Legionella pneumophila is a gram-negative bacillus that most often causes pneumonia with extrapulmonary features. Diagnosis requires specific growth media, serologic or urine antigen testing, or polymerase chain reaction analysis. Treatment is with macrolides, fluoroquinolones, or doxycycline.
Legionella pneumophila 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.
Transmission of Legionella species
Legionella organisms are often present in soil and fresh water. Amebas present in fresh water are a natural reservoir for these bacteria. Legionella organisms may enter a building's plumbing system via freshwater sources; a building’s water supply is often the source of a Legionella outbreak. Warm water temperatures of 25 to 40° C (77 to 104° F) support the highest concentrations of the organism in plumbing systems. 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, decorative fountains, whirlpool baths, hot tubs, or water cooling towers for air-conditioning).
Legionella infection is not transmitted from person to person, although one probable case was reported in 2016 (1).
Transmission reference
1. Correia AM, Ferreira JS, Borges V, et al: Probable person-to-person transmission of Legionnaires' disease. N Engl J Med 374(5):497–498, 2016. doi: 10.1056/NEJMc1505356
Diseases caused by Legionella species
Legionella infection is more frequent and more severe in the following:
Older adults
Patients with diabetes or chronic obstructive pulmonary disease (COPD)
Patients who smoke
Patients who are immunocompromised (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 and occurs mostly in patients who are immunocompromised. Manifestations include abscesses (eg, brain, spleen, muscle), sinusitis, myocarditis, pericarditis, native and prosthetic valve endocarditis, and infections of surgical wounds, vascular grafts, and native and prosthetic joints; most occur as disseminated complications of pneumonia, even though the primary pulmonary process may have already resolved. Primary cases without concurrent or recent pneumonia are rare.
Symptoms and Signs of Legionella Infections
Legionnaires’ disease has an incubation period of 2 to 10 days. It manifests as 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 case fatality for sporadic cases is approximately 10 to 15% but can reach 40% in patients with hospital-acquired infections, older adults, and patients who are immunocompromised (1).
Symptoms and signs reference
1. Jespersen S, Søgaard OS, Schønheyder HC, Fine MJ, Ostergaard L. Clinical features and predictors of mortality in admitted patients with community- and hospital-acquired legionellosis: a Danish historical cohort study. BMC Infect Dis. 2010;10:124. Published 2010 May 21. doi:10.1186/1471-2334-10-124
Diagnosis of Legionella Infections
Direct fluorescent antibody staining
Polymerase chain reaction (PCR) testing
Sputum or bronchoalveolar lavage fluid culture using specific growth media
Rapid urinary antigen test (for L. pneumophila serogroup 1 only)
Direct fluorescent antibody staining of sputum or bronchoalveolar lavage fluid is occasionally used but requires expertise. In addition, PCR testing with DNA probing is available and may help identify transmission pathways.
A urinary antigen test is 60 to 95% sensitive and > 98% specific 3 days after symptom onset but detects only L. pneumophila serogroup 1 (1), which accounts for 65% to 90% of cases (2, 3). 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 has a sensitivity of 20 to 95% depending on the severity of illness; blood cultures are unreliable. Specific growth media are required. Slow growth on laboratory media may delay identification for 3 to 5 days.
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Chest radiograph 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 and C-reactive protein levels.
Diagnosis references
1. Gassiep I, Armstrong M, Heather CS, Norton RE: Utility of the Legionella urinary antigen. Intern Med J 49(8):1050–1051, 2019. doi: 10.1111/imj.14381
2. Yu VL, Plouffe JF, Castellani Pastoris M, et al: Distribution of Legionella species and serogroups isolated by culture in patients with sporadic community-acquired legionellosis: An international collaborative survey. J Infect Dis 186(1):127–128, 2002. doi: 10.1086/341087
3. Avni T, Bieber A, Green H, et al: Diagnostic accuracy of PCR alone and compared to urinary antigen testing for detection of Legionella spp.: A systematic review. J Clin Microbiol 54(2):401–411, 2016. doi: 10.1128/JCM.02675-15
Treatment of Legionella Infections
Fluoroquinolones
Macrolides (preferably azithromycin)
Sometimes doxycycline
A respiratory fluoroquinolone (eg, levofloxacin or moxifloxacin) given IV or orally for 7 to 14 days and, for patients who are severely immunocompromised, sometimes up to 3 weeks is the preferred regimen.
Azithromycin (for 5 to 10 days) is effective; clarithromycin and erythromycin may be less effective and should be used only for mild pneumonia in patients who are not immunocompromised.
Doxycycline is an alternative for treating mild pneumonia in patients who are immunocompetent.
The addition of rifampin is not recommended because benefit has not been proved and there is potential for harm.
Pontiac fever goes away on its own without treatment and causes no lingering problems.
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 polymerase chain reaction testing; sputum or bronchoalveolar lavage fluid cultures are accurate but may take 3 to 5 days.
Treat using a respiratory fluoroquinolone (eg, levofloxacin) or azithromycin; doxycycline is an alternative for mild pneumonia in patients who are immunocompetent.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Centers for Disease Control and Prevention (CDC): Legionella (Legionnaires' Disease and Pontiac Fever)
