Legionnaires' disease outbreak investigation toolbox

Download Page

Outbreak sampling procedures

In an outbreak, it may not be possible or appropriate to take samples under the conditions used for routine monitoring, any relevant factors that might affect the sample result should be noted. Always note when the last and next biocide additions are in relation to the time the sample is taken [1].

A sample taken should be representative of the whole system and the situation at that time, though be prioritised by greatest likely risk. Therefore it is important to:

  • neutralise biocides at time of sampling (sufficient thiosulphate to neutralise up to 50 mg/l chlorine or appropriate for the system under review) [2]
  • sample if possible under the worst case conditions, such as where:
    • the minimum biocide concentration is likely to be found
    • the water is likely to be aerosolised
    • areas are most likely to be colonised
    • the system is most likely to have infected the patient
  • protect sample from changes during transport e.g. protect from light and heat
  • analyse as soon as possible after collection and definitely within 24 hours

Sampling typical cooling systems including cooling towers and evaporative condensers

Sampling a typical spa pool and associated cooling system

Sampling domestic hot and cold water systems

Nosocomial outbreaks

Every case of nosocomial legionellosis should constitute an alert, meaning that other cases may have occurred, or could occur, in the future in the healthcare facility (which would constitute an outbreak). Where there is a possibility of a nosocomial case, it should always be investigated [3].

Travel-associated outbreaks

If the outbreak appears to have international context, for additional support see here. Hotels and ships might have a higher concentration of water systems in one building and so the potential sources of Legionellae to be investigated might be increased and focussed prioritisation will be needed.

  1. J. LEE (2009) Sampling for Legionella HPA presentation Milton Keynes http
  2. ENVIRONMENT AGENCY (2005) The determination of Legionella bacteria in waters and other environmental samples http
  3. BARTRAM J. (2007) Legionella and the prevention of legionellosis WHO, Geneva ISBN 92 4 156297 http pdf