Legionella Case Definitions

Data is derived from laboratory confirmed reports and follow up surveillance forms that collect information on whether the case is community, hospital acquired or travel associated.

Legionnaires' disease is an uncommon form of pneumonia. The disease has no particular clinical features that clearly distinguish it from other types of pneumonia, and laboratory investigations must therefore be carried out in order to obtain a diagnosis. The following microbiological and epidemiological case definitions are used for surveillance purposes:

A. Microbiological definitions:

Confirmed case:

A clinical diagnosis of pneumonia with laboratory evidence of one or more of the following:

• Isolation (culture) of legionella species from clinical specimens.
• Seroconversion (a four-fold or greater increase in titre) determined using a validated indirect immunofluorescent antibody test (IFAT) incorporating a monovalent L. pneumophila serogroup 1 antigen.
• The presence of L.pneumophila urinary antigen determined using validated reagents/kits.

Presumptive case:

A clinical diagnosis of pneumonia with laboratory evidence of one or more of the following:

• A single high titre of 128 using IFAT as above (or a single titre of 64 in an outbreak).
• A positive direct fluorescence (DFA) on a clinical specimen using validated monoclonal antibodies (also referred to as a positive result by Direct Immunofluorescence (DIF)).

B. Clinical definitions:

1. Legionnaires' disease (pneumonic legionellosis)

a) a clinical diagnosis of pneumonia with microbiological evidence of infection with L. pneumophila serogroup 1.
b) a clinical diagnosis of pneumonia with microbiological evidence of infection with other L. pneumophila serogroups or other legionella species.

2. Legionellosis - non pneumonic

a) Cases with microbiological evidence of legionella infection (confirmed or presumptive) and symptomatic respiratory illness but without evidence of pneumonia.
b) Pontiac fever - cases with an acute respiratory influenza-like illness without pneumonia, frequently associated with outbreaks.

3. Asymptomatic legionella infection

Laboratory evidence of legionella (confirmed or presumptive) and evidence that the patient had no symptoms associated with the legionella infection.

4. Legionella infection - symptoms not known

Laboratory evidence of legionella (confirmed or presumptive) and symptom status not known.

5. Suspected legionellosis

A compatible illness - pneumonia or influenza-like illness and suspicious but not diagnostic laboratory evidence of legionella infection, ie:

a) an IFAT single titre of 1:16, 1:32 or 1:64 (1:16 or 1:32 in an outbreak)
b) a reported positive serology ("four-fold rise" or "single high titre") but figure and/or test not stated.

Summary of definitions used for epidemiological surveillance:

*When submitted to the Centre for Infections, all positive serum specimens are examined by the IFAT test in the presence of campylobacter blocking fluid, to eliminate cross reactions.
**This is a working definition: the decision to follow up cases is made locally.

Nosocomial (hospital acquired) Legionnaires’ disease:

The following sub-divisions are used for classifying nosocomial cases of Legionnaires’ disease:

  1. Definitely nosocomial:

Patients who spent all of the ten days in hospital before onset of symptoms.

  2. Probably nosocomial:

Patients who spent between one and nine of the ten days in hospital prior to onset of symptoms and either
a) became ill in a hospital associated with one or more cases of Legionnaires' disease
or
b) yielded an isolate that was indistinguishable by monoclonal antibody (mAb) subgrouping, or by molecular typing methods from isolates obtained from the hospital water system at about the same time.

  3. Possibly nosocomial:

Patients who spent between one and nine of the ten days in hospital prior to onset of symptoms, in a hospital not known to be associated with any other cases of Legionnaires' disease and where no microbiological link has been established between the infection and the hospital.

 

Epidemiological Data

• Epidemiological DataSource of the data is the National Surveillance Scheme for Legionnaires' disease in residents of England and Wales and ranges from 1980 to 2007.