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.
- Manifests almost exclusively in adults 
- Outbreaks are typical. 
- Locations at increased risk of a legionella outbreak include: 
- Nursing homes
- Confined travel accommodations (e.g., cruise ships, hotels, resorts) 
Epidemiological data refers to the US, unless otherwise specified.
- Pathogens 
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.
- Inhalation of contaminated aerosols
- High-risk groups 
- Incubation period: 2–10 days
- Fever, chills, headache
- Severe : dry cough which can become productive, shortness of breath, bilateral crackles
- Relative bradycardia
- Neurological features, especially confusion, agitation, and stupor
Subtypes and variants
Pontiac fever 
- Mild course of legionellosis without pneumonia
- Characterized by flu-like symptoms (e.g., fever, headache, myalgia)
- Incubation period: 1–2 days 
- Self-limiting disease; specific treatment is not needed.
Extrapulmonary legionellosis 
Extrapulmonary legionellosis has various possible manifestations, e.g.:
- Obtain routine studies and confirm a .
- Consider Legionnaires' disease in patients with:
- Confirm diagnosis using a legionella urine antigen test, PCR, and/or culture.
Routine may show nonspecific but supportive findings.
Laboratory studies 
- Serum studies
- Urine studies
- Chest x-ray
- Chest CT; findings may include:
Confirmatory studies 
- Legionella urinary antigen test 
- Legionella culture (gold standard) 
- Sample stains
General principles 
- Legionellosis is a notifiable disease. 
- Early antibiotic therapy is the mainstay of treatment.
- Affected individuals do not need to isolate because person-to-person transmission is uncommon.
- Offer as needed.
- See “ICU admission. ” for further details, including criteria for
Antibiotic therapy 
- Empiric antibiotics for pneumonia
- Once Legionnaires' disease is confirmed, initiate treatment with or switch to one of the following agents:
- Initial parenteral treatment is recommended for hospitalized patients.
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).