Legionella pneumophila - Legionnaire Disease

Legionellosis refers to two clinical syndromes caused by bacteria of the genus Legionella – Legionnaire disease and Pontiac fever.

Epidemiology

• Incidence – >10,000 annual cases of Legionnaire disease in U.S.
  • May be responsible for 3-8% of all community-acquired pneumonia (CAP)
• Gender – M>F
• Transmission
  • Most cases are sporadic
  • More common in summer months – outbreaks of pneumonia have occurred after inhalation of contaminated water droplets from water sources such as evaporative coolers, hot tubs, heating systems and respiratory therapy equipment
  • Person-to-person transmission has not been shown
  • Nosocomial cases are not uncommon, with sources of infection often being hospital showers, drinking fountains and respiratory-therapy equipment

Organism

• Legionella is a gram-negative bacterium that parasitizes freshwater amoeba in the environment and pulmonary macrophages in humans
• 42 species comprise 64 serogroups of Legionellae
• L. pneumophila is the major pathogen and the most commonly encountered member of the group

Risk Factors

• Compromised immune system (immunosuppressive therapy or T-cell dysfunction)
• Underlying chronic disease (e.g., chronic obstructive pulmonary disease)
• Recent surgery
• Older age
• Tobacco abuse

Clinical Presentation

• Pontiac fever
  • A self-limiting, febrile illness with a non pneumonic, influenza-like illness
    • Usually affects healthy individuals
• Legionnaire disease
  • Generally includes gastrointestinal symptoms - primarily loose stools, diarrhea and/or abdominal pain
  • Can be associated with a variety of other systemic problems, including kidney, liver and central nervous system dysfunction
  • The case-fatality rate for patients with legionellosis is 5-30%, with the elderly and immunocompromised patients at greater risk for death

Diagnosis

• Laboratory testing
  • Rapid testing is important for early initiation of treatment
    • PCR testing of respiratory secretions
    • DFA or urine antigen testing 
      • Urine antigen testing is less sensitive
    • Serological testing may be used for retrospective diagnosis

Differential Diagnosis

• Other atypical pneumonia (e.g. mycoplasma)

Treatment

• Supportive
• Antibiotic therapy

Indications for Ordering

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory

• Click on number for test-specific information in the ARUP Laboratory Test Directory