Legionnaires' disease outbreak investigation toolbox

Download Page

Legionnaires' disease outbreak case definitions

In order to categorise cases appropriately, definitions must be clear and specific as this will enable counting/inclusion of cases systematically and facilitate data sharing. Cases may be defined as sporadic or associated with a cluster or an outbreak (a cluster with a potential common source). Cases are linked with a cluster through clinical, microbiological and epidemiological criteria that are pragmatic and flexible as the situation develops. During the outbreak response it may be necessary and desirable to change case definitions, if this happens the justification (i.e. new evidence) should be documented.

The criteria for case inclusion/exclusion during an outbreak will be unique and locally determined by the outbreak control team. As shown in the diagram above, definitions are usually based on clinical features and/or the results of diagnostic tests and also specify the time and place of the putative exposure. A simplified flowchart for classifying cases is available here. For previously used exemplar outbreak definitions see here. To create an outbreak case definition consider the following criteria:

Clinical Criteria: Confirmed radiological or clinical pneumonia

Epidemiological Criteria: Case has visited location common to other cases, in 14 days prior to onset (this might simply be a town or broad geographic area or more focussed on a particular street or address if/once source is suspected) and case has onset date within defined window (set by Outbreak Control Team - perhaps over weeks or months if associated with broad community area or focussed over a few days or weeks if release is associated with a particular event).

Microbiological Criteria A: Laboratory evidence of at least one of the following:

  • Isolation (culture) of Legionella species from respiratory secretions or any normally sterile site
  • The presence of L. pneumophila urinary antigen determined using validated reagents/kits
  • L. pneumophila serogroup 1 specific antibody response.

Microbiological Criteria B: Laboratory evidence of at least one of the following:

  • Detection of Legionella pneumophila antigen in respiratory secretions or lung tissue e.g. by DFA staining using monoclonal-antibody derived reagents
  • Detection of Legionella spp. nucleic acid in a clinical specimen
  • L. pneumophila non-serogroup 1 or other Legionella spp. specific antibody response
  • L. pneumophila serogroup 1, other serogroups or other Legionella species: single high titre in specific serum antibody.
Then for an outbreak:

A confirmed case should be one that meets clinical, epidemiological and microbiological criteria A.

A probable case should be one that meets clinical, epidemiological and microbiological criteria B.

In an outbreak, case definitions may vary over the course of investigation as more information becomes available and will likely become more specific later on. An initially broad case definition, established at the outset of an investigation, will help identify anyone who might possibly be a case; it is better to gather too much information than too little. Such changes in case inclusion criteria should be carefully noted and registered with relevant agencies. If an analytical study is considered, then the definition might be further reviewed to allow inclusion/exclusion of cases as necessary.

In an ongoing outbreak response, following active case finding and microbiological matching of environmental source to initial cases, in certain situations the outbreak control team might find it necessary to simply define a probable case as one that meets only the clinical and epidemiological criteria above, see for example the outbreak described by Den Boer et al., 2002. [1]

The following supporting information is available:

  1. 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