LEGIONELLA - World Health Organization

Executive summary
Legionellosis is a collection of infections that emerged in the second half of the 20th century,
and that are caused by Legionella pneumophila and related bacteria. The severity of legionellosis
varies from mild febrile illness (Pontiac fever) to a potentially fatal form of pneumonia
(Legionnaires’ disease) that can affect anyone, but principally affects those who are susceptible
due to age, illness, immunosuppression and other risk factors, such as smoking.
Legionella is an important pathogen in health-care acquired (nosocomial) pneumonia, particularly
in immunocompromised patients. Legionella spp. can also cause community-acquired pneumonia,
which has a high rate of hospital admission. Legionnaires’ disease is recognized as a major
form of travel-associated pneumonia, and about 20% of the cases of legionellosis detected in
Europe are considered to be related to travel; these cases present a particular set of problems
because of difficulties in identifying the source of infection. Although Legionella is a wellrecognized
problem in developed nations, data are scarce from developing countries. Since
risk environments and susceptible populations are found worldwide, it is likely that the
problem of Legionella is under-appreciated in developing countries.
Chapter 1 describes the disease types caused by Legionella bacteria, including risk factors, prevalence
and outcomes of Legionnaires’ disease. Although all Legionella species are considered
potentially pathogenic for humans, Legionella pneumophila is the aetiological agent responsible
for most reported cases of Legionnaires’ disease.
Chapter 2 discusses the ecology and environmental sources of Legionella. Water is the major
natural reservoir for legionellae, and the bacteria are found worldwide in many different natural
and artificial aquatic environments and ranges of environmental conditions, such as cooling
towers; water systems in hotels, homes, ships and factories; respiratory therapy equipment;
fountains; misting devices; and spa pools.
The fact that legionellae are found in hot-water tanks or thermally polluted rivers emphasizes
that water temperature is a crucial factor in the colonization of water distribution systems.
L. pneumophila has been shown to be able to withstand temperatures of 50 °C for several hours,
but does not multiply below 20 °C (Fliermans, Soracco & Pope, 1981; Katz & Hammond,
1987; Colbourne et al., 1988; Bentham 1993). It is for this reason that the recommended
temperature for storage and distribution of cold water is below 25 °C and ideally below 20 °C.
Thus, the presence of Legionella in an aquatic environment and warm temperatures are two
factors that can increase the risk of Legionnaires’ disease.
The presence of biofilms is important for Legionella survival and growth in water systems.
Legionellae are found in sources such as distributed drinking-water supplies, which then feed
into water systems within buildings and cooling towers, accounting for the bacteria’s presence
and subsequent growth in these artificial environments.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS xx