Japanese encephalitis
Japanese Encephalitis (JE) is a viral infection of the central nervous system caused by the Japanese encephalitis virus (JEV), a mosquito-borne flavivirus. The disease is endemic in large parts of Asia and has expanded into certain areas of Australia. While most infections with the JEV are asymptomatic or mild, a small proportion progresses to severe neurological disease, which may be fatal or lead to lasting sequelae.
Transmission
Japanese encephalitis is transmitted through the bite of infected Culex spp mosquitoes. These mosquitoes are primarily active from dusk to dawn, meaning exposure risk is greatest during evening and night.
The JE virus is maintained in a cycle between mosquitos, and amplifying vertebrate hosts, primarily pigs and wading birds. Humans are accidental, dead-end hosts, they do not contribute to further transmission. Human-to-human transmission does not occur, except rarely through blood transfusion, organ transplantation or transplacental transmission.
Incubation Period
Typically ranges from 5 to 15 days after a mosquito bite.
Symptoms
Most people infected with JEV remain asymptomatic. Some people will experience non-severe symptoms like a non-specific febrile illness with headache, nausea, vomiting and malaise.
In less than 1% of cases, the infection progresses within a few days to a neurological disease with mental changes, general weakness and focal neurological symptoms. Classically, acute encephalitis develops with movement disorders resembling a Parkinson's-like syndrome with a mask-like facies, tremor, cogwheel rigidity, and choreoathetoid movements. Other symptoms include confusion, seizures, neck stiffness, focal neurological deficits and coma. Seizures are common, especially in young children. The mortality rate among patients with encephalitis is estimated at 20–30%. Of those who survive, approximately 30–50% are left with permanent neurological or cognitive sequelae, which may include seizures, hearing or vision loss, speech and language impairments, memory difficulties, or weakness in the limbs.
While acute encephalitis is the most severe presentation, milder neurological forms such as aseptic meningitis can also occur.
There is no antiviral treatment for Japanese encephalitis. Treatment is focussed on supportive care.
Risk areas
Japanese encephalitis is endemic in most parts of Asia and areas within the western Pacific. In recent years, the virus has emerged in new areas of Australia.
Transmission varies between and within countries by season, climate and local ecological conditions.
In more temperate regions, most cases occur during summer and early autumn, coinciding with the warmest weather. In tropical and subtropical regions, transmission can occur year-round, but it usually increases during the rainy season and around the time of the rice harvest when flooded fields create optimal breeding conditions for Culex mosquitoes and contact between vectors, amplifying hosts, and humans is intensified.
Transmission of Japanese encephalitis virus is most common in rural, agricultural settings, particularly in areas where pig farming, rice paddies, and flood-based irrigation systems are found in close proximity.
More detailed information on risk areas and transmission seasons can be found in the specific countrypages.
Risk for Travellers
For most travellers, the risk of Japanese encephalitis is very low, though it varies considerably depending on travel circumstances. For short trips, the estimated risk is about 1 in 1 million travellers. The risk increases for those who travel for longer periods, make frequent visits to endemic regions, or travel during high transmission season. The risk is also higher for travellers who stay overnight in rural areas near pig farms, rice fields, or wetlands with wading birds, particularly if they spend time outdoors after sunset and do not use mosquito protection measures.
Additional Information
- Wikitropica: background information on Japanese encephalitis for medical professionals.
- CDC Yellow Book: Japanese Encephalitis (CDC 2026)
- WHO Factsheet: Japanese Encephalitis ( WHO 2024)
References:
Yakob et al. Japanese Encephalitis Emergence in Australia: The Potential Population at Risk. Clin Infect Dis. 2023 Jan 13;76(2):335-337