Legionellosis Outbreak Toolbox
Update | November 2019
Key references documents
- Legionella and the prevention of legionellosis (Geneva: World Health Organization; 2007).
- WHO legionellosis fact sheet (Geneva; World Health Organization; 2018).
Case definitionsWHO suggested outbreak case definition1
Suspected case:
Relevant pneumonic or non-pneumonic illness but no supporting microbiological evidence.
Confirmed case:
Relevant pneumonic illness and microbiological evidence of infection.
Discarded case:
No microbiological evidence of infection.
1From: Legionella and the prevention of legionellosis. (Geneva: World Health Organization;2007)
WHO surveillance case definitions1
Combined microbiological and epidemiological case definitions are used for surveillance of legionellosis. Depending on the diagnostic method used and the result, cases are classified microbiologically as either confirmed or presumptive.
Based on the patient’s clinical history, cases are classified as one of the following:
- Legionnaires’ disease (relevant pneumonic illness and microbiological evidence of infection).
- Pontiac fever or similar illness (relevant non-pneumonic illness and microbiological evidence of infection).
- Asymptomatic legionellae infection (no illness compatible with microbiological result).
- Legionella infection (microbiological evidence of infection but symptoms not known).
- Suspected legionellosis (relevant pneumonic or non-pneumonic illness but no supporting microbiological evidence).
WHO other definitions:
Hospital acquired (nosocomial)
Date(s) of admission to hospital(s) before onset of symptoms.
Community acquired
Known exposure to cooling towers, whirlpool spas, showers, etc.
Travel associated
Countries visited, dates of stay, name and address of accommodation used, room number, name of tour operator, use of showers, spa pools, etc.
Domestically acquired
Use of domestic water system during incubation period, in absence of other risk exposures.
Data collections tools- Case report forms: Not available.
- Line list: Not available.
- Electronic tools: Not available.
Laboratory confirmationChapter 11, Legionella and the prevention of legionellosis (Geneva: World Health Organization; 2007).
Response tools and resources- Not available
Training- Not available
Other resources- Legionnaires' disease outbreak case definitions (Stockholm; European Centre for Disease Prevention and Control).
- Legionnaires' disease outbreak investigation toolbox (Stockholm: European Centre for Disease Prevention and Control; 2019).
- CDC Outbreak Investigation (Atlanta: Centers for Disease Control and Prevention; 2019).
- CDC Epidemiology Investigation Resources (Atlanta: Centers for Disease Control and Prevention, 2018).
- CDC Legionella fact sheet (Atlanta: Centers for Disease Control and Prevention, 2018).
- CDC Legionella materials (Atlanta: Centers for Disease Control and Prevention, 2018).
- European Technical Guidelines for the Prevention, Control and Investigation of Infections Caused by Legionella species, (Stockholm; The European Guidelines Working Group; 2017).