Japanese encephalitis virus, or JEV, is spread by mosquitoes and is an important cause of viral encephalitis in Asia. Most infections cause no apparent symptoms or only mild illness, but a small proportion affect the brain and can cause death or lasting disability.
WHO estimates about 100,000 clinical cases globally each year. Most cases occur in children below 15 years, although a person of any age can be affected.
How is Japanese encephalitis transmitted?
JEV reaches people through bites from infected Culex mosquitoes, mainly Culex tritaeniorhynchus. The virus circulates in a cycle involving mosquitoes, pigs and/or water birds.
Humans do not develop enough virus in the blood to infect mosquitoes that feed on them. This means an infected person is not the source that maintains the usual mosquito transmission cycle.
Where and when does transmission occur?
WHO identifies transmission risk in 24 countries in its South-East Asia and Western Pacific regions. Disease is found predominantly in rural and periurban areas where people may live close to pigs or water birds.
In temperate parts of Asia, transmission occurs mainly during the warm season and large epidemics can occur. In tropical and subtropical areas, it can occur throughout the year and may intensify during the rainy season and before harvest in rice-growing regions. Risk can vary within a country, so current destination-specific advice matters.
What happens after infection?
Most JEV infections are mild, causing fever and headache, or produce no apparent symptoms. WHO estimates that about 1 in 250 infections leads to severe clinical illness. The incubation period is 4–14 days.
In children, abdominal pain and vomiting may be the main early symptoms. These common symptoms alone do not point to Japanese encephalitis. Exposure location, progression and examination are essential.
What are the warning signs of severe disease?
WHO describes severe disease as a rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures and spastic paralysis. Encephalitis means inflammation of the brain and requires urgent medical attention.
A child with altered awareness, seizure, marked neck stiffness, new weakness, difficulty waking or another sign of severe neurologic illness needs emergency assessment. Do not wait for every listed symptom or try to confirm the diagnosis at home.
How serious can the illness be?
Among people who develop encephalitis, WHO states that the case-fatality rate can be as high as 30%. Survivors may have permanent cognitive, behavioural or neurological effects. The fact sheet lists seizures, hearing or vision loss, speech and language problems, memory and communication difficulties, and limb weakness.
These figures describe groups with symptomatic encephalitis and should not be applied to every mosquito bite or mild fever. Most JEV infections do not cause severe disease.
How is the diagnosis confirmed?
WHO considers encephalitis in someone who lives in or has travelled to an endemic area a suspected case. Clinical assessment is followed by testing. WHO recommends testing for JEV-specific IgM antibodies in cerebrospinal fluid or serum using an IgM-capture ELISA, with cerebrospinal fluid preferred to reduce false-positive results from previous infection or vaccination.
Testing also helps rule out other causes of encephalitis. The decision to perform a lumbar puncture and the interpretation of results belong to the treating clinical team.
What treatment is available?
WHO states that there is no specific antiviral cure for Japanese encephalitis. Hospital treatment is supportive and focuses on stabilizing the patient, relieving severe clinical signs and helping the person overcome the infection.
This is not a condition for home treatment when encephalitis is suspected. Emergency care may be needed for breathing, seizures, consciousness and other complications.
What care may survivors need?
Some people who survive encephalitis need long-term treatment, rehabilitation and support. The effects may involve health, education, communication, social participation and family life. WHO notes that access to these services is often insufficient, especially in low- and middle-income countries.
Follow-up may involve several services according to the person’s needs. Families can ask the clinical team for a written plan covering rehabilitation, hearing or vision assessment, seizures, development, school support and available disability services.
What does WHO say about vaccination?
Safe and effective vaccines are available. WHO recommends integrating Japanese encephalitis vaccination into national immunization schedules wherever the disease is recognized as a public-health issue, together with surveillance and reporting.
Several vaccine types are used. Product, schedule, age eligibility and national recommendations differ. Parents and travellers should use advice from an authorized vaccination or travel-health service rather than choose a product from a general article.
What should travellers consider?
WHO says the risk for travellers to endemic areas is normally low. Travellers spending extensive time in endemic areas are recommended to receive vaccination before travel. The actual decision depends on destination, season, length of stay, activities and current guidance.
Plan advice early enough for a travel-health professional to review the itinerary and any vaccine schedule. A city stopover and a long stay in a rural rice-growing area may not carry the same exposure pattern.
How can mosquito bites be reduced?
WHO lists mosquito repellents, long-sleeved clothing, coils and vaporizers as personal preventive measures. These precautions can reduce bites but do not provide the same population protection as vaccination in areas where JE is a public-health priority.
Use repellents and other products according to their labels and age restrictions. Families should combine practical bite prevention with current vaccination advice when relevant.
What should families remember?
Severe Japanese encephalitis is rare among infections, but it is dangerous when it occurs. Location and mosquito exposure inform risk; neurological warning signs require emergency care. Vaccination programmes and bite prevention are the main preventive tools described by WHO.
References
- World Health Organization. Japanese encephalitis, 6 August 2024. Source reviewed 4 October 2026.