Last updated: November 26, 2020

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Legionellosis is an infection caused by Legionella pneumophila, a gram-negative rod that thrives in warm aqueous environments such as drinking-water systems, hot tubs, and air-conditioning units. Transmission occurs by inhaling contaminated, aerosolized water droplets. Legionellosis is a common nosocomial infection and outbreaks are typical. Notable risk factors include smoking, chronic lung disease, advanced age, and immunosuppressive conditions. There are two forms of legionellosis: Legionnaires' disease and Pontiac fever. Patients with Legionnaires' disease present with atypical pneumonia (shortness of breath, cough), commonly in combination with various other symptoms, including gastrointestinal (e.g., diarrhea) and neurological (e.g., confusion). Laboratory abnormalities are common, especially hyponatremia. Pontiac fever is a milder, self-limiting, flu-like illness. A urine antigen test is used to confirm infection with L. pneumophila. Fluoroquinolones are the treatment of choice for Legionnaires' disease. In the US, legionellosis is a notifiable disease and steps should be taken to eliminate contaminated sources and to prevent future outbreaks.

  • Frequency: : occurs rarely in infants, almost solely in adults (of any age) and typically in outbreaks
  • High-risk groups [1][2]

Epidemiological data refers to the US, unless otherwise specified.

Causative organism

  • Legionella pneumophila (gram-negative, obligate aerobic, facultative intracellular rod) causes over 90% of Legionnaires' disease outbreaks.
  • The optimal water temperature for Legionella is 25–42°C (77–108°F). [3][4]
  • Pontiac fever is generally due to lesser-known types of Legionella.

Path of infection

  • Inhalation of contaminated aerosols
    • Cold and hot water systems; (e.g., those found in hotels, hospitals, and retirement homes)
    • Whirlpools/hot tubs, swimming pools, showers
    • Air-conditioning systems with contaminated condensed water
  • Person-to-person transmission is uncommon.

Since transmission from person to person is uncommon, isolation is unnecessary.

Locations at particular risk of outbreak

  • Nursing homes
  • Hospitals
  • Confined travel accommodations (e.g., cruise ships, hotels, resorts)


Legionnaires' disease

Legionellosis should always be considered in patients with signs of atypical pneumonia and diarrhea in combination with possible exposure (e.g., cruise ship travel, use of a whirlpool).

Pontiac fever


See “Diagnosis of pneumonia”.

Laboratory findings

The legionaries drew their iron swords, donned their silver helmets, and jumped off the ship to burn the town to coal: legionellosis, iron buffered medium, silver stain, history of cruise ship travel, charcoal yeast extract agar.

Imaging [9]

Legionnaires' disease should be treated early with antibiotics, especially because of its high mortality rate of 10%. [10]

In any case of atypical pneumonia, antibiotic treatment needs to cover Legionella.

  • Legionellosis is a notifiable disease.
  • Course of action when contaminated water sources are detected in medical facilities
    • Contaminated water systems should be disinfected .
    • Use terminal tap water filters, especially for high-risk patients (e.g., immunocompromised or the elderly).


  1. Farnham A, Alleyne L, Cimini D, Balter S. Legionnaires' Disease Incidence and Risk Factors, New York, New York, USA, 2002–2011. Emerging Infectious Diseases. 2014; 20 (11). doi: 10.3201/eid2011.131872 . | Open in Read by QxMD
  2. Causes, How it Spreads, and People at Increased Risk. http://www.cdc.gov/legionella/about/causes-transmission.html. Updated: May 31, 2016. Accessed: August 16, 2016.
  3. What Owners and Managers of Buildings and Healthcare Facilities Need to Know about the Growth and Spread of Legionella. https://www.cdc.gov/legionella/wmp/overview/growth-and-spread.html. Updated: April 30, 2018. Accessed: October 31, 2020.
  4. Epalle T, Girardot F, Allegra S, Maurice-Blanc C, Garraud O, Riffard S. Viable but Not Culturable Forms of Legionella pneumophila Generated After Heat Shock Treatment Are Infectious for Macrophage-Like and Alveolar Epithelial Cells After Resuscitation on Acanthamoeba polyphaga. Microb Ecol. 2014; 69 (1): p.215-224. doi: 10.1007/s00248-014-0470-x . | Open in Read by QxMD
  5. Legionella (Legionnaires' Disease and Pontiac Fever). http://www.cdc.gov/legionella/about/prevention.html. Updated: June 7, 2016. Accessed: August 18, 2016.
  6. Authors Victor L Yu, MD Janet E Stout, PhD Nieves Sopena Galindo, MD. Clinical manifestations and diagnosis of Legionella infection. undefined. 2016 .
  7. Diederen BMW, Kluytmans JAJW, Vandenbroucke-Grauls CM, Peeters MF. Utility of real-time PCR for diagnosis of legionnaires' disease in routine clinical practice. J Clin Microbiol. 2007; 46 (2): p.671-677. doi: 10.1128/jcm.01196-07 . | Open in Read by QxMD
  8. Diagnosis, Treatment, and Prevention. http://www.cdc.gov/legionella/clinicians/diagnostic-testing.html. Updated: June 14, 2016. Accessed: August 18, 2016.
  9. Poirier R, Rodrigue J, Villeneuve J, Lacasse Y. Early Radiographic and Tomographic Manifestations of Legionnaires' Disease.. Canadian Association of Radiologists journal = Journal l'Association canadienne des radiologistes. 2017; 68 (3): p.328-333. doi: 10.1016/j.carj.2016.10.005 . | Open in Read by QxMD
  10. Dooling KL, Toews KA, Hicks LA, et al.. Active Bacterial Core Surveillance for Legionellosis–United States, 2011–2013. Morbidity and Mortality Weekly Report (MMWR). 2015; 64 (42): p.1190-3.
  11. Yu VL, Galindo NS. Treatment and prevention of Legionella infection. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. http://www.uptodate.com/contents/treatment-and-prevention-of-legionella-infection?source=machineLearning&search=legionella+treatment&selectedTitle=1~101§ionRank=2&anchor=H21954908#H21954908.Last updated: January 21, 2016. Accessed: August 18, 2016.
  12. Bergallo C, Jasovich A, Teglia O, et al. Safety and efficacy of intravenous tigecycline in treatment of community-acquired pneumonia: results from a double-blind randomized phase 3 comparison study with levofloxacin. Diagn Microbiol Infect Dis. 2009; 63 (1): p.52-61. doi: 10.1016/j.diagmicrobio.2008.09.001 . | Open in Read by QxMD

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