A basic working definition for a cluster of Legionnaires' disease cases is
essentially multiple cases that may be linked in space and time. An outbreak
of Legionnaires' disease is then a special case of a cluster where a common
source is suspected.
Identification of multiple cases of Legionnaires' disease, linked by common exposure, is
reliant on a high index of suspicion and should be considered when the number of cases in an
area is clearly in excess of the normal frequency expected for a specific time period. These
excesses of Legionnaires' disease cases may present as a cluster of two or more cases following
exposure to a single environmental source during a short period of time or as a number of
apparently sporadic cases over a prolonged period of time in an area in which it is highly
endemic. A cluster of two or more cases linked in time and place is therefore the starting
point for epidemiological and environmental investigations of potential links that may
eventually lead to the detection of an outbreak associated with an environmental source of
infection. If no links are found, the environmental actions in response to a single case are
determined locally. Clusters and outbreaks may occur within a hospital or community setting or
be linked to travel either in the home Member State or abroad.
It may be that the initial cases are from other member states and were identified via travel
history reported through ELDSNet
An outbreak is defined as two or more cases where the onset of illness is closely linked
in time (weeks rather than months) and in space, where there is suspicion of, or
evidence of, a common source of infection, with or without microbiological support (i.e.
common spatial location of cases from travel history). An outbreak control team should always
be convened to investigate outbreaks.
Two or more cases that initially appear to be linked by space (for example area of
residence, work, or other setting [i.e. healthcare or community]) and which have
sufficient proximity in dates of onset of illness (e.g. six months) to warrant further
investigation. The geographical area used to define excess based in spatial proximity should
take account of population size and density when investigations are planned. Such excesses over
baseline will vary by member state and local discretion should be used. If, after
investigation, no common exposures to a potential source of infection are identified for these
cases, other than the links mentioned above, then they should be classified as sporadic
community-acquired cases. Consideration should be given to convening an outbreak control team
if a cluster is identified.
An alternative classification of cluster is one where cases are linked by close onset dates but
only by a very broad geographic proximity. Such clusters are not likely to cause convening of
outbreak control teams as they are likely to arise due to, for example, changes in
meteorological conditions rather than common source of infection.
Clusters might occur with two or more cases having stayed overnight at the same accommodation
site in the 14 days before onset of illness and whose illness is within say the same two-year
period. Such travel associated cases may be identified at their home location and so should be
reported to the public health authorities in the region of travel to aid their local
Where outbreaks are associated with more than one country
countries involved may take part in an investigation through exchanging clinical and
environmental specimens or sequence typing data from an outbreak and this is being done more
and more. International collaborations help to validate diagnostic tests and the
microbiological association between cases and sources of infection.
Similar data collection algorithms and facilitation of data sharing will aid cross-border