Legionnaires' Disease

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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 previously unrecognized causative bacterium 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).
    • It is one of the three most common causes of pneumonias and the most common atypical pneumonia.
  • System(s) affected: pulmonary, gastrointestinal (GI)
  • 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 reported in 2015 (1)
  • Outbreaks most common in late summer/early fall
  • ~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; grow 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 for 30 minutes (2)[C].
  • Ultraviolet light and 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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