Which factors increase the risk for Legionella infection?

Updated: Nov 13, 2018
  • Author: Mobeen H Rathore, MD, CPE, FAAP, FIDSA; Chief Editor: Russell W Steele, MD  more...
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Most nosocomial infections and hospital outbreaks have been linked to contaminated hot water supply. However, contamination of cold-water supply has also been reported. [7] Nosocomial Legionnaires disease associated with water birth is reported in a few neonates, but the risk appears to be low. [8, 9] Person-to-person transmission has not been demonstrated.

Mucociliary action clears Legionella organisms are cleared from the upper respiratory tract. Any process that compromises mucociliary clearance (eg, smoking tobacco) increases risk of infection. Virulence varies between strains of L pneumophila. For example, some strains can adhere to the respiratory epithelial cells via pili, whereas strains with a mutated gene that encodes for the pili show reduced adherence in vitro. [10]

Organisms that reach the alveoli undergo phagocytosis by the alveolar macrophages but are not actively killed. Macrophages may actually support the growth of Legionella organisms. The bacteria multiply intracellularly until the cell ruptures. Liberated bacteria then infect other macrophages. Additional virulence factors include genes that potentiate infection of macrophages and inhibit phagosomal fusion, allowing intracellular growth. [11]

Cell-mediated immunity appears to be the primary host defense mechanism against Legionella infection. Activation of macrophages produces cytokines that regulate antimicrobial activity against Legionella organisms. Individuals with certain deficiencies in cell-mediated immunity are at increased risk for legionellosis. [4] Complicated cases have been reported in children treated with steroids. [12, 13]

The role of neutrophils in host defense against Legionella infection is unclear; neutropenia does not appear to predispose patients to legionellosis. Humoral immunity may play a secondary role.

Once infection is established, Legionella organisms cause an acute fibrinopurulent pneumonia with alveolitis and bronchiolitis. In addition to the lungs, Legionella organisms may infect the lymph nodes, brain, kidney, liver, spleen, bone marrow, and myocardium. [14]

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