Legionella pneumophila most often causes pneumonia with extrapulmonary features. Diagnosis requires specific growth media, serologic testing, or PCR analysis. Treatment is with doxycycline, macrolides, or fluoroquinolones.
The first appearance of this organism was in 1976 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 patients < 1 yr, the elderly, patients with diabetes or COPD, cigarette smokers, and 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 foci of infection occur most frequently in hospitalized patients and most commonly involve the heart. Other sites include the CNS, liver, spleen, lymph nodes, and intestines.
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 of sputum or lavage fluid is frequently used. In addition, PCR with DNA probing is available. A urinary antigen test is 70% sensitive and 100% 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 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.
A fluoroquinolone given IV or po for 2 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 and trimethoprim/sulfamethoxazole (TMP/SMX) are other alternatives. Rifampin may be added for severe infections.
Last full review/revision February 2014 by Larry M. Bush, MD; Maria T. Perez, MD
Content last modified March 2014