Guidance

Legionnaires' disease: case definitions

Information for health professionals on case definitions for a confirmed, presumptive and travel associated case of Legionnaires' disease.

The national surveillance scheme collect enhanced surveillance data derived from laboratory confirmed reports and follow up surveillance forms in order to determine whether an individual meets the case definition for Legionnaires’ disease.

Legionellosis is the collective term for all cases of legionella infection and includes Legionnaires’ disease and Pontiac fever.

Legionnaires’ disease is the pneumonic form of the disease caused by L. pneumophila or other Legionella species. Pontiac fever is an acute, self-limiting influenza-like illness without pneumonia. It has a high attack rate and is usually detected when outbreaks of this infection occurs.

Non-pneumonic Legionellosis is the term used to describe illness that is urinary antigen positive for L. pneumophila with clinical symptoms that do not fit the definition of Pontiac fever and with no evidence of pneumonia.

Clinical and microbiological case definitions

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 definitions integrate the clinical and microbiological criteria that classify an individual as a ‘confirmed’ or a ‘presumptive’ case of Legionnaires’ disease and is used for surveillance purposes.

Confirmed case of Legionnaires’ disease

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

  • isolation (culture) of legionella species from clinical specimens
  • the presence of L.pneumophila urinary antigen determined using validated reagents or kits
  • seroconversion (a 4-fold or greater increase in titre) determined using a validated indirect immunofluorescent antibody test (IFAT) incorporating a monovalent L. pneumophila serogroup 1 antigen. (When submitted to the reference laboratory at Colindale, all positive serum specimens are examined by the IFAT test in the presence of campylobacter blocking fluid, to eliminate cross reactions)

Presumptive case of Legionnaires’ disease

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

  • detection of Legionella spp. nucleic acid (eg by PCR) in a clinical specimen.
  • a positive direct fluorescence (DFA) on a clinical specimen using validated L. pneumophila monoclonal antibodies (also referred to as a positive result by Direct Immunofluorescence (DIF)).
  • a single high titre of 128 using IFAT or over (or a single titre of 64 in an outbreak) using IFAT incorporating a monovalent L. pneumophila serogroup 1 antigen. (When submitted to the reference laboratory at Colindale, all positive serum specimens are examined by the IFAT test in the presence of campylobacter blocking fluid, to eliminate cross reactions)
  • a 4-fold rise increase in antibodies against other Legionella species or L. pneumophila non-serogroup 1 infections.

Category of exposure

One of the main objectives for the national surveillance scheme is to determine the source of infection in order to implement control measures and prevent further cases. The enhanced surveillance data is used to scrutinise an individuals movements during the incubation period to identify possible sources of exposure. The source of exposure is broadly classified into 3 categories; community, hospital and travel. The definition for these categories and sub-categories are as follows:

Community

Cases with no history of overnight stays in holiday or business accommodation or travel abroad or hospital admission or association with a healthcare facility during the incubation period prior to the onset of illness.

Travel (abroad and UK)

Cases who spend 1 or more overnight stays in holiday or business accommodation in the UK or abroad in the incubation period prior to the onset of symptoms. Overnight stays include accommodation in hotels, camp sites, ships, rented holiday apartments or other tourist facilities. (This definition is used throughout Europe for managing the follow up of travel associated cases reported to ELDSNet)

Nosocomial

Definite nosocomial: cases of Legionnaires’ disease who were in a hospital or nursing home or other healthcare facility for at least 10 days prior to the onset of symptoms.

Probable nosocomial: cases of Legionnaires’ disease who stayed or spent time (eg as an outpatient or a healthcare worker) in a hospital or other healthcare facility for part of the incubation period and where the facility has been associated with 1 or more previous cases of Legionnaires’ disease.

Possible nosocomial: cases of Legionnaires’ disease who stayed or spent time (eg as an outpatient) or who worked in a hospital or other healthcare facility for part of the incubation period but where there have been no previous cases of Legionnaires’ disease or isolates from the hospital water system at about the same time.

Epidemiological definitions of clusters and outbreaks

A further objective of the national surveillance scheme for Legionnaires’ disease is to identify clusters and outbreaks and to initiate investigations to identify the source of infection and implement control measures to prevent further cases. An outbreak of Legionnaires’ disease may present as a cluster 2 or more cases exposed to a point source over a short period of time or as a number of apparently sporadic cases over a prolonged period of time in a highly endemic area. Therefore a cluster of 2 or more cases of Legionnaires’ disease linked in time and place is considered the point where epidemiological and environmental investigations are initiated. Cluster and outbreak definitions are:

Cluster

Two or more cases that initially appear to be linked by area of residence or work, including a healthcare or other type of community setting and which have sufficient proximity in dates of onset of illness (eg 6 months) to warrant further investigation (this is a working definition: the decision to follow up cases is made locally). The area of residence should take account of population size and density when investigations are planned. If after investigation no common exposures to a potential source of infection are identified for the cases, other than the links mentioned above, then they should be classified as sporadic community acquired cases. Consideration should be given to convening an incident control team if a cluster is identified.

Outbreak

Two or more cases where the onset of illness is closely linked in time (weeks rather than months) and where there is epidemiological evidence of a common source of infection, with or without microbiological evidence. An incident control team should always be convened to investigate outbreaks.

Travel associated cluster

Two or more cases who stayed overnight at the same accommodation site in the 2 to 10 days before onset of illness and whose illness is within the same 2 year period. (This definition is used throughout Europe for managing the follow up of travel associated cases reported to ELDSNet).

Travel associated cases of Legionnaires’

The case definitions indicate that an individual diagnosed clinically and microbiologically with Legionnaires’ disease and travelled, either abroad or within the UK, during their incubation period, are categorised as ‘travel associated cases of Legionnaires’ disease’.

European Surveillance Scheme

The general objective of the European Surveillance Scheme is to monitor trend and detect clusters and outbreaks of Legionnaires’ disease associated with commercial accommodation sites used by cases who travelled across the Member States. In turn, the European Surveillance Scheme contributes to the evaluation and monitoring of control and prevention programmes in collaboration with the Member State.

More detailed information can be found on the European Legionnaires’ Disease Surveillance Network (ELDSNet) page of the European Centre for Disease Prevention and Control (ECDC) website.

Minimising risk: Leaflet for managers of tourist accommodation

As a member of the European Legionnaires’ disease Surveillance Network (ELDSNet), it is a requirement to ensure that any accommodation site associated with a single case of Legionnaires’ disease is issued with a Leaflet for managers of tourist accommodation on how to reduce the risk of Legionnaires’ disease. The leaflet includes a 15 point checklist for reducing the risk from Legionella infection and advises on the criteria that should be included in an active risk management program.

Investigation reporting forms A and B

If 2 or more cases of Legionnaires’ disease (a cluster) are identified as being associated with any accommodation site within a 2 year period, a risk assessment needs to be carried out and the findings reported to the ELDSNet.

A preliminary report needs to be completed within 2 weeks of a cluster being identified, using the Investigation reporting form A (MS Word Document, 117KB) .

A full report needs to be completed within 6 weeks of a cluster being identified using the Investigation reporting form B (MS Word Document, 121KB) .

Published 31 January 2013
Last updated 11 July 2016 + show all updates
  1. Minimising risk: leaflet for managers of tourist accommodation has been updated.
  2. Updated investigation reporting forms A and B.
  3. First published.