ID Board Review/Bacterial Diseases/Gram-negative Rods (GNR)/Legionella spp

Histology

• Facultative Intracellular Gram-negative bacilli

Clinical Presentations

• Elderly patients, classically nursing home residents
  • Risk Factors: Age, Hx of COPD
• Confusion, high fever, bradycardia
• CXR often unilobar infiltrate that progresses to consolidation
• Often failure to respond to broad spectrum/appropriate antibiotics

Natural Environments

• Aquatic bodies e.g. lakes, streams, man-made reservoirs, standing water
  • "Air conditioners in retirement homes"

Diagnosis

• Legionella Urine Antigen test (only detects L. pneumophila serogroup 1)
• Sputum or BAL sample for Legionella Cx
• Hyponatremia, thrombocytopenia, elevated LFTs are all suggestive

Management

• Proven, or suspected Legionella PNA requires either:
  • Respiratory fluoroquinolone (e.g. Levofloxacin)
  • Newer macrolide (e.g. Azithromycin)

NOTE: Both have good lung penetration with potent intracellular activity