Guidance

Legionnaires' disease: case definitions

Updated 6 February 2020

The national Legionnaires’ disease surveillance scheme collects 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 types of illness caused by legionella bacteria, and includes:

• Legionnaires’ disease – atypical pneumonia caused by legionella bacteria, which can be fatal
• Pontiac fever – a milder, self-limiting respiratory illness
• non-pneumonic legionellosis – an uncommon syndrome with microbiological confirmation of legionella infection from a clinical specimen with no evidence of pneumonia or Pontiac fever

Clinical and microbiological case definitions

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 a clinical lower respiratory tract specimen
• detection of Legionella pneumophila antigen in a urine specimen
• detection of Legionella species nucleic acid (such as via PCR) in a lower respiratory tract specimen (such as sputum, bronchoalveolar lavage (BAL)

Probable case of Legionnaires’ disease (following declaration of an outbreak only)

A case with clinical or radiological diagnosis of pneumonia but no microbiological evidence for confirmation of Legionella infection (above) is considered a probable case where the location and onset date(s) meet the outbreak-specific case definition for the exposure category (described in the section below).

For more information on Legionnaires’ disease: guidance, data and analysis is available.

Category of exposure

The national surveillance scheme supports the public health investigation of sources of infection for cases of legionellosis; this allows control measures to be implemented to prevent further cases. Potential sources of exposure can broadly be divided into 3 categories; community-acquired, healthcare-associated and travel-associated. Definitions for these categories and sub-categories are:

Community-acquired

A case where the most likely potential source of infection is in the community or where there is no evidence of travel or healthcare-associated exposure during the 2 to 10 days before symptom onset

Travel-associated

A case who either stayed overnight or was considered to have had significant exposure to the water system during a visit (for example the use of spa and leisure facilities) to holiday or business accommodation during the 2 to 10 days before symptom onset. Accommodation sites include hotels, campsites, ships, rented holiday apartments or other tourist facilities.

Healthcare-associated

A case who either stayed overnight or was considered to have had significant exposure to a healthcare associated premises for some or all the 2 to 10 days prior to onset of symptoms.

Healthcare associated cases can be sub-categorised based on the type (whether the site has been associated to previous cases) and amount (time spent at the site) of exposure the case was subjected to in the 2 to 10 days prior to onset of symptoms.

Type A: a case who stayed overnight in healthcare associated premises for the entire 2 to 10 days before symptom onset.

Type B: a case who stayed overnight or worked as a regular employee in healthcare associated premises for any of the 2 to 10 days before symptom onset AND the premises have been associated with any previous case(s) of Legionnaires’ disease or other legionellosis (if the previous cases occurred less than 2 years previously, this should be investigated as a cluster).

Type C: a case who stayed overnight or worked as a regular employee in healthcare associated premises for any of the 2 to 10 days before symptom onset AND the premises have not been associated with any previous cases of Legionnaires’ disease or other legionellosis

Type D: a case who visited healthcare associated premises during the 2 to 10 days before symptom onset, including as an occasional, transient worker but did not stay overnight and did not work at the premises as a regular employee; and a risk assessment indicates investigation of the case as healthcare associated due to, for example, frequency of visits, nature of treatment, previous cases associated with the site or documented exposure to a source known to be contaminated.

Further information on these subgroups is included in Investigation of cases, clusters and outbreaks of Legionnaires’ disease.

Epidemiological definitions of clusters and outbreaks

Clusters

Two or more cases with symptom onset close in time, within months or years depending on the category of exposure (see below), close in space and/or share an epidemiological link according to the following exposure-specific definitions:

Healthcare-associated cluster:

Two or more cases of Legionnaires’ disease where each case stayed/visited/worked at the same healthcare facility during the 2 to 10 days before symptom onset, and the onset date of each case is within 2 years of another case in the cluster.

Travel-associated cluster:

Two or more cases of Legionnaires’ disease where each case either stayed overnight or were considered to have had significant exposure to the water system during a visit (such as the use of spa or leisure facilities) to the same accommodation site during the 2 to 10 days before symptom onset, and the onset date of each case is within 2 years of another case in the cluster.

Community cluster:

Two or more cases of Legionnaires’ disease that were geographically linked by a common setting or within 6 kilometres of each other by places of residence, work or other community location during the 2 to 10 days before symptom onset; the onset of each case should be within 6 months of another case in the cluster.

Important notes about clusters

1. A case can be included in more than one cluster until and unless microbiological evidence from clinical and environmental specimens has identified or excluded a specific location as the likely source.

2. Consult the PHE guidance on investigation of cases, clusters and outbreaks of Legionnaires’ disease for further information and examples of how these definitions may be applied.

Outbreaks

Two or more cases of Legionnaires’ disease where each case, during the 2 to 10 days before symptom onset, either:

a) stayed, or visited or worked at the same healthcare premises (Healthcare-associated)

OR

b) stayed overnight or were considered to have had a significant exposure to the water system during a visit to the same accommodation site (Travel-associated)

OR

c) was geographically linked by a common setting or within 6 kilometres of each other by places of residence, work, or other community location (Community)

AND

The symptom onset date for each case is within 28 days of at least one other case associated with the exposure site (either healthcare facility, travel site or community setting)

AND

One or more of the following apply:

a) isolates from clinical AND environmental specimens are indistinguishable

b) isolates from lower respiratory tract specimens from at least 2 cases are indistinguishable

c) strong epidemiological evidence for link(s) between ALL cases.

For example:

• cases who stayed on the same hospital ward
• cases who used the same spa pool in travel or leisure facility
• cases live in a multi-occupancy residence with a common water supply (or any other evidence of a common source)

Important notes about outbreaks

1. It is possible to have one or more outbreaks within a larger cluster.

2. Outbreaks are defined as healthcare-associated, travel-associated or community-acquired. Which definition is used will depend on the source of exposure type identified.

3. Consult the PHE guidance on investigation of cases, clusters and outbreaks of Legionnaires’ disease for further information and examples of how these definitions may be applied.