Legionnaires' disease outbreak investigation toolbox

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Environmental investigation

In an outbreak investigation, it is important to prevent further cases and make sure that the source of infection has been located. Ideally, the investigation would obtain environmental isolates which can then be matched with those of the cases (if available). Evidence obtained from environmental investigations on its own does not give strong evidence to associate a source with the outbreak, but would add weight to the epidemiological evidence in the absence of microbiological evidence. It is extremely important that potential sources be sampled before any precautionary disinfection.

Dynamic Risk Assessment

In an outbreak situation, a risk assessment should be carried out at the start of each site visit to identify risks and to assess that controls are in place to protect field scientists. A clinical risk assessment has to be carried out in hospital settings to assess the risk to the most vulnerable patients and prioritise remedial work. However, in some instances in an outbreak investigation, there may be no information available on the design of the system or of previous system risk assessments; therefore one would need to be conducted to support the outbreak investigation and the health and safety interests of sampling staff.

The environmental investigation assesses the source system's risk of causing legionellosis and includes assessment on the likelihood of contamination, multiplication and dissemination of Legionella. The specifics of the investigation depend on the risk of the source. This may include:

  • The risk management document for the site and the "significant findings"
  • Details of the appointed person and deputy and other competent persons involved in the control regime
  • Communications arrangements and training requirements
  • A schematic plan of the water system(s)
  • A description of the correct and safe operation of the system
  • The precautions to be taken (the control regime)
  • A records of checks that have been carried out to ensure the efficacy of the control regime (i.e. monitoring and test results, inspection and checks data)
  • Details of the remedial action that should be taken in the event of failure of the control regime.
  • The chemical or physical (e.g. temperature) water treatment programme
  • Water quality/system control data
  • Remedial measures to be taken when control parameters are exceeded
  • Cleaning and disinfection procedures
  • Record of any audits that have been carried out

Environmental sampling in an outbreak situation

The type and volume of sampling that is required during an investigation is directed by the epidemiological information that is available at the outset of the investigation. It is essential to prioritise the sampling to target high risk processes or equipment first, followed by sampling of lower risk processes. The samples taken normally comprise of samples of water taken from a water distribution system and swabs of the biofilms found on the surfaces of water systems.