Developing Understanding

Legionnaires' disease

Legionnaires' disease is an atypical form of pneumonia caused by bacteria of the genus Legionella. Diagnosis of cases is based on clinical identification of pneumonia and microbiological confirmation. The majority of cases are reported as single (isolated or sporadic) cases that occur throughout the year but with a seasonal variation with most cases occurring in late summer early and autumn. However, clusters (cases associated in space and time) and outbreaks (clusters with a suspected common source) can also occur. People become infected when they inhale Legionella bacteria which have been released into the air in aerosolised form from a contaminated source. As such, when faced with an outbreak or cluster, careful consideration should be given to any potential sources in the locality. Given that the infectious agent is airborne, if cases appear close to a national border or have a history of travel, protocols should be enacted to enable swift cooperation between states, such as data sharing and effective communication strategies. A schematic showing a potential outbreak with major events and decision points is available.

Following a search of the scientific and technical literature, articles relevant to outbreak scenarios have been reviewed and summarised on subsequent pages. The publication name, citation and any public access link are provided:

• Legionnaires' disease background
• Legionella in the environment
  • Potential sources
  • Environmental effects on Legionella
• Legionella effects on individuals and diagnosis challenges
  • Infectiousness of aerosolised organisms
  • Incubation period
  • Immune response to a Legionella infection
  • Diagnosis of Legionnaires' disease - underdiagnosis and underreporting
  • Experimental studies of animal infection with Legionella
  • Background immunity/antibody level in population
• A review of past outbreaks
  • Previous use of GIS-based analysis during an outbreak

Outbreak Preparedness

Prior to any outbreak occurring in a locality, suitable preparation should be made to ensure staff are aware of their expected roles and responsibilities during an outbreak (training and safety considerations) and the context of the legal issues. Guidance on the constitution and membership of an outbreak control team is available.

Outbreaks and clusters might involve travel-associated cases (maybe as heralds of subsequent local cases). In this situation, staff should be aware of their responsibilities should a history of international travel become apparent from a case's interview history. For further information on outbreaks with potential international implications.

Staff involved in environmental investigations should be aware of the requirements of environmental sampling procedures and potential engineering issues (specimen engineering designs for cooling systems and spa pools and associated water systems). A thorough risk assessment should be carried out prior to site visits, see here for additional information on creating a risk management plan for a site. The outbreak control team first reviews the epidemiological information provided by each case to decide where to focus initial environmental investigations and control measures. If the patients are all associated with a particular building, the initial investigations should be targeted at all the water uses in that building, otherwise alternative potential sources should be considered. Similarly, prior to departure to a potential source location, guidance is offered here.

It might be helpful to consider a list of equipment and materials that field staff will find helpful prior to site deployment. An indicative list is available here but as each outbreak is unique this should be reviewed given the particular outbreak demands.