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Legionella Infections

By Larry M. Bush, MD, Affiliate Professor of Clinical Biomedical Sciences; Affiliate Associate Professor of Medicine, Charles E. Schmidt College of Medicine, Florida Atlantic University; University of Miami-Miller School of Medicine
Maria T. Perez, MD, Associate Pathologist, Department of Pathology and Laboratory Medicine, Wellington Regional Medical Center, West Palm Beach

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Legionella pneumophila most often causes pneumonia with extrapulmonary features. Diagnosis requires specific growth media, serologic testing, or PCR analysis. Treatment is with macrolides fluoroquinolones or doxycycline.

This organism was first recognized in 1976 after an outbreak at a convention of the American Legion in Philadelphia, Pennsylvania—thus, the name legionnaires’ disease. This disease is the pneumonic form of an infection usually caused by Legionella pneumophila serogroup 1. Nonpneumonic infection is called Pontiac fever, which manifests as a febrile, viral-like illness.

The organisms are often present in soil and freshwater. Amebas present in freshwater are a natural reservoir for these bacteria. A building’s water supply is often the source of a Legionella outbreak. Legionella organisms are embedded in a biofilm that forms on the inside of water pipes and containers. The infection is usually acquired by inhaling aerosols (or less often aspiration) of contaminated water (eg, as generated by shower heads, misters, whirlpool baths, or water cooling towers for air-conditioning). Nosocomial infection usually involves a contaminated hot water supply. The infection is not transmitted from person to person.

Diseases caused by Legionella sp

Legionella infection is more frequent and more severe in the following:

  • Patients < 1 yr

  • The elderly

  • Patients with diabetes or COPD

  • Cigarette smokers

  • Immunocompromised patients (typically with diminished cell-mediated immunity)

The lungs are the most common site of infection; community- and hospital-acquired pneumonia may occur.

Extrapulmonary legionellosis is rare; manifestations include sinusitis, hip wound infection, myocarditis, pericarditis, and prosthetic valve endocarditis, frequently in the absence of pneumonia.

Symptoms and Signs

Legionnaires’ disease is a flu-like syndrome with acute fever, chills, malaise, myalgias, headache, or confusion. Nausea, loose stools or watery diarrhea, abdominal pain, cough, and arthralgias also frequently occur. Pneumonic manifestations may include dyspnea, pleuritic pain, and hemoptysis. Bradycardia relative to fever may occur, especially in severe cases.

Overall mortality is low (about 5%) but can reach 40% in patients with hospital-acquired infections, the elderly, and immunocompromised patients.


  • Direct fluorescent antibody staining

  • Sputum culture

  • Rapid urinary antigen test (for serogroup 1 only)

Direct fluorescent antibody staining of sputum or lavage fluid is occasionally used but requires expertise. In addition, PCR with DNA probing is available and may help identify transmission pathways. A urinary antigen test is 60 to 95% sensitive and > 99% specific 3 days after symptom onset but detects only L. pneumophila (serogroup 1) and not non-pneumophila Legionella. Paired acute and convalescent antibody assays may yield a delayed diagnosis. A 4-fold increase or an acute titer of 1:128 is considered diagnostic.

Diagnosis of legionnaires' disease is by culture of sputum or bronchoalveolar lavage fluid; blood cultures are unreliable. Slow growth on laboratory media may delay identification for 3 to 5 days.

Chest x-ray should be done; it usually shows patchy and rapidly asymmetrically progressive infiltrates (even when effective antibiotic therapy is used), with or without small pleural effusions.

Laboratory abnormalities often include hyponatremia, hypophosphatemia, and elevated aminotransferase levels.


  • Fluoroquinolones

  • Macrolides (preferably azithromycin)

  • Sometimes doxycycline

A fluoroquinolone given IV or po for 7 to 14 days and, for severely immunocompromised patients, sometimes up to 3 wk is the preferred regimen. Azithromycin (for 5 to 10 days) is effective, but erythromycin may be less effective. Erythromycin should be used only for mild pneumonia in patients who are not immunocompromised. Doxycycline is an alternative for immunocompetent patients with mild pneumonia. The addition of rifampin is no longer recommended because benefit has not been proved and there is potential for harm.

Key Points

  • L. pneumophila usually causes pulmonary infection; it rarely causes extrapulmonary infections (most often involving the heart).

  • L. pneumophila infection is typically acquired by inhaling aerosols (or less often by aspiration) of contaminated water; it is not transmitted from person to person.

  • Diagnose using direct fluorescent antibody staining or PCR testing; sputum cultures are accurate but may take 3 to 5 days.

  • Treat using a fluoroquinolone or azithromycin; doxycycline is an alternative.

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