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Legionnaires' Disease

Legionnaires' Disease is a topic covered in the 5-Minute Clinical Consult.

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  • Legionnaires’ disease was named for an epidemic of lower respiratory tract disease at an American Legion convention in Philadelphia in 1976. The causative bacterium was previously unrecognized. It was isolated, identified, and named Legionella pneumophila. The organism primarily causes pneumonia and flulike illness. Legionella preferentially colonizes commercial water systems (e.g., hotels, hospitals, air conditioning cooling towers).
    • Among the three most common clinical pneumonias
    • Most common atypical pneumonia
  • System(s) affected: pulmonary, gastrointestinal
  • Synonym(s): Legionella pneumonia; legionellosis


  • Predominant age: 15 months to 84 years; 74–91% of patients are >50 years old.
  • Predominant gender: male > female

  • U.S. cases of Legionnaires’ disease have increased 4-fold since 2000; >6,000 cases were reported in 2015 (1).
  • Outbreaks are most common in late summer and early fall.
  • Represents 2–9% of all cases of pneumonia in the United States

Etiology and Pathophysiology

  • L. pneumophila is a weak gram-negative aerobic saprophytic freshwater bacterium. It is widely distributed in soil and water. Bipolar flagella provide motility; grows optimally at 40–45°C
  • Exists in nature as protozoan parasite and within fresh water biofilms
  • Serogroups 1 to 6 account for clinical disease.
  • Serogroup 1 represents 70–92% of all clinical cases of Legionella in the United States.
  • In the lung, Legionella infects alveolar macrophages.
  • The organism is transmitted by breathing in contaminated water droplets or by aspiration of contaminated water (e.g., contaminated shower water responsible for the inaugural Philadelphia outbreak).
  • Recently, community outbreaks associated with whirlpools, spas, fountains, and aboard cruise ships

Risk Factors

  • Impaired cellular immunity (Legionella are intracellular pathogens.)
  • Male gender
  • Smoking; alcohol abuse
  • Immunosuppression; HIV; diabetes; organ transplant recipients; corticosteroid use
  • Chronic cardiopulmonary disease
  • Advanced age
  • Use of antimicrobials within the past 3 months

General Prevention

  • Not transmitted person to person (respiratory isolation is unnecessary)
  • Superheat and flush water systems: Heat water to 70°C and flush outlets with hot water for 30 minutes (2)[C].
  • Ultraviolet light or copper–silver ionization are bactericidal.
  • Monochloramine disinfection of municipal water supplies decreases risk for Legionella infection.
  • 0.2 micron water filters—change regularly.
  • Keep water heaters >60°C, cold water <20°C.

Commonly Associated Conditions

Pontiac fever: self-limited flulike illness without pneumonia caused by Legionella species

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Stephens, Mark B., et al., editors. "Legionnaires' Disease." 5-Minute Clinical Consult, 27th ed., Wolters Kluwer, 2019. Medicine Central, im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116344/all/Legionnaires'_Disease.
Legionnaires' Disease. In: Stephens MB, Golding J, Baldor RA, et al, eds. 5-Minute Clinical Consult. 27th ed. Wolters Kluwer; 2019. https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116344/all/Legionnaires'_Disease. Accessed April 20, 2019.
Legionnaires' Disease. (2019). In Stephens, M. B., Golding, J., Baldor, R. A., & Domino, F. J. (Eds.), 5-Minute Clinical Consult. Available from https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116344/all/Legionnaires'_Disease
Legionnaires' Disease [Internet]. In: Stephens MB, Golding J, Baldor RA, Domino FJ, editors. 5-Minute Clinical Consult. Wolters Kluwer; 2019. [cited 2019 April 20]. Available from: https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116344/all/Legionnaires'_Disease.
* Article titles in AMA citation format should be in sentence-case
TY - ELEC T1 - Legionnaires' Disease ID - 116344 ED - Stephens,Mark B, ED - Golding,Jeremy, ED - Baldor,Robert A, ED - Domino,Frank J, BT - 5-Minute Clinical Consult, Updating UR - https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116344/all/Legionnaires'_Disease PB - Wolters Kluwer ET - 27 DB - Medicine Central DP - Unbound Medicine ER -