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Tuesday, August 12, 2025

Japanese encephalitis


Definition

Japanese encephalitis (JE) is a mosquito-borne viral infection caused by the Japanese encephalitis virus (JEV), a flavivirus related to dengue, yellow fever, and West Nile viruses. It primarily affects the central nervous system, leading to encephalitis, meningitis, or meningoencephalitis. It is a major cause of viral encephalitis in Asia and parts of the Western Pacific.

Epidemiology

  • Endemic in South and Southeast Asia, parts of China, Korea, Japan, and the Western Pacific.

  • Transmission peaks during the rainy season in tropical regions and summer/autumn in temperate zones.

  • Children are most affected in endemic areas due to lack of immunity, but unvaccinated adults are also at risk.

  • Annually, an estimated 68,000 clinical cases occur worldwide, with high mortality and neurological sequelae rates.

Transmission and Vector

  • Vector: Primarily Culex mosquitoes (especially Culex tritaeniorhynchus).

  • Reservoir hosts: Water birds (herons, egrets).

  • Amplifying hosts: Pigs (develop high viraemia without illness).

  • Humans: Dead-end hosts (do not develop sufficient viraemia to transmit the virus).

Pathophysiology

  • After a mosquito bite, the virus replicates locally, spreads via lymphatic and hematogenous routes, and crosses the blood–brain barrier.

  • Causes direct neuronal injury and induces inflammatory cytokine release, leading to cerebral edema and neurodegeneration.

  • Predilection for the thalamus, basal ganglia, brainstem, hippocampus, and spinal cord.

Incubation Period

  • 5–15 days (usually 6–8 days).

Clinical Features

1. Asymptomatic Infection

  • Over 99% of infections are subclinical or present as a mild febrile illness.

2. Symptomatic Disease

  • Prodromal stage (2–5 days):

    • Fever, chills, headache, malaise.

    • Myalgia, nausea, vomiting.

  • Encephalitic stage:

    • Altered mental status (confusion, disorientation).

    • Seizures (especially in children).

    • Focal neurological deficits.

    • Movement disorders (parkinsonian features, tremors, dystonia).

    • Signs of meningeal irritation (neck stiffness, photophobia).

  • Severe complications:

    • Coma, decerebrate/decorticate posturing.

    • Respiratory failure due to brainstem involvement.

Diagnosis

Clinical

  • Acute encephalitic syndrome in a patient from/traveling to an endemic area within the incubation period.

Laboratory

  • Definitive test: Detection of JEV-specific IgM antibodies in serum or cerebrospinal fluid (CSF) by ELISA.

  • CSF findings: Lymphocytic pleocytosis, mild protein elevation, normal/slightly low glucose.

  • RT-PCR or viral isolation possible but less commonly used due to short viraemic phase.

Imaging

  • MRI: Bilateral thalamic hyperintensities on T2-weighted and FLAIR sequences (characteristic but not pathognomonic).

Treatment

There is no specific antiviral treatment for Japanese encephalitis. Management is supportive and symptomatic.

Supportive Measures

  • Hospitalization for all suspected encephalitis cases.

  • Airway protection and respiratory support if required.

  • Control of fever with paracetamol (acetaminophen) – adults: 500–1000 mg orally every 4–6 hours (max 4 g/day); children: 10–15 mg/kg every 4–6 hours.

  • Anticonvulsants for seizures (e.g., diazepam 0.1–0.3 mg/kg IV slowly, max 10 mg/dose; or phenytoin 15–20 mg/kg IV loading dose).

  • Osmotic therapy for raised intracranial pressure: mannitol 0.25–1 g/kg IV over 20 minutes, repeated every 6–8 hours if needed.

  • Fluid and electrolyte balance maintenance.

  • Nutritional support in prolonged illness.

Corticosteroids

  • Not routinely recommended; no conclusive benefit.

Experimental / Adjunctive Therapies

  • Interferon-alpha and ribavirin have been tried but lack proven efficacy.

Complications

  • Mortality: 20–30% in hospitalized cases.

  • Neurological sequelae in 30–50% of survivors:

    • Cognitive impairment.

    • Speech and language deficits.

    • Motor weakness, paralysis.

    • Seizure disorders.

    • Movement disorders (parkinsonism, dystonia).

Prevention

1. Vaccination – Primary preventive measure.

Inactivated Vero Cell–Derived Vaccines (e.g., IXIARO, JESPECT)

  • Schedule: 2 doses intramuscularly, 28 days apart.

    • Adults: 0.5 mL per dose (first at day 0, second at day 28; complete at least 1 week before exposure).

    • Children (2 months–<3 years): 0.25 mL per dose.

Live Attenuated SA 14-14-2 Vaccine (widely used in Asia)

  • Single subcutaneous dose.

  • In endemic areas: Given to children at 9 months or 12 months, with booster as per national schedules.

Live Recombinant Chimeric Vaccine (JE-CV, IMOJEV)

  • Single dose, subcutaneous; for children ≥9 months and adults up to 65 years.

2. Vector Control

  • Mosquito control measures: Larviciding, drainage of breeding sites, use of insecticides.

3. Personal Protection

  • Insect repellents (DEET-containing).

  • Long-sleeved clothing.

  • Bed nets, especially in rural agricultural areas.

Public Health Considerations

  • Vaccination campaigns have drastically reduced JE incidence in countries like Japan, South Korea, and China.

  • Travelers to rural Asia during transmission season should be vaccinated.

  • Surveillance, vector control, and public education are key components of JE control programs.




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