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For further information, see CMDT Part 32-03: Other Neurotrophic Viruses

Key Features

Essentials of Diagnosis

  • Most important vaccine-preventable cause of encephalitis in the Asia-Pacific region

  • The virus is transmitted by mosquitoes, especially Culex species

  • Wide symptom spectrum; most infections asymptomatic

General Considerations

  • Most common cause of encephalitis in East Asia with over 68,000 estimated cases and up to 20,400 deaths each year

  • Most cases occur in the summer and late fall although in tropical and subtropical areas transmission occurs throughout the year

  • Major outbreaks every 2–15 years often correlate with patterns of agricultural development

  • Virus is transmitted by mosquitoes, especially Culex species

  • Wading birds and pigs more commonly sustain the infection as reservoirs; viremia in humans is transient and not usually high enough to sustain transmission

  • In endemic countries, Japanese encephalitis is primarily a disease of children

  • Travelers to major urban areas for < 1 month are at minimal risk

  • Transmission by blood transfusion is documented

Clinical Findings

Symptoms and Signs

  • Incubation is 5–15 days

  • Most persons asymptomatically seroconvert

  • Sudden onset headaches and nausea and vomiting followed by

    • Mental status changes

    • Parkinsonian movement disorders

    • Seizures, typically in children

Differential Diagnosis

  • St Louis encephalitis

  • West Nile encephalitis


Laboratory Findings

  • Leukocytosis

  • Mild anemia

  • Hyponatremia

  • Cerebrospinal fluid typically shows

    • Mild to moderate pleocytosis with a lymphocytic predominance

    • Slightly elevated protein

    • Normal glucose

  • Enzyme-linked immunosorbent assay (ELISA) for anti-Japanese encephalitis IgM performed on cerebrospinal fluid or serum

  • Definitive diagnosis requires fourfold increase in virus-specific IgG confirmed by plaque reduction neutralization assay


  • In severe disease, brain imaging reveals thalamic lesions, with the hippocampus, midbrain, basal ganglia, and cerebral cortex affected to varying degrees


  • Supportive, including antipyretics, analgesics, bedrest, and fluids

  • Corticosteroids may be needed for rare complications such as the opsoclonus myoclonus syndrome



  • Cognitive, neurologic, and psychiatric complications, including memory and intellectual impairment, occur in as many as 50% of survivors and persist at least 1–2 years after acute infection

  • Opsoclonus myoclonus syndrome and the Kasabach-Merritt phenomenon of consumptive coagulopathy are reported rarely


  • Use of mosquito repellents

  • Wearing long sleeves, long pants, and socks

  • Use of air-conditioned facilities and bed nets are essential means of protection

  • Four effective types of vaccine against Japanese encephalitis are available worldwide, including live attenuated and inactivated vaccines

  • In the United States, inactivated Vero-cell culture-derived Japanese encephalitis vaccine (IXIARO) is licensed for the prevention of Japanese encephalitis in persons 2 months of age and older

    • No studies about its ...

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