Legionnaires' disease outbreak investigation toolbox

Download Page

Outbreak and cluster definitions

A basic working definition for a cluster of Legionnaires' disease cases is essentially multiple cases that may be linked in space and time. An outbreak of Legionnaires' disease is then a special case of a cluster where a common source is suspected.

Identification of multiple cases of Legionnaires' disease, linked by common exposure, is reliant on a high index of suspicion and should be considered when the number of cases in an area is clearly in excess of the normal frequency expected for a specific time period. These excesses of Legionnaires' disease cases may present as a cluster of two or more cases following exposure to a single environmental source during a short period of time or as a number of apparently sporadic cases over a prolonged period of time in an area in which it is highly endemic. A cluster of two or more cases linked in time and place is therefore the starting point for epidemiological and environmental investigations of potential links that may eventually lead to the detection of an outbreak associated with an environmental source of infection. If no links are found, the environmental actions in response to a single case are determined locally. Clusters and outbreaks may occur within a hospital or community setting or be linked to travel either in the home Member State or abroad.

It may be that the initial cases are from other member states and were identified via travel history reported through ELDSNet

Outbreak definition

An outbreak is defined as two or more cases where the onset of illness is closely linked in time (weeks rather than months) and in space, where there is suspicion of, or evidence of, a common source of infection, with or without microbiological support (i.e. common spatial location of cases from travel history). An outbreak control team should always be convened to investigate outbreaks.

Cluster definition

Two or more cases that initially appear to be linked by space (for example area of residence, work, or other setting [i.e. healthcare or community]) and which have sufficient proximity in dates of onset of illness (e.g. six months) to warrant further investigation. The geographical area used to define excess based in spatial proximity should take account of population size and density when investigations are planned. Such excesses over baseline will vary by member state and local discretion should be used. If, after investigation, no common exposures to a potential source of infection are identified for these cases, other than the links mentioned above, then they should be classified as sporadic community-acquired cases. Consideration should be given to convening an outbreak control team if a cluster is identified.

An alternative classification of cluster is one where cases are linked by close onset dates but only by a very broad geographic proximity. Such clusters are not likely to cause convening of outbreak control teams as they are likely to arise due to, for example, changes in meteorological conditions rather than common source of infection.

International considerations

Clusters might occur with two or more cases having stayed overnight at the same accommodation site in the 14 days before onset of illness and whose illness is within say the same two-year period. Such travel associated cases may be identified at their home location and so should be reported to the public health authorities in the region of travel to aid their local investigations.

Where outbreaks are associated with more than one country[1] [2] the countries involved may take part in an investigation through exchanging clinical and environmental specimens or sequence typing data from an outbreak and this is being done more and more. International collaborations help to validate diagnostic tests and the microbiological association between cases and sources of infection[3]. Similar data collection algorithms and facilitation of data sharing will aid cross-border working.

  1. GAIA V., FRY N.K., HARRISON T.G., PEDUZZI R. (2003) Sequence-based typing of Legionella pneumophila serogroup 1 offers the potential for true portability in legionellosis outbreak investigation Journal of Clinical Microbiology 41, pp.2932-2939 http pdf
  2. JOSEPH C., MORGAN D., BIRTLES R., PELAZ C., MARTIN-BOURGON C., BLACK M., GARCIA-SANCHEZ I., GRIFFIN M., BORNSTEIN N. & BARTLETT C. (1996) An international investigation of an outbreak of Legionnaires' disease among UK and French tourists European Journal of Epidemiology 12(3), pp.215-219; http
  3. BARTRAM J. (2007) Legionella and the prevention of legionellosis WHO, Geneva ISBN 92 4 156297 http pdf