Japanese encephalitis

Last updated: April 19, 2023

Summarytoggle arrow icon

Japanese encephalitis is a mosquito-borne viral disease endemic in Asia and the Western Pacific. Disease transmission occurs through the bite of infected Culex tritaeniorhynchus mosquitoes. Clinically, Japanese encephalitis virus (JEV) infection can range from asymptomatic disease to acute encephalitis, which occurs in < 1% of patients. Despite its rarity, acute encephalitis with altered mental status and neurological deficits remains the most important clinical manifestation; it typically develops following a short period of non-specific febrile illness. Seizures are common, especially in children. Other known clinical manifestations include acute psychosis and spastic or flaccid paralysis. Elevations in white blood cell count and CSF pleocytosis are often present, along with characteristic thalamic lesions on brain MRI. Definitive diagnosis is made through serology. Vaccinations are available for travelers and as part of childhood immunization programs in some endemic areas.

Epidemiologytoggle arrow icon

  • Distribution: endemic throughout most of Asia and parts of the Western Pacific region
  • Incidence
    • A significant cause of viral encephalitis in Asia
    • ∼ 68,000 cases occur every year
    • Typically affects individuals < 15 years old
    • The incidence in travelers from non-endemic regions is estimated to be < 1 case per million.


Epidemiological data refers to the US, unless otherwise specified.

Etiologytoggle arrow icon

  • Pathogen: Japanese encephalitis virus (JEV), a mosquito-borne flavivirus
  • Transmission
    • Two main patterns of transmission
      • During the warmer months in temperate areas of Asia (e.g., China, Japan, South Korea)
      • Year-round transmission in tropical areas (e.g., Cambodia, Thailand) with peaks during the rainy season
    • The primary mosquito vector is Culex tritaeniorhynchus .
    • Pigs and wading birds (e.g., herons and egrets) are major hosts in the JEV cycle.
      • Pigs are particularly important, as they develop high levels of viremia and are often kept in close proximity to human dwellings.
      • Humans are dead-end hosts.


Clinical featurestoggle arrow icon


Diagnosticstoggle arrow icon

Laboratory findings


Diagnostic testing


Differential diagnosestoggle arrow icon

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon

Prognosistoggle arrow icon

  • ∼ 30% of patients who develop acute encephalitis die.
  • In survivors, neurologic, cognitive, and psychiatric sequelae are common.

Preventiontoggle arrow icon

Japanese encephalitis vaccine

Mosquito-bite prevention

  • Wear light-colored clothing
  • Use insect repellants
  • Avoid activities between dusk and dawn (when Culex tritaeniorhynchus is most active)
  • Place screens at doors and windows
  • Remove open water containers to minimize mosquito breeding


Referencestoggle arrow icon

  1. Hills SL, Fischer M, Solomon T. Japanese Encephalitis. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. Last updated: October 11, 2018. Accessed: January 9, 2019.
  2. Török E, Cooke FJ, Moran E. Oxford Handbook of Infectious Diseases and Microbiology. Oxford University Press ; 2017
  3. $Summary of the WHO Position Paper on Vaccines against Japanese encephalitis (JE).
  4. Japanese Encephalitis Vaccine. Updated: August 5, 2015. Accessed: January 9, 2019.

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