Overview
Mosquito-borne viral brain infection endemic in Asia-Pacific. Leading cause of vaccine-preventable encephalitis in Asia with ~68,000 clinical cases and ~17,000 deaths annually.
Symptoms
Symptoms | Frequency | Severity | Onset |
|---|---|---|---|
| Headache | 90% | Moderate | Early |
| Chills | 45% | Mild | Early |
| Loss of appetite | 60% | Mild | Early |
| Malaise | 75% | Mild | Early |
| Nausea | 60% | Mild | Early |
| Vomiting | 55% | Moderate | Early |
| Abdominal pain | 25% | Mild | Early |
| Back pain | 20% | Mild | Early |
| Diarrhea | 20% | Mild | Early |
| Dizziness | 35% | Mild | Early |
| Myalgia | 50% | Mild | Early |
| Altered consciousness | 75% | Critical | Peak |
| Confusion | 65% | Severe | Peak |
| High fever | 80% | Severe | Peak |
| Neck stiffness | 70% | Moderate | Peak |
| Photophobia | 50% | Mild | Peak |
| Seizures | 65% | Critical | Peak |
| Tremor | 40% | Moderate | Peak |
| Irritability | 55% | Moderate | Peak |
| Paralysis | 30% | Severe | Peak |
| Fever | 95% | Severe | Any phase |
| Fatigue | 70% | Mild | Any phase |
Transmission
Overview
Viral brain infection transmitted by Culex mosquitoes.
Overview
Japanese encephalitis (JE) is caused by the Japanese encephalitis virus (JEV), a flavivirus transmitted by Culex tritaeniorhynchus and related mosquitoes that breed in rice paddies and other flooded agricultural areas. The transmission cycle involves waterbirds (herons, egrets) as amplifying hosts and pigs as the principal peridomestic amplifying host (high viremia). Humans are dead-end hosts. JEV is the most important cause of viral encephalitis in Asia. The vast majority of infections (~99%) are asymptomatic or cause mild febrile illness; however, when encephalitis develops, it is devastating — mortality is 20–30% and 30–50% of survivors have permanent neuropsychiatric sequelae.
Emergency Signs
Seek emergency medical care immediately if:
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Severe headache with neck stiffness (meningeal signs)
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Confusion, disorientation, or altered consciousness
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Seizures or convulsions
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Sudden onset of paralysis or weakness in limbs
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High fever (>39°C) with any neurological symptoms
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Inability to swallow or drooling
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In children: unusual irritability, bulging fontanelle, high-pitched cry, opisthotonus
Detailed Symptoms
Most common signs and symptoms
Incubation: 5–15 days. Most infections (~99%) are asymptomatic or cause mild febrile illness.
Symptomatic JE progresses through phases:
Prodromal phase (2–3 days):
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Sudden onset high fever (39–40°C)
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Headache, malaise, and myalgia
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Nausea and vomiting
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Diarrhea in children
Acute encephalitic phase:
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Altered mental status: confusion → obtundation → coma
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Seizures (especially in children — up to 85%)
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Movement disorders: parkinsonian features (rigidity, mask-like face, tremor), dystonia, choreoathetosis
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Neck stiffness and meningeal signs
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Cranial nerve palsies (facial weakness, dysphagia)
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Flaccid paralysis (polio-like anterior horn cell involvement)
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Abnormal posturing: decorticate or decerebrate
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In children: opisthotonus, high-pitched cry
Recovery (weeks to months):
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Slow neurological recovery; many deficits persist
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30–50% of survivors have permanent sequelae: cognitive impairment, motor deficits, seizure disorder, behavioral changes
Knowing the symptoms is the first step to a quick response.
Course of Disease
Typical disease course (encephalitic form):
- Incubation period: 5–15 days.
- Prodromal phase (1–6 days): Fever, headache, malaise, nausea, vomiting.
- Encephalitic phase (3–14 days): Altered consciousness, seizures (especially in children), movement disorders (tremor, Parkinsonism), focal neurological deficits, neck stiffness.
- Acute phase resolution: Fever subsides over 1–2 weeks. Neurological deficits may persist.
- Convalescence (weeks to months): Slow neurological recovery. Many survivors have permanent sequelae.
Spectrum: Infection ranges from asymptomatic viremia (most common) to mild febrile illness, aseptic meningitis, or full encephalitis. Encephalitis develops in <1% of infections but carries devastating consequences.
Diagnosis
How this disease is identified
Diagnosis is based on clinical presentation in an endemic area with laboratory confirmation:
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CSF analysis: Lymphocytic pleocytosis (10–500 cells/mm³), elevated protein, normal glucose. Opening pressure often elevated.
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Anti-JEV IgM in CSF: Most reliable — positive in >90% by day 7; highly specific when measured in CSF (less cross-reactive than serum)
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Serum anti-JEV IgM (ELISA): Positive from day 4–7; cross-reacts with other flaviviruses (dengue, West Nile, Zika) — confirm with PRNT (plaque reduction neutralization test)
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RT-PCR: Low sensitivity in CSF (viremia is brief and low-level in humans)
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Neuroimaging (MRI): Bilateral thalamic T2/FLAIR hyperintensities are characteristic (seen in 30–50%); also basal ganglia, substantia nigra, midbrain involvement
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EEG: Diffuse slowing; burst suppression in severe cases
Differential: herpes simplex encephalitis, cerebral malaria, tuberculous meningitis, bacterial meningitis, enterovirus encephalitis.
Treatment
Available treatment methods
No specific antiviral therapy exists for JE. Treatment is entirely supportive:
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ICU care: Required for most encephalitis cases; airway protection for decreased consciousness
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Seizure management: IV benzodiazepines (lorazepam/diazepam) for acute seizures; phenytoin or levetiracetam for maintenance
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Raised intracranial pressure: Mannitol, head elevation, controlled hyperventilation if indicated
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Fluid and electrolyte management: SIADH (syndrome of inappropriate ADH) is common — monitor sodium closely; fluid restriction may be needed
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Nutritional support: Nasogastric feeding for patients with dysphagia
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Aspiration prevention: Positioning, suction; consider intubation if GCS <8
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Rehabilitation: Early physical, occupational, and speech therapy for survivors with neurological deficits
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No proven benefit: Interferon-alpha, steroids, and immunoglobulin have not shown consistent benefit in clinical trials
Recovery may take months. ~50% of survivors have significant long-term disability.
Most cases are effectively treated with early diagnosis.
Prevention Details
How to protect yourself
Vaccination (highly effective):
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Ixiaro (Vero cell-derived, inactivated): Licensed in US/EU for age ≥2 months. 2-dose schedule (days 0 and 28); booster at 12–24 months if ongoing risk. Efficacy >95% after primary series. Well tolerated.
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SA 14-14-2 (live attenuated): Most widely used globally (>500 million doses in China); single dose ~85–95% effective; used in endemic countries
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IMOJEV (chimeric live — YF-JEV): Single dose; licensed in Australia, Thailand; boosted at 1–2 years
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WHO recommends JE vaccine in national immunization programs in all endemic areas.
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Traveler recommendation: Vaccinate if spending ≥1 month in rural endemic areas during transmission season. Consider for shorter trips if extensive outdoor/rural exposure (camping, cycling, rice field visits).
Mosquito bite prevention:
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Culex mosquitoes bite primarily at dusk and nighttime (different from Aedes!)
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Sleep under insecticide-treated bed nets
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Apply DEET-based repellent in the evening
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Avoid rice paddy areas and pig farms at dusk
Preparation is the best protection.
Travel Advice
Risk to travelers:
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High risk: Rural areas of South and Southeast Asia during monsoon season, especially near rice paddies and pig farms
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Moderate risk: Urban areas in endemic countries; short-term travelers with limited outdoor exposure
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Low risk: Brief stays in major cities; Japan, Korea, Taiwan (successful vaccination programs)
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Vaccinate if: – Spending ≥1 month in endemic rural areas – Repeat travel to endemic zones – Extensive outdoor activities (camping, hiking, cycling) even for shorter trips – Working in rice fields, animal agriculture, or laboratory exposure
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Culex mosquitoes bite at night — bed nets and evening repellent are critical
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JE is a disease of rural agricultural areas — urban travelers have lower but non-zero risk
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Children should be prioritized for vaccination (higher susceptibility and severity)
How common is it?
Statistics and geographic data
JE is the leading cause of viral encephalitis in Asia and the Western Pacific. WHO estimates ~68,000 clinical cases and ~17,000 deaths annually, with an estimated 1.8 billion people living in endemic areas:
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Highest endemic areas: India (largest absolute burden), China (decreasing with vaccination), Nepal, Bangladesh, Myanmar, Vietnam, Cambodia, Laos, Philippines, Indonesia
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Seasonal transmission: Correlates with monsoon/rainy season (rice paddy flooding); in temperate regions (Japan, Korea, northern China) transmission is seasonal (May–October); in tropical regions, year-round with monsoon peaks
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Japan/Korea/Taiwan: Very low incidence now due to successful vaccination programs (<10 cases/year)
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India: Reports ~1,500–3,000 cases/year (vast undercount; true burden estimated 10–15× higher)
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Expanding range: Climate change and rice cultivation expansion may push JE into new areas (Central Asia, higher altitudes)
Risk to unvaccinated travelers: estimated 1 per 1 million for typical itineraries; up to 1 per 5,000 per week for prolonged rural exposure during transmission season.
Risk Factors
Who is most at risk
Traveling to endemic areas in Asia and the Western Pacific during transmission season, rural and agricultural settings near rice paddies and pig farms, prolonged outdoor exposure during evening and nighttime hours, lack of vaccination, children under 10 years of age.
Complications Details
Potential complications
JE has the highest complication and mortality rate among arboviral encephalitides:
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Mortality: 20–30% of encephalitis cases (higher in children and elderly
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up to 50% in outbreaks with limited ICU access)
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Neuropsychiatric sequelae (30–50% of survivors): – Cognitive impairment (memory, attention, executive function) – Motor deficits: parkinsonism, dystonia, hemiparesis, quadriparesis – Seizure disorder (20–30% of survivors develop epilepsy) – Behavioral changes: personality alterations, aggression – Speech and language deficits
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Flaccid paralysis: Anterior horn cell destruction mimicking poliomyelitis
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may be permanent
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Pregnancy risks: Transplacental transmission documented
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associated with miscarriage and fetal abnormalities
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Long-term care burden: Many survivors require lifelong assistance — JE accounts for enormous disability burden in Asia
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Children: More susceptible to encephalitis than adults
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more severe seizures
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higher rate of long-term sequelae
Recovery & Outlook
Expected outcomes and recovery
Overall: Most JEV infections (>99%) are asymptomatic. Of those who develop encephalitis:
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CFR: 20–30%.
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Permanent neurological sequelae in 30–50% of survivors (cognitive impairment, motor deficits, epilepsy, psychiatric symptoms).
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Children <10 years are disproportionately affected and have worse outcomes.
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No specific antiviral treatment — management is supportive.
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Full recovery occurs in only 20–30% of encephalitis cases.
