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
- Isolation (culture) of Legionella species from respiratory secretions or any
normally sterile site
- The presence of L. pneumophila urinary antigen determined using validated
L. pneumophila serogroup 1 specific antibody response.
Microbiological Criteria B: Laboratory evidence of at least one of
- 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
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.,
The following supporting information is available: