Legionnaires Disease

Key Features

• 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

Diagnosis

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

• Gram stain of sputum: polymorphonuclear leukocytes and no organisms

• Culture of Legionella species has 80–90% sensitivity

• Testing sputum samples using PCR is a highly sensitive method for diagnosing Legionella

• 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

• Chest radiograph: focal patchy infiltrates or consolidation

Treatment

• Drugs of choice

  • Azithromycin, 500 mg orally once daily

  • Clarithromycin, 500 mg orally twice daily

  • A fluoroquinolone (eg, levofloxacin, 750 mg orally once daily)

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