Japanese encephalitis


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 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.


  • 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.


  • 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 features



Laboratory findings


Diagnostic testing


Differential diagnoses

The differential diagnoses listed here are not exhaustive.



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



  • Indications
    • Travelers spending > 1 month in endemic areas
    • Travelers spending < 1 month in endemic areas during periods of JEV transmission
    • Children < 15 years who live in areas with high JEV transmission
  • Administration
    • 2 doses, 28 days apart
    • Given at least one week before exposure
    • Booster if last dose ≥ 1 year ago
  • Contraindications: See general contraindications for vaccination.

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