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

Key Features

Essentials of Diagnosis

  • Patients are often immunocompromised, smoke cigarettes, or have chronic lung disease

  • Scant sputum production, pleuritic chest pain, toxic appearance

  • Chest radiograph: focal patchy infiltrates or consolidation

  • Gram stain of sputum: polymorphonuclear leukocytes and no organisms

General Considerations

  • 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


  • An estimated 8000–18,000 cases occur each year in the United States

  • Most cases are sporadic; 10–20% can be linked to outbreaks, and about 20% are hospital-associated

Clinical Findings

Symptoms and Signs

  • 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

Differential Diagnosis

  • Other infectious pneumonia

  • Pulmonary embolism

  • Aspiration pneumonia

  • Myocardial infarction

  • Pleurodynia (coxsackievirus)


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

  • Gram-stained smear of sputum does not show 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

  • Polymerase chain reaction is a highly sensitive method for detecting Legionella in sputum samples


  • Following regimens all administered for 10–14 days

    • Levofloxacin, 750 mg once daily orally or intravenously

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

    • Clarithromycin, 500 mg orally twice daily

  • Azithromycin may be effective as a 5-day regimen

  • A 21-day course of treatment and combination therapy (eg, addition of rifampin 300 mg twice daily or macrolide-fluoroquinolone) are recommended in the immunocompromised patient; improved outcome is not certain

  • Tetracyclines and trimethoprim-sulfamethoxazole are used for the patient who cannot be treated with either a macrolide or a fluoroquinolone



  • Improved design and maintenance of cooling towers and plumbing systems to limit the growth and spread of Legionella organisms

  • Person-to-person transmission does not occur

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