• Acute inflammation of the brain
  • 20,000 cases in the U.S. annually
  • Mortality: 10%
  • Inflammatory reaction occurs within brain parenchyma with destruction of neurons, parenchymal edema, and petechial hemorrhages
  • Route of CNS infection usually hematogenous:
    • Respiratory or GI tract
    • Blood transfusion
    • Organ transplant
  • Neural migration occurs with rabies, herpes simplex virus (HSV), and varicella zoster virus (VZV) encephalitis


  • Viral is most common
  • Noninfectious
    • Autoimmune, paraneoplastic, collagen vascular disease
  • 50% of cases have no identifiable cause

Specific Viruses
  • HSV:
    • 10–20% of all encephalitides
    • Primary or reactivation
    • Early treatment improves prognosis
  • Arbovirus:
    • 10–15% of all encephalitides
    • Zoonotic transmission (mosquitoes, ticks) in warm months
    • Eastern equine causes fulminant encephalitis:
      • Tropism for the hippocampus
      • Abrupt onset of headache, fever, vomiting progressing to coma
    • Western equine occurs mostly in the western 2/3 of the U.S.:
      • Often preceded by nonspecific upper respiratory/GI tract symptoms
    • Japanese – most prevalent arboviral encephalitis worldwide:
      • Indolent course of fever, headache, myalgias, and fatigue followed by confusion, delirium, masklike facies, and parkinsonism, seizures, brainstem dysfunction, coma, and death
  • Flavivirus:
    • West Nile virus – increased incidence in North America:
      • Found in mosquitoes and birds
      • Febrile illness, often with rash
      • Headache
      • Lymphadenopathy
      • Polyarthropathy
      • Increased morbidity/mortality in elderly patients
    • Flaccid paralysis can lead to respiratory failure with 50% mortality
  • Enteroviral:
    • Occurs mainly in children <10 yr old
    • Relatively benign course with little or no long-term sequelae
  • Measles encephalitis:
    • Occurs several days to 2–3 wk after primary infection and rash, or after years of latent infection
    • Abrupt onset and rapid progression to coma
    • Seizures common (50–60%)
    • Postimmunization incidence of 1 per 1 million vaccinated
  • HIV encephalitis:
    • Lower CD4 counts predispose to encephalitis
    • Typical features include motor spasticity and dementia
    • Involvement of white matter with extensive neural degeneration
  • Rhabdovirus: Rabies
    • CNS infection in the absence of systemic infection

  • Mycoplasma pneumoniae
  • Toxoplasma gondii
  • Rickettsia rickettsii
  • Mycobacterium tuberculosis
  • Borrelia burgdorferi
  • Bartonella henselae

Immunocompromised/HIV Patients
  • Histoplasma
  • Cryptococcus neoformans
  • VZV
  • Listeria monocytogenes
  • Cytomegalovirus (CMV)
  • T. gondii
  • Human herpesvirus type 6 (HHV-6)

  • Anti-LGI1 encephalitis
  • Anti-N-methyl-D-aspartate receptor (anti-NMDAR) encephalitis

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