responsible for between 0.5% and 15% of community acquired pneumonias2.
  • High risk groups include, diabetics, the elderly, the immunocompromised, persons with chronic lung disease, heavy alcohol users and smokers.
  • Case fatality for LD is 10-15% of cases (higher during nosocomial outbreaks).


  • Environmental water.
  • Legionellae grow at temperatures between 25 and 450 C, and so the highest risk occurs with water systems that lead to the aerosolisation of water stored at these temperatures2.
  • Legionella is chlorine resistant.

Mode of transmission
Inhalation of contaminated aerosols from devices including cooling towers, air conditioning cooling towers, hot water systems, humidifiers and Jacuzzis.

Incubation period
LD -  2-10 days, commonly 5-6 days.
PF -   5-66 hours, commonly 24-48 hours.

Period of Communicability
Person to person spread has not been demonstrated.

Prevention and control
Methods of prevention include2.

  • Appropriate design, maintenance and monitoring of water systems.
  • Maintenance and hygiene of wet cooling systems.
  • Disinfection, regular cleaning and changing of water in indoor fountains and whirlpool spas.
  • Use of sterile water for respiratory therapy devices.


  1. Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association; 2004.
  2. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.

© CM Kirwan 2006