Japanese Encephalitis

Japanese encephalitis is an infection of the brain caused by Japanese encephalitis virus (JEV), a mosquito-borne flavivirus belonging to the same genus as dengue, Zika, yellow fever, and West Nile viruses. First documented as a clinical entity in Japan in 1871, JEV is the etiologic agent responsible for this zoonotic viral encephalitis. The virus is maintained in an enzootic transmission cycle involving Culex mosquitoes, pigs, and water birds, with humans serving as incidental dead-end hosts who do not develop sufficient viraemia to infect feeding mosquitoes.

The clinical spectrum of JEV infection ranges from asymptomatic or mild febrile illness to severe encephalitis. Most infections are subclinical or present only with fever and headache; however, approximately 1 in 250 infected individuals develops severe neurological disease. The incubation period ranges from 4 to 14 days, with children frequently presenting initially with gastrointestinal pain and vomiting. Severe disease is characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, and spastic paralysis. Among symptomatic patients, the case fatality rate can reach 30%, and among survivors, 20–30% suffer permanent cognitive, behavioural, or neurological sequelae including seizures, hearing or vision loss, speech and language impairment, and memory or communication problems.

Japanese encephalitis is endemic across 24 countries in the WHO South-East Asia and Western Pacific Regions, exposing more than 3 billion people to transmission risk. The disease is predominantly found in rural and periurban settings where humans live in closer proximity to vertebrate reservoir hosts, particularly domestic pigs. Annual incidence varies across and within endemic countries, ranging from 10 per 100,000 population during inter-epidemic periods to higher rates during outbreaks. A literature review and modelling study estimates approximately 100,000 clinical cases globally each year, with about 25,000 deaths. The disease primarily affects children, as most adults in endemic countries acquire natural immunity after childhood infection, though individuals of any age may be affected.

JEV is transmitted to humans through the bites of infected mosquitoes, principally Culex species, with Culex tritaeniorhynchus being the primary vector. The virus persists in nature through an enzootic transmission cycle involving mosquitoes, pigs, and water birds as vertebrate amplifying hosts. Humans become infected incidentally and do not develop sufficient viraemia to infect feeding mosquitoes, thereby serving as dead-end hosts. Transmission intensity follows seasonal patterns, with most cases occurring during warm months in temperate areas of Asia. The disease occurs predominantly outside of urban centres in agricultural areas where rice cultivation and pig rearing create favourable conditions for vector proliferation.

Children are the primary population affected by Japanese encephalitis, as most adults in endemic countries acquire natural immunity through childhood infection. However, individuals of any age may be affected, particularly those without prior exposure who enter endemic areas. Children under five years of age experience higher mortality rates, and immunocompromised individuals face elevated risk of severe outcomes. Residents of rural and periurban areas in endemic regions, particularly those living in close proximity to pig-rearing operations and agricultural landscapes supporting vector breeding, constitute the population at greatest exposure risk.

Prevention and control of Japanese encephalitis relies primarily on vaccination, with safe and effective vaccines available and WHO recommending immunization in all regions where the disease constitutes a recognized public health priority. Most endemic countries have implemented country-wide or targeted vaccination programmes, and Gavi supports catch-up campaigns and co-finances the vaccine for routine immunization in eligible countries. A decline in disease incidence has been reported in recent years, likely attributable in part to vaccination efforts. Beyond immunization, prevention activities include strengthening surveillance and reporting mechanisms, though source-backed detail on additional vector-control measures is not yet available.

Surveillance for Japanese encephalitis is primarily syndromic, focusing on acute encephalitis syndrome cases. Individuals who reside in or have travelled to endemic areas and present with encephalitis are considered suspected cases. Diagnosis requires laboratory confirmation, with WHO recommending testing for JEV-specific IgM antibody in a single sample of cerebrospinal fluid or serum using IgM-capture ELISA. Testing of cerebrospinal fluid is preferred to reduce false-positive results from previous infection or vaccination. If the initial test is negative, a convalescent sample may be tested. Confirmatory neutralizing antibody testing is recommended for patients with positive IgM results. Effective surveillance enables detection of outbreaks and monitoring of vaccination programme impact.

1. 1 WHO Fact Sheet. Japanese encephalitis.
2. 2 WHO Q&A. News.
3. 3 Wikidata. Japanese encephalitis.
4. 4 Wikipedia. Japanese encephalitis - Wikipedia.

ICD-10

A83.0

ICD-11

1C83.0

Total cases

16K

Total deaths

723

Peak month

2010-08

Coverage

3 reporting countries · 2000-01-01 → 2026-05-02