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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 annual cases

  • 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

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

Diagnosis

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

Imaging

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

Treatment

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

Outcome

Complications

  • 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

Prevention

  • 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 safety in pregnant women

    • Therefore, administration to pregnant women should be deferred, unless the risk of infection outweighs the risk of vaccine complications

  • Primary vaccination requires two doses administered 28 days apart, to be completed more than 1 week before travel

  • For adults, a booster dose is recommended in case of potential reexposure or of continued risk for infection if the primary series of ...

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