Legionnaire's Disease

Author: Michelle Lin, MD
Updated: 9/16/2011

Legionnaire's Disease

Pneumonia caused by any Legionella species


  • Outbreaks related to exposure to colonized water
  • Air travel
  • Water puddles
  • Excavation
  • Construction sites
  • Most commonly found in patients age > 50 years, smokers, immunocomprised
  • Differential diagnosis often involves other atypical pneumonia and zoonotic organisms
  • Requires reporting to your state's health department to help track for outbreaks

Clinical Presentation

Acute presentation of severe community acquired pneumonia (CAP)

Classic Chest X-Ray: Rapidly progressive asymmetrical patchy infiltrates

  • Besides Legionella, the differential diagnosis for such a CXR finding includes:
    1. Strep pneumoniae in patients with impaired splenic function
    2. Zoonotic atypical pathogens (Q fever, tularemia, adenovirus)

Extrapulmonary organ involvement:

  • CNS: Mental confusion, encephalopathic, headache
  • Cardiac: Relative bradycardia
  • GI: Loose stools, watery diarrhea
  • Renal: Microscopic hematuria, renal insufficiency
Temperature Appropriate HR (bpm) Relative Bradycardia (bpm)
102°F (38.9°C) 110 < 100
103°F (39.4°C) 120 < 110
104°F (40.0°C) 130 < 120
105°F (40.6°C) 140 < 130

Legionella clinical pathway

Classic Laboratory Findings

  • Na: Low in CAP but mostly frequently associated with Legionella
  • AST/ALT: Mildly increased 2-5x normal
  • Phosphate: Decreased
  • CK: Increased
  • CRP: > 35 mg/L
  • Ferritin: Increased > 2x normal

Legionella Diagnostic Triad (Cunha et al.)

Need All 3 Criteria Key Clinical Feature Key Lab Features (Any 3)
Signs/sx of CAP T >102F with unexplained relative bradycardia Hypophophatemia
New infiltrate on CXR | Ferritin> 2x normal
No zoonotic contact hx | Mild transaminitis
| Relative lymphopenia

Diagnostic Test

  • Urine Legionella antigen: Only detects serotypes 1-6

Antibiotic Treatment

  • Macrolides: First-line agent, although a growing resistance
  • Other antibiotics: Doxycycline, fluoroquinolones
  • Duration of treatment: 2 weeks


  • Cunha BA, Mickail N, Syed U, Strollo S, Laguerre M. Rapid clinical diagnosis of Legionnaires' disease during the "herald wave" of the swine influenza (H1N1) pandemic: the Legionnaires' disease triad. Heart Lung. 2010 May-Jun;39(3):249-59. [PubMed]
  • Cunha BA. Legionnaires' disease: clinical differentiation from typical and other atypical pneumonias. Infect Dis Clin N Am. 2010;24(1):73-105. [PubMed]