• A 40-year-old female presents with a 3 day history of fever, cough, headache and arthralgia after returning from a spa 5 days prior. The patient is a smoker. Temperature is 101.8°F (38.8°C), respirations are 18/min, pulse is 90/min, BP is 100/60 mmHg with oxygen saturation 90% on room air. Chest exam reveals fine crackles on auscultation. Chest radiograph reveals patchy infiltrates in both lungs. Complete blood count shows a leukocytosis. Complete metabolic panel reveals a serum sodium of 133 mEq/L and a serum creatinine of 0.9 mg/dL (baseline). AST/ALT levels were mildly elevated. Urinary antigen testing was positive, and a sputum sample was stained with silver and sent for culture after a Gram stain revealed only neutrophils. .
  • Classification
    • bacteria
      • other gram-negative
        • Legionella
          • L. pneumophila
  • Pathogenesis
    • transmission
      • airborne
        • air-conditioning
      • no human-to-human transmission
    • reservoir
      • water
      • contaminated air-conditioners
    • molecular biology
      • facultative intracellular pathogen
        • inhibits phagosome-lysosome fusion in macrophages
      • endotoxin
      • can form biofilm
  • Legionnaire's disease 
    • severe atypical pneumonia
      • "fever, cough, diarrhea, especially in a smoker"
    • high fever
    • low heart rate
    • confusion
    • hyponatremia
      • can infect kidneys, causing interstitial nephritis → ↓ renin → ↓ RAAS → hyponatremia
    • diarrhea
      • although no bacteria seen in GI tract
  • Pontiac fever
    • mild flu-like syndrome
  • Weakly gram-negative
    • stain with silver instead
  • Fastidious
  • Culture on charcoal yeast extract (CYE) with increased iron and cysteine 
  • Diagnose via detection of antigen in the urine
  • Erythromycin
  • Penicillin is not effective (intracellular organism)
  • Prevent by decontaminating air-conditioners routinely

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