Legionnaires' disease

Definition

Legionellosis: an infection caused by Legionella spp.
Legionnaires' disease: a type of legionellosis that manifests as Pneumonia#Atypical pneumonia
Pontiac fever: a type of legionellosis that manifests with flu-like symptoms without pneumonia.

Epidemiology

Manifests almost exclusively in adults
Locations at increased risk of a legionella outbreak include:
- Nursing homes
- Hospitals
- Confined travel accommodations (e.g., cruise ships, hotels, resorts)

Clinical features

Incubation period: 2–10 days
Fever >39 C (102.2 F)
Bradycardia relative to high fever
Severe atypical pneumonia: dry cough which can become productive, shortness of breath, bilateral crackles
Several days of gastrointestinal symptoms (eg, vomiting, diarrhea) prior to developing pulmonary symptoms
Neurological features, especially confusion, agitation, and stupor

Tip

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).

Diagnostics

Legionella urinary antigen test
- Most important diagnostic tool
- High specificity, high sensitivity, fast (results available within 15 minutes)
Microbiological studies
- Legionella culture (gold standard)
  - Requires buffered charcoal yeast extract agar with iron and cysteine
  - Slow (results typically after 3–5 days)
- Sample stains
  - Gram stain of respiratory secretions typically shows many neutrophils but no organisms.
  - Visualization of Legionella requires silver stain.