Legionnaires' Disease



  • Legionnaires’ disease was named for an epidemic of lower respiratory tract disease at the 1976 American Legion convention in Philadelphia. 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, apartment buildings, 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; Pontiac Fever (self-limited flulike illness without pneumonia caused by Legionella spp.)


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


  • Cases of Legionnaires’ disease have increased 4-fold in the United States since 2000; almost 10,000 cases are reported in 2018.
  • Outbreaks are most common in late summer/early fall (1).
  • ~2–9% of all cases of pneumonia in the United States; Legionnaires’ disease is fatal in 1 of every 10 cases.

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 (2)
  • Exists in nature as a protozoan parasite, often within fresh water biofilms
  • Serogroups 1 to 6 account for clinical disease.
  • Serogroup 1 represents 70–92% of all clinical cases of Legionella infections 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 was responsible for the inaugural Philadelphia outbreak).
  • Community outbreaks have been associated with whirlpools, spas, fountains, and aboard cruise ships.

Risk Factors

  • Impaired cellular immunity (Legionellae are intracellular pathogens.)
  • Male gender
  • Smoking; alcohol abuse
  • Immunosuppression; HIV; diabetes; organ transplant recipients; chronic or high-dose 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 at least 158°F and flush for 30 minutes.
  • Ultraviolet light and coppersilver ionization are bactericidal.
  • Monochloramine disinfection of municipal water supplies decreases risk for Legionella infection.
  • 0.2 micron water filters—must change regularly
  • Keep water heaters >140°F; cold water <68°F

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