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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 doxycycline, macrolides, or fluoroquinolones.
The 1st appearance of this organism was in 1976 at a convention of the American Legion, thus the name Legionnaires' disease. Nonpneumonic infection is called Pontiac fever. The organisms can be found in soil and freshwater. Manufactured water-storage containers, including water-cooled air-conditioning units, enhance its growth. Spread is most likely by aerosols of potable water.
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, cigarette smokers, the elderly, and those with chronic lung disease are principally affected.
Symptoms,
Signs, and Diagnosis
Legionnaires' disease is a flu-like syndrome with acute fever, chills, malaise, myalgias, headache, or confusion. Frequently nausea, loose stools/watery diarrhea, abdominal pain, cough, and arthralgias also occur. Pneumonic manifestations may include dyspnea, pleuritic pain, and hemoptysis.
Diagnosis is by examination of sputum or bronchoalveolar lavage fluid; blood cultures are unreliable. Slow growth on laboratory media may delay identification for 3 to 5 days. 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. Chest x-rays usually show nonspecific changes such as infiltrates and pleural effusions.
Treatment
Doxycycline , macrolides, and fluoroquinolones are highly effective for Legionnaires' disease. Any respiratory quinolone (given IV or po) for 7 to 14 days is the recommended regimen. Rifampin may be added for severe infections. Mortality is low in otherwise healthy people but can reach 50% in hospitalacquired outbreaks.
Last full review/revision November 2005
Content last modified November 2005
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