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

Histology

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  • Facultative Intracellular Gram-negative bacilli

Pathophysiology

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Clinical Presentations

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  • 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

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

Diagnosis

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

Differential Diagnoses

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Management

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  • 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