Encephalitis, Viral



  • Inflammation of the meninges and brain associated with clinical evidence of neurologic dysfunction
  • System(s) affected: nervous
  • Synonym(s): meningoencephalitis


3.5 to 7.4/100,000 per year


  • Seasonal variation (e.g., arboviruses, enteroviruses)
  • Nonseasonal (e.g., herpes simplex virus [HSV], varicella zoster virus [VZV])
  • Vaccines have altered prevalence and seasonality (e.g., mumps and measles were common in winter before routine immunization).

Etiology and Pathophysiology

  • Most enter through respiratory or GI tract with subsequent hematogenous spread.
  • Other modes of entry include:
    • Neurotropic spread (e.g., rabies)
    • Mosquito vectors (e.g., Japanese encephalitis, dengue virus encephalitis)
    • Tick vectors (e.g., tick-borne encephalitis virus, Powassan virus, and Colorado tick fever virus)
  • Specific symptom complexes:
    • Neuronal involvement: seizures
    • Oligodendroglia: may cause demyelination, cortical infection, or reactive parenchymal swelling; altered levels of consciousness
    • Brainstem neurons: coma, respiratory failure
    • Microglia, macrophages: neurologic dysfunction
  • Perivascular mononuclear infiltrates seen in postinfectious encephalomyelitis
  • Most common etiologies in the United States are HSV, West Nile virus (WNV), and enteroviruses. Zika virus has gained attention (associated with congenital microcephaly); most U.S. cases reported in travelers to Caribbean, Central and South America
  • Despite extensive evaluations, the etiologic agent is often not identified in viral encephalitis.

Risk Factors

  • Increased incidence in infants and elderly
  • Impaired immunity
  • Occupation (e.g., lab or animal care workers)
  • Recreational activities (e.g., camping, hunting—mosquito; arthropod exposure)
  • Transfusion and transplantation
  • Travel to endemic areas
  • Recent vaccinations/unvaccinated status

General Prevention

  • Use of mosquito and tick repellents (DEET, picaridin) and appropriate clothing to prevent arthropod bites
  • Avoid travel to endemic areas. Promptly remove ticks.
  • Eliminate mosquito-breeding sources.
  • Vaccination (e.g., mumps, measles, polio, rabies, Japanese encephalitis)

Commonly Associated Conditions

  • Seizures
  • Hyperthermia
  • Increased intracranial pressure (ICP)
  • Inappropriate antidiuretic hormone (ADH) secretion

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