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Legionellosis refers to the two clinical syndromes caused by bacteria of the genus Legionella. Pontiac fever is an acute, febrile, self-limited illness that has been serologically linked to Legionella species, whereas Legionnaires’ disease is the designation for pneumonia caused by these species. Legionnaires’ disease was first recognized in 1976, when an outbreak of pneumonia took place at a Philadelphia hotel during an American Legion convention.


The family Legionellaceae comprises more than 50 species with more than 70 serogroups. The species Legionella pneumophila causes 80–90% of human infections and includes at least 16 serogroups; serogroups 1, 4, and 6 are most commonly implicated in human infections. To date, 18 species other than L. pneumophila have been associated with human infections, among which L. micdadei (Pittsburgh pneumonia agent), L. bozemanii, L. dumoffii, and L. longbeachae are the most common. Members of the Legionellaceae are aerobic gram-negative bacilli that do not grow on routine microbiologic media. Buffered charcoal yeast extract (BCYE) agar is the medium used to grow Legionella.


The natural habitats for L. pneumophila are aquatic bodies, including lakes and streams. L. longbeachae has been isolated from natural soil. Commercial potting soil has been suggested as the reservoir for L. longbeachae infections in Australia and New Zealand. Legionella can survive under a wide range of environmental conditions; for example, the organisms can live for years in refrigerated water samples. Natural bodies of water contain only small numbers of legionellae. However, once the organisms enter human-constructed aquatic reservoirs (such as drinking-water systems), they can grow and proliferate. Factors known to enhance colonization by and amplification of Legionella include warm temperatures (25–42°C) and the presence of scale and sediment. L. pneumophila can form microcolonies within biofilms; its eradication from drinking-water systems requires disinfectants that can penetrate the biofilm. The presence of symbiotic microorganisms, including algae, amebas, ciliated protozoa, and other water-dwelling bacteria, promotes the growth of Legionella. The organisms can invade and multiply within free-living protozoa.

image Heavy rainfall and flooding can result in the entry of high numbers of legionellae into water-distribution systems, leading to an upsurge of cases. Climate change may be a factor in the apparent increase in incidence of Legionnaires’ disease worldwide.

Large buildings over three stories high are commonly colonized with Legionella. Sporadic community-acquired Legionnaires’ disease has been linked to colonization of hotels, office buildings, factories, and even private homes. Drinking-water systems in hospitals and extended-care facilities have been the source for health care–associated Legionnaires’ disease.

In contrast, cooling towers and evaporative condensers have been overestimated as sources of Legionella causing human illness. Early investigations that implicated cooling towers antedated the discovery that the organism could also exist in drinking water. In many outbreaks attributed to cooling towers, cases of Legionnaires’ disease continued to occur despite ...

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