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IV. Encephalitis

A. Pathogenesis:

  • Hematogenous spread after viremia
  • Spread along nerves (rabies, Herpes simplex)

B. Etiologic Agents:   

  • Usually viral:   important organisms include HIV, polio, rabies, Herpes simplex, Herpes zoster, equine encephalitis and St. Louis encephalitis viruses. 
  • Rickettsia (typhus and Rocky Mountain spotted fever) and protozoa (toxoplasma) also cause a diffuse inflammatory reaction in the central nervous system.
  • Bacterial invasion of the CNS typically results in abscess formation, not encephalitis or encephalomyelitis.