I. Clinical features and pathophysiology

  1. Caused by Legionella sp. (80% of the time is L. pneumophila), an aerobic bacillus that is an intracellular parasite, primarily of macrophages and monocytes
  2. Clinical disease is typically either pneumonia (“Legionanaires’ disease”) or a self-limited acute febrile illness (“Pontiac Fever”).  Also can get SIADH with hyponatremia
  3. Pneumonia onset typically acute but can take days
  4. Clinical features of Legionnaire’s disease:
  1. Symptoms of pneumonia; can have pleural effusion
  2. Relative bradycardia (blunting of the usual increase in heart rate associated with fever)
  3. Diarrhea
  4. Nausea and vomiting
  5. Headache
  6. Confusion
  7. Microscopic hematuria and proteinuria
  8. Renal failure
  9. Leukocytosis with relative lymphopenia
  10. Thrombocytopenia
  11. Transaminase elevations to 2-5x normal
  12. Hyponatremia
  13. Hypophosphatemia
  14. Elevated serum ferritin
  1.  Transmission
    1. Thought primarily to occur through inhalation of mist from infected water sources, e.g. showers, air-conditioning, etc.
    2. Person-to-person transmission has not been documented

II. Risk factors

  1. Advanced age
  2. Chronic pulmonary disease
  3. Renal failure
  4. Hepatic failure
  5. Transplant recipient
  6. Smoking
  7. Immunosuppressed state

III. Diagnosis

  1. Urine for Legionella antigen (sens/spec 70%/100%)
  2. Culture of respiratory secretions (sens/spec 80%/100%)
  3. Paired acute/convalescent serology (sens/spec 70-80%/90%)
  4. Direct fluorescent antibody stain of respiratory secretions (sens/spec 25-75%/95%)
  5. CXR findings are nonspecific

IV. Treatment

  1. (Azithromycin x 7-10d) or (moxifloxacin or levofloxacin x 10-14d) are treatments of choice (better efficacy than erythromycin).
  2. Treat longer for more severe disease or immunocompromise.
(Sources include Core Content Review of Family Medicine, 2012)