Legionnaires' disease

Last updated: June 28, 2023

Summarytoggle arrow icon

Legionnaires' disease is a type of legionellosis that manifests with atypical pneumonia (shortness of breath, cough), typically in combination with gastrointestinal (e.g., diarrhea) and neurological (e.g., confusion) symptoms. The condition is typically 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 via inhalation of contaminated aerosolized water droplets; disease outbreaks are common. Laboratory abnormalities are common, especially hyponatremia. Diagnosis can be confirmed using a urine antigen test, PCR, or microbiological studies. Fluoroquinolones or macrolides are the treatment of choice. Legionellosis is a notifiable disease in the US, and steps should be taken to eliminate contaminated sources and prevent outbreaks.

Pontiac fever is a more mild, flu-like type of legionellosis that is self-limiting.

Definitiontoggle arrow icon

Epidemiologytoggle arrow icon

  • Manifests almost exclusively in adults [1]
  • Outbreaks are typical. [2]
  • Locations at increased risk of a legionella outbreak include: [1]
    • Nursing homes
    • Hospitals
    • Confined travel accommodations (e.g., cruise ships, hotels, resorts) [2]

Epidemiological data refers to the US, unless otherwise specified.

Etiologytoggle arrow icon

  • Pathogens [2]
  • Transmission [3]
    • Inhalation of contaminated aerosols
      • Cold and hot water systems
      • Whirlpools/hot tubs, swimming pools, showers
      • Air conditioning systems with contaminated condensed water
    • Person-to-person transmission is uncommon.
  • High-risk groups [1]

Clinical featurestoggle arrow icon

Legionnaires' disease 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).

Subtypes and variantstoggle arrow icon

Pontiac fever [3]

Extrapulmonary legionellosis [3]

Extrapulmonary legionellosis has various possible manifestations, e.g.:

Extrapulmonary legionellosis is rare and most commonly affects immunosuppressed patients.

Diagnosticstoggle arrow icon

Approach [2][4]

Routine pneumonia diagnostics may show nonspecific but supportive findings.

Routine studies

Laboratory studies [2][5]

Hyponatremia may be caused by SIADH or by dilution and is more commonly associated with Legionnaires' disease than with other causes of pneumonia. [2][5]

Imaging [5][6]

Confirmatory studies [2][7]

Treatmenttoggle arrow icon

General principles [2]

Legionnaires' disease has a mortality rate of ∼10% in patients without high-risk factors. [1][2]

Antibiotic therapy [2][4]

Empiric treatment of atypical pneumonia and antibiotic regimens for inpatient treatment of CAP need to cover Legionella species.

Preventiontoggle arrow icon

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 individuals, older adults).


Referencestoggle arrow icon

  1. Soda EA, Barskey AE, Shah PP, et al. Vital Signs: Health Care-Associated Legionnaires’ Disease Surveillance Data From 20 States and a Large Metropolitan Area-United States, 2015. Am J Transplant. 2017; 17 (8): p.2215-2220.doi: 10.1111/ajt.14407 . | Open in Read by QxMD
  2. Natalia E. Castillo, Arun Rajasekaran, Sayed K. Ali. Legionnaires' Disease. Infectious Diseases in Clinical Practice. 2016; 24 (5): p.248-253.doi: 10.1097/ipc.0000000000000401 . | Open in Read by QxMD
  3. Jameson JL, Fauci AS, Kasper DL, Hauser SL, Longo DL, Loscalzo J. Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2). McGraw-Hill Education / Medical ; 2018
  4. Metlay JP, Waterer GW, Long AC, et al. Diagnosis and Treatment of Adults with Community-acquired Pneumonia. An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America. Am J Respir Crit Care Med. 2019; 200 (7): p.e45-e67.doi: 10.1164/rccm.201908-1581st . | Open in Read by QxMD
  5. Burke A. Cunha. Legionnaires' Disease: Clinical Differentiation from Typical and Other Atypical Pneumonias. Infect Dis Clin North Am. 2010; 24 (1): p.73-105.doi: 10.1016/j.idc.2009.10.014 . | Open in Read by QxMD
  6. Poirier R, Rodrigue J, Villeneuve J, Lacasse Y. Early Radiographic and Tomographic Manifestations of Legionnaires' Disease.. Can Assoc Radiol J. 2017; 68 (3): p.328-333.doi: 10.1016/j.carj.2016.10.005 . | Open in Read by QxMD
  7. Mercante JW, Winchell JM. Current and Emerging Legionella Diagnostics for Laboratory and Outbreak Investigations. Clin Microbiol Rev. 2015; 28 (1): p.95-133.doi: 10.1128/cmr.00029-14 . | Open in Read by QxMD
  8. Legionella (Legionnaires' Disease and Pontiac Fever): How to Report Cases. Updated: March 25, 2021. Accessed: November 15, 2022.
  9. Legionella (Legionnaires' Disease and Pontiac Fever). Updated: June 7, 2016. Accessed: August 18, 2016.

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