Legionnaires' disease outbreak investigation toolbox

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Potential sources

Outbreaks of Legionnaires' disease are usually caused by the inhalation, by susceptible persons, of contaminated aerosols generated by artificial water systems (although aspiration has been a proven method of infection - particularly for nosocomial cases).

The literature shows that a large number of outbreaks occur in an outdoor setting, and are the result of releases of legionella-contaminated aerosols from poorly maintained cooling towers associated with commercial and industrial air-conditioning systems [1-7]. There have also been outbreaks that have occurred at particular events [8-10] or attractions [11; 12] and at which the cases were at the source of infection only once. The types of sources in outbreaks can vary e.g. fountains [8], spa pools [10; 13] and wet air conditioning systems [14; 15]. Legionnaires' disease has also recently been linked to compost bought at garden centres [16] and to working in manholes [17]. The number of cases associated with an outbreak, and the interpretation of initial epidemiological investigation, might indicate a potential source. For example, if cases appear over a broad area with no clear common location, then large industrial sources might be prioritised as an initial focus for investigation, but if all cases appear linked to a single location, then the source might be more obvious. Indicative potential sources are discussed in [18] and references therein. The table below presents these potential sources to help investigators consider various sites in a locality.

Consider other sources

Even when, after review of initial epidemiological evidence, a building is indicated as a source, the possibility of a source outside, but close to, the building should also be considered. In the United Kingdom, investigations have usually concentrated on all potential sources within a 500m radius of the epicentre of an outbreak (should the setting enable the epicentre be clearly defined). Once all potential sources within the 500m radius have been identified and visited, the radius may be increased to 1000m or more. Transmission is usually only considered likely up to about 2000m, although in an outbreak in Lens in the north of France in 2003-2004, transmission up to 8km has been suggested [5]. Such investigations are aided if the local authorities have a register of cooling towers in their area. It is usually easiest to investigate each water system systematically by starting at the water supply into the property and working forwards through storage tanks and any intermediate equipment, such as water heaters and softeners, to the outlets [18; 19].

Indicative list of potential sources of Legionella [18]

Any system or site under investigation, that is capable of storing water and is connected to a device that is capable of generating aerosols or sprays, should be considered as a potential source of legionella. This list is by definition, therefore, neither exhaustive nor prioritised, athough it is ordered such that later items might be associated with larger outbreaks.

Ice cubes (infection via aspiration)

Domestic plumbing

Hot and cold-water systems (distribution system, storage tanks, shower heads, thermostatic mixer valves, taps and other outlets)

Wash basins (e.g. hairdressers)

Respiratory devices (including nebulizers and ventilatory machines)

Drinking water dispensers

Toilet cisterns

Balance tanks and associated water systems


Car screen wash

Car washes


Potting mixes and compost

Fire-fighting systems

Food humidifiers (e.g. on food display units)

Mist machines

Medical humidifiers filled and rinsed with tap water

Air-conditioning and handling units

Air-conditioning humidifiers

Condensation trays in air-conditioners and fan coils

Poorly maintained plant/boiler rooms

Birthing pools and associated water system

Hot tubs and spa pools

Swimming pools (both cold water and heated pools)

Thermal pools and springs

Asphalt machines

Insufficiently disinfected bunkered water, on ships and similar

Irrigation systems

Ornamental fountains

Biological treatment plants

Cooling towers and evaporative condensers

Industrial air scrubbers

Industrial works that generate aerosols

Reference List

  1. BROWN C. M., NUORTI P. J., BREIMAN R. F., HATHCOCK A. L., FIELDS B. S., LIPMAN H. B., LLEWELLYN G. C., HOFMANN J. & CETRON M. (1999) A community outbreak of Legionnaires' disease linked to hospital cooling towers: An epidemiological method to calculate dose of exposure International Journal of Epidemiology 28, pp.353-359 http pdf
  2. CASTILLA J., BARRICARTE A., ALDAZ J., GARCIA CENOZ M., FERRER T., PELAZ C., PINEDA S., BALADRON B., MARTIN I., GONI B., ARATAJO P., CHAMORRO J., LAMEIRO F., TORROBA L., DORRONSORO I., MARTINEZ-ARTOLA V., ESPARZA M. J., GASTAMINZA M. A., FRAILE P. & ALDAZ P. (2008) A large Legionnaires' disease outbreak in Pamplona, Spain: early detection, rapid control and no case fatality Epidemiology and Infection 136, pp.823 - 832 http
  3. GARCIA-FULGUEIRAS A., NAVARRO C., FENOLL D., GARCIA J., GONZALEZ-DIEGO P., JIMENEZ-BUNUALES T., RODRIGUEZ M., LOPEZ R., PACHECO F., RUIZ J., SEGOVIA M., BALADRON B. & PELAZ C. (2003) Legionnaires' disease outbreak in Murcia, Spain Emerging Infectious Diseases 9(8), pp.915 - 921 http pdf
  4. KIRRAGE D., REYNOLDS G., SMITH G.E. & OLOWOKURE B; HEREFORD LEGIONNAIRES' OUTBREAK CONTROL TEAM (2007) Investigation of an outbreak of Legionnaires' disease: Hereford, UK 2003 Respiratory Medicine 101(8), pp.1639-1644 http
  5. NGUYEN T. M. N., ILEF D., JARRAUD S., ROUIL L., CAMPESE C., CHE D., HAEGHEBAERT S., GANIAYRE F., MARCEL F., ETIENNE J. & DESENCLOS J.C. (2006) A community-wide outbreak of Legionnaires' disease linked to industrial cooling towers: How far can contaminated aerosols spread? Journal of Infectious Diseases 193, pp.102-111 http
  6. SABRIA M., ALVAREZ J., DOMINGUEZ A., PEDROL A., SAUCA G., SALLERAS L., LOPEZ A., GARCIA-NUNEZ M. A., PARRON I. & BARRUFET M. P. (2006) A community outbreak of Legionnaires' disease: evidence of a cooling tower as the source Clinical Microbiology and Infectious Diseases 12, pp.642-647 http pdf
  7. WATSON J. M., MITCHELL E., GABBAY J., MAGUIRE H., BOYLE M., BRUCE J., TOMLINSON M., LEE J. V., HARRISON T. G., UTTLEY A., O'MAHONY M. C., & CUNNINGHAM D. (1994) Piccadilly Circus Legionnaires' disease outbreak Journal of Public Health Medicine 16 (3), pp. 341-347. http
  8. CORREIA A. M., GONCALVES G., REIS J., CRUZ J. M. & CASTRO E FREITAS J. A. (2001) An outbreak of Legionnaires' disease in a municipality in northern Portugal Eurosurveillance 6(7), pp.21-24. http
  9. DE SCHRIJVER K., DIRVEN K., VAN BOUWEL K., MORTELMANS L., VAN ROSSOM P., DE BEUKELAAR T., VAEL C., FAJO M., RONVEAUX O., PEETERS M. F., VAN DER ZEE A., BERGMANS A., IEVEN M. & GOOSSENS H. (2003) An outbreak of Legionnaire's disease among visitors to a fair in Belgium in 1999 Journal of the Royal Institute of Public Health 117, pp.117 - 124 http
  10. DEN BOER J. W, YZERMAN P. F.E., SCHELLEKENS J., LETTINGA K. D, BOSHUIZEN H. C, VAN STEENBERGEN J. E, BOSMAN A., VAN DEN HOF S., VAN VLIET H. A, PEETERS M. F., VAN KETEL R. J., SPEELMAN P., KOOL J.L., & CONYN VAN SPAENDONCK M. A. E. (2002) A large outbreak of Legionnaires' disease at a flower show, the Netherlands, 1999 Emerging Infectious Diseases 8(1), pp.37-43 http pdf
  11. GREIG J. E., CARNIE J. A., TALLIS G. F., RYAN N. J., TAN A. G., GORDON I. R., ZWOLAK B., LEYDON J. A., GUEST C. S. & HART W. G. (2004) An outbreak of Legionnaires' disease at the Melbourne Aquarium, April 2000: investigation and case-control studies The Medical Journal of Australia. 180(11), pp.566-572. http
  12. OKADA M., KAWANO K., KURA F., AMEMURA-MAEKAWA J., WATANABE H., YAGITA K., ENDO T. & SUZUKI S. (2005) The largest outbreak of legionellosis in Japan associated with spa baths: epidemic curve and environmental investigation Journal of the Japanese Association for Infectious Diseases 79, pp.365 - 374 http
  13. BEYRER K., LAI S., DREESMAN J., LEE J. V., JOSEPH C., HARRISON T., SURMAN-LEE S., LUCK C., BRODHUN B. & BUCHHOLZ U. & WINDORFER A. (2007) Legionnaires' disease outbreak associated with a cruise liner, August 2003: Epidemiological and microbiological findings Epidemiology and Infection 135, pp.802 - 810 http
  14. EDMOND-SMITH J. (2006) Report of the public meetings into the Legionella outbreak in Barrow-in-Furness, August 2002 Health and Safety Executive http pdf
  15. O'MAHONY M. C., STANWELL-SMITH R. E., TILLETT H. E., HARPER D., HUTCHINSON J. G. P., FARRELL I. D., HUTCHINSON D. N., LEE J. V., DENNIS P. J., DUGGAL H. V., SCULLY J. A. & DENNE C. (1990) The Stafford outbreak of Legionnaires' disease Epidemiology and Infection 104, pp.361 - 380 http
  16. PRAVINKUMAR S. J., EDWARDS G., LINDSAY D., REDMOND S., STIRLING J., HOUSE R., KERR J., ANDERSON E., BREEN D., BLATCHFORD O., MCDONALD E. & BROWN A. (2010) A cluster of Legionnaires' disease caused by Legionella longbeachae linked to potting compost in Scotland, 2008-2009 Eurosurveillance 15 (8), http pdf
  17. RICCI M. L., FONTANA S., BELLA A., GAGGIOLI A., CASCELLA R., CASSONE A. & SCATURRO M. (2010) A preliminary assessment of the occupational risk of acquiring Legionnaires' disease for people working in telephone manholes, a new workplace environment for Legionella growth American Journal of Infection Control 38(7), pp.540-545 http
  18. BARTRAM J. (2007) Legionella and the prevention of legionellosis WHO, Geneva ISBN 92 4 156297 http pdf
  19. HEALTH AND SAFETY EXECUTIVE (2000) Legionnaires' disease HSE, UK http pdf