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Legionnaires' disease, also known as legionellosis, is a form of atypical pneumonia caused by any type of Legionella bacteria.[1] An estimated eight thousand to eighteen thousand cases a year in the United States require hospitalization.[2]Outbreaks of disease account for a minority of cases.[3]

Signs and symptoms

Those with Legionnaires' disease usually have fever, chills, and a cough, which may be dry or may produce sputum. Almost all experience fever, while around half have cough with sputum, and one-third cough up blood or bloody sputum. Some also have muscle aches, headache, tiredness, loss of appetite,ataxia, chest pain, or diarrhea and vomiting.[4]


Legionella pneumophila is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative bacterium of the genus Legionella.L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of Legionnaires' disease, also known as legionellosis[5]


Legionnaires' disease is usually spread by the breathing in of aerosolized water or soil contaminated with the Legionella bacteria.[4] Experts have stated that Legionnaires' disease is not transmitted from person to person.[6]


Legionella spp. enter the lungs either by aspiration of contaminated water or inhalation of aerosolized contaminated water or soil. In the lung, the bacteria are consumed by macrophages, a type of white blood cell, inside of which the Legionella bacteria multiply, causing the death of the macrophage. Once the macrophage dies, the bacteria are released from the dead cell to infect other macrophages. .[7]


The most useful diagnostic tests detect the bacteria in coughed-up mucus, find Legionella antigens in urine samples, or allow comparison of Legionella antibody levels in two blood samples taken 3 to 6 weeks apart. A urine antigen test is simple, quick, and very reliable, but only detects L. pneumophila serogroup 1, which accounts for 70 percent of disease caused by L. pneumophila, which means use of the urine antigen test alone may miss as many as 30 percent of cases.[8][9]


Effective antibiotics include most macrolides, tetracyclines, ketolides, and quinolones.[4] Legionella spp. multiply within the cell, so any effective treatment must have excellent intracellular penetration. Current treatments of choice are the respiratory tract quinolones levofloxacin, moxifloxacin, gemifloxacin, or newer macrolides like azithromycin.[10][11]


The fatality rate of Legionnaires' disease has ranged from 5 to 30 percent during various outbreaks and approaches 50 percent for nosocomial infections, especially when treatment with antibiotics is delayed.[12] Hospital-acquired Legionella pneumonia has a fatality rate of 28 percent, and the principal source of infection in such cases is the drinking-water distribution system.[13]


Legionnaires' disease acquired its name in July 1976, when an outbreak of pneumonia occurred among people attending a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia. Of the 182 cases reported, 29 individuals died from the bacterial infection.[14]


  1. "Legionella (Legionnaires' Disease and Pontiac Fever) About the Disease". Centers for Disease Control and Prevention (CDC). 26 January 2016. Archived from the original on 25 March 2016. Retrieved 21 March 2016.
  2. "Legionnaires Disease Signs and Symptoms | CDC". 22 July 2022. Retrieved 4 August 2022.
  3. "Legionella (Legionnaires' Disease and Pontiac Fever) Prevention". Centers for Disease Control and Prevention (CDC). 28 October 2015. Archived from the original on 12 March 2016. Retrieved 21 March 2016.
  4. 4.0 4.1 4.2 Cunha, Burke A.; Burillo, Almudena; Bouza, Emilio (23 January 2016). "Legionnaires' disease". Lancet (London, England). 387 (10016): 376–385. doi:10.1016/S0140-6736(15)60078-2. ISSN 1474-547X. Retrieved 5 August 2022.
  5. Farris, Alton B.; Selig, Martin K.; Nielsen, G. Petur (1 January 2010). "Chapter 5 - Ultrastructural Diagnosis of Infection". Diagnostic Pathology of Infectious Disease. W.B. Saunders. pp. 77–98. ISBN 978-1-4160-3429-2. Retrieved 6 August 2022.
  6. "Legionella (Legionnaires' Disease and Pontiac Fever)". Centers for Disease Control and Prevention (CDC). 22 January 2016. Archived from the original on 1 February 2016. Retrieved 10 February 2016.
  7. Legionella and the prevention of legionellosis. Geneva: World Health Organization (WHO). 2007. hdl:10665/43233. ISBN 978-9241562973.
  8. "Legionnaires Disease Diagnosis, Treatment, and Prevention | CDC". 2 August 2022. Archived from the original on 12 March 2016. Retrieved 5 August 2022.
  9. Fields, B. S.; Benson, R. F.; Besser, R. E. (2002). "Legionella and Legionnaires' Disease: 25 Years of Investigation". Clinical Microbiology Reviews. 15 (3): 506–526. doi:10.1128/CMR.15.3.506-526.2002. ISSN 0893-8512. PMC 118082. PMID 12097254.
  10. Amsden, Guy W. (2005). "Treatment of Legionnaires' disease". Drugs. 65 (5): 605–614. doi:10.2165/00003495-200565050-00003. ISSN 0012-6667. Archived from the original on 19 June 2022. Retrieved 6 August 2022.
  11. Sencen, Lisa. "Legionnaires' Disease". NORD (National Organization for Rare Disorders). Archived from the original on 13 May 2022. Retrieved 9 August 2022.
  12. "Legionnaire disease: MedlinePlus Medical Encyclopedia". Retrieved 12 August 2022.
  13. Stout JE, Muder RR, Mietzner S, Wagener MM, Perri MB, DeRoos K, Goodrich D, Arnold W, Williamson T, Ruark O, Treadway C, Eckstein EC, Marshall D, Rafferty ME, Sarro K, Page J, Jenkins R, Oda G, Shimoda KJ, Zervos MJ, Bittner M, Camhi SL, Panwalker AP, Donskey CJ, Nguyen MH, Holodniy M, Yu VL (July 2007). "Role of environmental surveillance in determining the risk of hospital-acquired legionellosis: A national surveillance study with clinical correlations" (PDF). Infect Control Hosp Epidemiol. 28 (7): 818–824. doi:10.1086/518754. PMID 17564984. S2CID 13774784. Archived from the original (PDF) on 24 January 2009.
  14. "Legionnaire disease". Encyclopaedia Britannica. Archived from the original on 1 November 2013. Retrieved 30 October 2013.