Legionella Pneumonia: Etiology and Treatment
Etiology
Legionella pneumonia is caused by Legionella species, most commonly Legionella pneumophila, a gram-negative bacterium that naturally inhabits freshwater environments and man-made aquatic systems. 1
Environmental Sources and Transmission
The bacteria colonize and multiply in water systems including cooling towers, evaporative condensers, heated potable-water distribution systems, and locally produced distilled water within hospitals. 1
Optimal growth conditions include water temperatures of 25-42°C, stagnation, presence of scale and sediment, and free-living aquatic amoebae that support intracellular bacterial growth. 1
Transmission occurs through inhalation of contaminated aerosols from man-made water systems; person-to-person transmission has never been documented. 2
High-Risk Patient Populations
Patients with severe immunosuppression, chronic underlying illnesses, hematologic malignancies, end-stage renal disease, organ transplant recipients, HIV disease, diabetes mellitus, and underlying lung disease face markedly increased risk. 1, 3
Nosocomial vs. Community-Acquired
In North America, Legionella accounts for 0-14% of nosocomial pneumonias, though this likely underestimates true incidence due to underdiagnosis. 1
Among 196 nosocomial cases in England and Wales (1980-1992), 69% occurred during outbreaks, and when one case is identified, additional cases should be suspected. 1
Hospital-acquired legionellosis is more common in facilities where the organism colonizes the water supply or during ongoing construction. 1
Treatment
First-Line Antibiotic Therapy
For hospitalized patients with Legionella pneumonia, fluoroquinolones—particularly levofloxacin 750 mg IV/PO once daily—are the preferred first-line treatment due to superior clinical outcomes including more rapid defervescence, fewer complications, and shorter hospital stays compared to macrolides. 4
Alternative first-line options include:
Treatment Duration
The standard treatment duration for immunocompetent patients is 7-10 days. 4
Extended treatment of 21 days should be considered for:
- Immunosuppressed patients 4
- Severely ill patients at presentation 4
- Patients with significant comorbidities 4
Route of Administration
Initiate intravenous therapy for hospitalized patients, switching to oral therapy once clinical improvement occurs and temperature has been normal for 24 hours. 4
Treatment Initiation Timing
Treatment should be initiated as rapidly as possible after diagnosis, even with negative diagnostic test results if epidemiologic evidence suggests Legionella infection. 4
Severe Disease and Special Populations
For severe Legionellosis, nosocomial legionellosis, and immunosuppressed patients, intravenous fluoroquinolones are first-choice drugs regardless of severity. 6
Combination therapy may be considered for:
- Patients with severe disease 4
- Significant comorbidities 4
- Immunocompromised hosts 4
- Patients refractory to conventional monotherapy 4
Historical Context and Evidence Quality
While erythromycin was historically used, observational data demonstrate fluoroquinolones achieve superior outcomes with fatality rates of approximately 18.8% with erythromycin versus significantly higher rates with other antibiotics. 7
Diagnostic Considerations
Urinary antigen testing for L. pneumophila serogroup 1 provides rapid diagnosis (within 15 minutes) with 80% sensitivity and 100% specificity, though it only detects serogroup 1. 3, 6
Culture of respiratory secretions or tissues on selective media remains the most specific diagnostic test. 3
Common Pitfalls
Do not delay antibiotic administration beyond 4 hours after admission. 8
Legionnaires' disease cannot be distinguished clinically or radiographically from other pneumonias, requiring high clinical suspicion and appropriate diagnostic testing. 3
Because diagnostic tests are not performed routinely in most U.S. hospitals, the true incidence is likely underestimated. 1
When one nosocomial case is identified, actively search for additional cases as 69% occur during outbreaks. 1