Legionella infection (legionellosis)
Legionella pneumophila is an intracellular, water-borne gram-negative bacillus causing severe community-acquired pneumonia with prominent gastrointestinal and hepatic features, confirmed by urinary antigen testing and treated with a macrolide or fluoroquinolone rather than a beta-lactam.
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What this article covers
- Overview
An intracellular gram-negative bacillus causing severe atypical pneumonia: Legionella pneumophila is a fastidious, water-borne pathogen causing 2% to 15% of community-acquired pneumonia (CAP), likely underrecognized.
- Epidemiology
Accounts for a meaningful minority of CAP: Legionella causes 2% to 15% of CAP illnesses, with likely underrecognition due to reliance on serogroup 1-limited urinary antigen testing.
- Risk factors
Immunocompromise, especially long-term corticosteroid therapy: patients on chronic corticosteroids and other immunocompromised hosts are especially susceptible to Legionella infection.
- Pathophysiology
Inhalation of contaminated aerosols delivers the organism to the alveoli: Legionella is acquired by inhaling aerosolized water droplets containing the bacterium rather than by person-to-person spread.
- Clinical features
Fever, weakness, malaise, and a nonproductive (dry) cough dominate the presentation: these constitutional and respiratory symptoms typically develop over 2 to 10 days after exposure (the incubation period) and progress to pneumonia.
- Differential diagnosis
Streptococcus pneumoniae pneumonia: the most common typical bacterial cause of CAP, generally presenting with lobar consolidation and productive cough rather than the diarrhea/hyponatremia pattern…
- Diagnosis
The diagnosis is confirmed by a positive Legionella urinary antigen test (sensitive for L.
- Management
First, determine the site of care using the IDSA/ATS severe CAP criteria or Pneumonia Severity Index, since this determines whether empiric therapy is outpatient, inpatient non-ICU, or ICU-level.
- Complications
Respiratory failure requiring mechanical ventilation: severe cases can progress to acute respiratory distress syndrome and invasive ventilatory support.
- Prognosis
Outcome depends heavily on severity at presentation and host immune status: patients meeting ICU-level severity criteria or who are immunocompromised have substantially higher mortality than those with milder disease.
- Prevention
Water system maintenance is the primary preventive measure: hospitals and large buildings should maintain cooling towers and hot water systems above the temperature ranges that favor Legionella…
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Community-acquired pneumonia is a lower respiratory tract infection diagnosed by fever with cough, dyspnea, or chest pain plus a new infiltrate on chest imaging, with Streptococcus pneumoniae, respiratory viruses, and atypical organisms as leading causes. Site of care is guided by the Pneumonia Severity Index or IDSA/ATS criteria, outpatient therapy is amoxicillin or doxycycline, and hospitalized patients receive a beta-lactam plus a macrolide or a respiratory fluoroquinolone, escalated to cover Pseudomonas or MRSA when risk factors are present.
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