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How serious?
Risk of death
Yes
Vaccine available?
Time to symptoms
Countries affected
Active outbreaks
Risk is very low for short-term urban travelers. Vaccination is recommended for stays >1 month in rural endemic areas of East/Southeast Asia, especially during the monsoon season. Mosquito bite prevention (dusk-to-dawn) reduces risk significantly.
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 | 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 |
Viral brain infection transmitted by Culex mosquitoes.
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.
Seek emergency medical care immediately if:
Severe headache with neck stiffness (meningeal signs)
Confusion, disorientation, or altered consciousness
Seizures or convulsions
Sudden onset of paralysis or weakness in limbs
High fever (>39°C) with any neurological symptoms
Inability to swallow or drooling
In children: unusual irritability, bulging fontanelle, high-pitched cry, opisthotonus
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):
Sudden onset high fever (39–40°C)
Headache, malaise, and myalgia
Nausea and vomiting
Diarrhea in children
Acute encephalitic phase:
Altered mental status: confusion → obtundation → coma
Seizures (especially in children — up to 85%)
Movement disorders: parkinsonian features (rigidity, mask-like face, tremor), dystonia, choreoathetosis
Neck stiffness and meningeal signs
Cranial nerve palsies (facial weakness, dysphagia)
Flaccid paralysis (polio-like anterior horn cell involvement)
Abnormal posturing: decorticate or decerebrate
In children: opisthotonus, high-pitched cry
Recovery (weeks to months):
Slow neurological recovery; many deficits persist
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.
Typical disease course (encephalitic form):
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.
How this disease is identified
Diagnosis is based on clinical presentation in an endemic area with laboratory confirmation:
CSF analysis: Lymphocytic pleocytosis (10–500 cells/mm³), elevated protein, normal glucose. Opening pressure often elevated.
Anti-JEV IgM in CSF: Most reliable — positive in >90% by day 7; highly specific when measured in CSF (less cross-reactive than serum)
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)
RT-PCR: Low sensitivity in CSF (viremia is brief and low-level in humans)
Neuroimaging (MRI): Bilateral thalamic T2/FLAIR hyperintensities are characteristic (seen in 30–50%); also basal ganglia, substantia nigra, midbrain involvement
EEG: Diffuse slowing; burst suppression in severe cases
Differential: herpes simplex encephalitis, cerebral malaria, tuberculous meningitis, bacterial meningitis, enterovirus encephalitis.
Available treatment methods
No specific antiviral therapy exists for JE. Treatment is entirely supportive:
ICU care: Required for most encephalitis cases; airway protection for decreased consciousness
Seizure management: IV benzodiazepines (lorazepam/diazepam) for acute seizures; phenytoin or levetiracetam for maintenance
Raised intracranial pressure: Mannitol, head elevation, controlled hyperventilation if indicated
Fluid and electrolyte management: SIADH (syndrome of inappropriate ADH) is common — monitor sodium closely; fluid restriction may be needed
Nutritional support: Nasogastric feeding for patients with dysphagia
Aspiration prevention: Positioning, suction; consider intubation if GCS <8
Rehabilitation: Early physical, occupational, and speech therapy for survivors with neurological deficits
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.
How to protect yourself
Vaccination (highly effective):
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.
SA 14-14-2 (live attenuated): Most widely used globally (>500 million doses in China); single dose ~85–95% effective; used in endemic countries
IMOJEV (chimeric live — YF-JEV): Single dose; licensed in Australia, Thailand; boosted at 1–2 years
WHO recommends JE vaccine in national immunization programs in all endemic areas.
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:
Culex mosquitoes bite primarily at dusk and nighttime (different from Aedes!)
Sleep under insecticide-treated bed nets
Apply DEET-based repellent in the evening
Avoid rice paddy areas and pig farms at dusk
Preparation is the best protection.
Risk to travelers:
High risk: Rural areas of South and Southeast Asia during monsoon season, especially near rice paddies and pig farms
Moderate risk: Urban areas in endemic countries; short-term travelers with limited outdoor exposure
Low risk: Brief stays in major cities; Japan, Korea, Taiwan (successful vaccination programs)
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
Culex mosquitoes bite at night — bed nets and evening repellent are critical
JE is a disease of rural agricultural areas — urban travelers have lower but non-zero risk
Children should be prioritized for vaccination (higher susceptibility and severity)
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:
Highest endemic areas: India (largest absolute burden), China (decreasing with vaccination), Nepal, Bangladesh, Myanmar, Vietnam, Cambodia, Laos, Philippines, Indonesia
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
Japan/Korea/Taiwan: Very low incidence now due to successful vaccination programs (<10 cases/year)
India: Reports ~1,500–3,000 cases/year (vast undercount; true burden estimated 10–15× higher)
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.
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.
Potential complications
JE has the highest complication and mortality rate among arboviral encephalitides:
Mortality: 20–30% of encephalitis cases (higher in children and elderly
up to 50% in outbreaks with limited ICU access)
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
Flaccid paralysis: Anterior horn cell destruction mimicking poliomyelitis
may be permanent
Pregnancy risks: Transplacental transmission documented
associated with miscarriage and fetal abnormalities
Long-term care burden: Many survivors require lifelong assistance — JE accounts for enormous disability burden in Asia
Children: More susceptible to encephalitis than adults
more severe seizures
higher rate of long-term sequelae
Expected outcomes and recovery
Overall: Most JEV infections (>99%) are asymptomatic. Of those who develop encephalitis:
CFR: 20–30%.
Permanent neurological sequelae in 30–50% of survivors (cognitive impairment, motor deficits, epilepsy, psychiatric symptoms).
Children <10 years are disproportionately affected and have worse outcomes.
No specific antiviral treatment — management is supportive.
Full recovery occurs in only 20–30% of encephalitis cases.
This disease is vaccine-preventable. Effective protection is available through vaccination.
Talk to a travel health specialist about the recommended schedule before your trip.
Find a vaccination clinic →The content on this page is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment recommendations. If you have health concerns, consult a qualified healthcare professional. Medova is not a medical service provider.
Full terms of useGeographic distribution and active outbreaks
Recent epidemiological data from the World Health Organization Global Health Observatory.
Source: WHO GHO OData ↗
This data is provided for informational purposes. Please consult official WHO sources for the most current information.
View WHO data source →| Flag | Country | Risk level |
|---|---|---|
 | Laos | High risk |
 | Myanmar | High risk |
 | India | High risk |
 | China | High risk |
 | Indonesia | High risk |
 | Vietnam | High risk |
 | Philippines | High risk |
 | Nepal | High risk |
 | Cambodia | High risk |
 | Bangladesh | High risk |
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