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Encephalitis, Viral

Encephalitis, Viral is a topic covered in the 5-Minute Clinical Consult.

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  • 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 recent attention (associated with 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

  • Age: increased incidence in infants and elderly
  • Contact with animals or insect vectors
  • Impaired immune status
  • Occupation (e.g., lab or animal care workers)
  • Recreational activities (e.g., camping, hunting)
  • 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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Stephens, Mark B., et al., editors. "Encephalitis, Viral." 5-Minute Clinical Consult, 27th ed., Wolters Kluwer, 2019. Medicine Central, im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116203/all/Encephalitis__Viral.
Encephalitis, Viral. In: Stephens MB, Golding J, Baldor RA, et al, eds. 5-Minute Clinical Consult. 27th ed. Wolters Kluwer; 2019. https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116203/all/Encephalitis__Viral. Accessed April 20, 2019.
Encephalitis, Viral. (2019). In Stephens, M. B., Golding, J., Baldor, R. A., & Domino, F. J. (Eds.), 5-Minute Clinical Consult. Available from https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116203/all/Encephalitis__Viral
Encephalitis, Viral [Internet]. In: Stephens MB, Golding J, Baldor RA, Domino FJ, editors. 5-Minute Clinical Consult. Wolters Kluwer; 2019. [cited 2019 April 20]. Available from: https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116203/all/Encephalitis__Viral.
* Article titles in AMA citation format should be in sentence-case
TY - ELEC T1 - Encephalitis, Viral ID - 116203 ED - Stephens,Mark B, ED - Golding,Jeremy, ED - Baldor,Robert A, ED - Domino,Frank J, BT - 5-Minute Clinical Consult, Updating UR - https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116203/all/Encephalitis__Viral PB - Wolters Kluwer ET - 27 DB - Medicine Central DP - Unbound Medicine ER -