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Legionnaires’ disease is a unique bacterial pneumonia that is transmitted to individuals from environmental water sources contaminated with Legionella spp. The disease can be associated with both complex building water systems (e.g., in hospitals, institutions, long-term care facilities, hotels) and with community settings. Outbreaks involving community-associated Legionnaires’ disease cases can escalate rapidly and gain significant attention. In other situations, cases of Legionnaires’ disease may continue within buildings for extended periods of time before an outbreak is detected.

When an outbreak is detected, a multidisciplinary approach involving epidemiology, laboratory science, and environmental health can be key to identifying and controlling the source of an outbreak. In settings with complex water systems, the expertise of environmental health specialists can be useful to understanding parts of the system that may be at risk for Legionella growth. In many outbreaks, source attribution can involve analysis of both epidemiological data and molecular subtyping of Legionella strains in the laboratory. Primary prevention efforts, such as the development and review of water management plans, also require a multidisciplinary approach. Communication specialists can be particularly helpful in designing public health messages on Legionnaires’ disease prevention and providing complex messaging on the progress of public health investigations during outbreaks.


Legionella spp. are Gram-negative aerobic rods with fastidious growth requirements. Approximately 60 species of Legionella are currently recognized.1 At least one-third of these species have been associated with disease.2 L. pneumophila causes the vast majority (~90%) of disease, although L. longbeachae is a major cause of illness in Australia and New Zealand.3 While L. pneumophila is commonly found in aquatic sources, L. longbeachae has been found in potting soils.4 Nearly all species of Legionella require cysteine for growth and the use of specialized media such as buffered charcoal yeast extract (BCYE) is one of the most commonly used media for cultivation of these organisms. Supplementation of BCYE with various antimicrobial agents helps to reduce competing microorganisms present in samples that can hinder isolation of Legionella spp.5

At least 15 serogroups of L. pneumophila have been reported; however, L. pneumophila serogroup 1 causes more than 80% of Legionnaires’ disease cases in the United States.4,6 Serogroup 1 strains can be further characterized using panels of monoclonal antibodies.7,8 Monoclonal antibody 2 (MAb2), also called 3/1 when part of the Dresden panel, recognizes a component of the lipopolysaccharide and strains reacting with this reagent have been shown to contain the gene lag-1.9 Interestingly, MAb2 strains are more commonly isolated from cases of sporadic disease and recovered during outbreak investigations.10 Although it remains unclear if lag-1 provides a selective advantage for isolates causing human disease, it is possible that strains expressing this gene are capable of surviving inhospitable environmental niches or able to evade immune responses.

Clinical disease due to non-Legionella pneumophila spp. is most frequently attributed ...

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