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—thus, the name Legionnaires' disease. This disease is the pneumonic form of an infection usually caused by Legionella pneumophila serogroups 1 through 6. Nonpneumonic infection is called Pontiac fever.
The organisms are often present in soil and freshwater. 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 of contaminated water (eg, as generated by shower heads, misters, whirlpool baths, or water cooling towers for air-conditioning).
Diseases caused by Legionella sp
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, and intestines. Immunocompromised patients, patients with diabetes mellitus, cigarette smokers, the elderly, and patients with chronic lung disease are principally affected.
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.
Mortality is low in otherwise healthy people but can reach 50% in hospital-acquired outbreaks.
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 (serogroups 1 through 6) 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.
A fluoroquinolone given IV or po for 2 to 3 wk is the preferred regimen. Doxycycline is also highly effective. Azithromycin is effective, but erythromycin may be ineffective. Rifampin may be added for severe infections.
Last full review/revision August 2009 by Burke A. Cunha, MD