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For further information, see CMDT Part 33-15: Legionnaires Disease

Key Features

  • Patients are often immunocompromised, smokers, or have chronic lung disease

  • Ranks among the three or four most common causes of community-acquired pneumonia

  • Classically, this pneumonia is caused by Legionella pneumophila, though other species can cause identical disease

  • Occurs more commonly in immunocompromised persons, in smokers, and in those with chronic lung disease

  • Outbreaks have been associated with contaminated water sources, such as shower heads and faucets in patient rooms, and air conditioning cooling towers

Clinical Findings

  • Many features of typical pneumonia, with high fevers, a toxic appearance, pleurisy, and grossly purulent sputum

  • Nausea, vomiting and diarrhea may be prominent

  • There may be relative bradycardia


  • Hyponatremia, hypophosphatemia, elevated liver enzymes, and elevated creatine kinase

  • Gram stain of sputum: polymorphonuclear leukocytes and no organisms

  • Culture of Legionella species has a 80–90% sensitivity

  • Culture onto charcoal-yeast extract agar or similar enriched medium

    • Most sensitive method for diagnosis

    • Permits identification of infections caused by species and serotypes other than L pneumophila serotype 1

  • Dieterle silver staining of tissue, pleural fluid, or other infected material is also a reliable method for detecting Legionella species

  • Direct fluorescent antibody stains and serologic testing such as urinary antigen are less sensitive because these will detect only L pneumophila serotype 1

  • Testing sputum samples using polymerase chain reaction is highly sensitive method for diagnosing Legionella

  • Chest radiograph: focal patchy infiltrates or consolidation


  • Following regimens all administered for 10–14 days

    • Azithromycin, 500 mg then 250 mg once daily, orally or intravenously

    • Clarithromycin, 500 mg orally twice daily

    • Levofloxacin, 750 mg once daily orally or intravenously

  • Duration of therapy is 10–14 days, although a 21-day course of therapy is recommended for immunocompromised patients

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