Legionnaires' disease outbreak investigation toolbox

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Definition of a case for surveillance purposes

Case definitions during an outbreak are likely to be more flexible (in terms of clinical evidence/diagnosis) and more organic (in terms of shifting evidence base) yet more constrained (in terms of location/time) than definitions for Legionnaires' disease for surveillance and monitoring purposes. The case definitions below are taken from [1] and are included for contextual background.

Case definitions for confirmed and probable cases of Legionnaires' disease[1]

Confirmed Case

Any person with a diagnosis of pneumonia AND with 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

Probable case

Any person with pneumonia

AND either 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.

OR any person with pneumonia and at least one of the following epidemiological links:

· Environmental exposure[2]

· Exposure to the same common source[3]


[2] for example, persons with pneumonia could have had the same environmental exposure through staying in a hospital or hotel with laboratory-confirmed presence of Legionella in the water system. Although these people have not been tested for the disease there is an assumption that their pneumonia could be due to the same organism through the epidemiological link.

[3] for example, persons with pneumonia who were in the vicinity of a common source outbreak but did not get tested for the disease. For example, outbreaks on cruise ships or where exposure to infection may involve residents from more than one country could include cases with an epidemiological link who fall into this category but who return home and do not get tested for Legionella infection.