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  • 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.


Legionella infection ranks among the three or four most common causes of community-acquired pneumonia and is considered whenever the etiology of a pneumonia is in question. Legionnaires disease is more common in persons who smoke cigarettes and in those with chronic lung disease or who are immunocompromised. Outbreaks have been associated with contaminated water sources, such as showerheads and faucets in patient rooms and air conditioning cooling towers.


A. Symptoms and Signs

Legionnaires disease is one of the atypical pneumonias, so called because a Gram-stained smear of sputum does not show organisms. However, many features of Legionnaires disease are more like typical pneumonia, with high fevers, toxic appearance, pleurisy, and grossly purulent sputum. Nausea, vomiting, and diarrhea may be prominent. There may be relative bradycardia. Classically, this pneumonia is caused by Legionella pneumophila, though other species can cause identical disease.

B. Laboratory Findings

There may be hyponatremia, hypophosphatemia, elevated liver enzymes, and elevated creatine kinase. Culture of Legionella species has an 80–90% sensitivity. Testing sputum samples using polymerase chain reaction is as a highly sensitive method for diagnosing Legionella. The use of charcoal-yeast extract agar or similar enriched medium is the most sensitive method for diagnosis and 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.


Azithromycin (500 mg orally once daily), clarithromycin (500 mg orally twice daily), or a fluoroquinolone (eg, levofloxacin, 750 mg orally once daily), and not erythromycin, are the drugs of choice for treatment of legionellosis. These agents have excellent intracellular penetration and in vitro activity, as well as desirable pharmacokinetic properties that permit oral administration and once or twice daily dosing. Duration of therapy is 10–14 days, although a 21-day course of therapy is recommended for immunocompromised patients.

Cunha  BA  et al. Legionnaire's disease: a clinical diagnostic approach. Infect Dis Clin North Am. 2017;31:81.
[PubMed: 28159178]  
Herwaldt  LA  et al. Legionella: a reemerging pathogen. Curr Opin Infect Dis. 2018;31:325.
[PubMed: 29794542]  

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