Description

• The term Legionnaires’ disease was coined for an epidemic of lower respiratory tract disease occurring among people attending an American Legion convention in Philadelphia in 1976. Until this outbreak, the causative bacterium was unknown. It was isolated, identified, and named Legionella pneumophila, due to its ability to cause pneumonia and flulike illnesses. The bacteria inhabit manmade water system in hotels, hospitals, and air conditioning cooling towers:
  • Ranks among the 3 most common pneumonias in the clinical setting
  • Most common atypical pneumonia
• System(s) affected: Gastrointestinal; Pulmonary
• Synonym(s): Legionella pneumonia; Legionellosis

Epidemiology

• Predominant age: 15 months–84 years; increased >50 years
• Predominant gender: Male > Female

• Reported cases increased from 1,310 in 2002, to 2,223 in 2003, with >2,000 cases/year from 2003–2005
• Outbreaks occur most often at the end of the summer and early fall.

Risk Factors

• Impaired cellular immunity (Legionella are intracellular pathogens)
• Smoking
• Alcohol abuse
• Immunosuppression/HIV
• Chronic cardiopulmonary disease
• Surgery
• Advanced age
• Transplant recipients
• Diabetes mellitus
• Use of antimicrobials within the past 3 months

General Prevention

• Not transmitted person to person (isolation is unnecessary)
• Superheat and flush water systems: Water is heated to 70°C, and distal outlets are flushed with hot water for 30 minutes.
• Ultraviolet light or copper-silver ionization are bactericidal.
• Monochloramine disinfection of municipal water supplies is associated with decreased risk for Legionella infection.

Etiology

• Legionella pneumophila, a weak gram-negative aerobic organism, is a saprophytic freshwater bacterium, widely distributed in soil and water. Bacteria are motile by bipolar flagella. Optimum temperature for growth is 40–45°C.
• Exists as an intracellular parasite of protozoa, colonizes surfaces and grows in biofilms, which persist in nematodes
• Serogroups 1–6 account for cases of disease.
• In the lung, Legionella infects alveolar macrophages.
• Mode of transmission:
  • Aspiration
  • Direct transmission into the lungs by equipment, such as respiratory equipment
  • Most important mode: Aerosolization and airborne dissemination of contaminated water, such as inhaling organisms while showering
• Recently, community outbreaks have been associated with whirlpools, spas, and fountains.
• Not spread between humans

Commonly Associated Conditions

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