Japanese Encephalitis

Japanese encephalitis is a viral encephalitis caused by Japanese encephalitis virus, a mosquito-borne flavivirus of the same genus as dengue, Zika, yellow fever, and West Nile viruses ^[1][2]. It is transmitted to humans through the bite of infected Culex mosquitoes and represents the leading global cause of virus-induced encephalitis ^[1][2].

Most infections are mild, presenting as fever and headache, or are asymptomatic, yet roughly 1 in 250 infections progresses to severe illness ^[1]. The incubation period spans 4–14 days, and in children gastrointestinal pain and vomiting may dominate early presentation ^[1]. Severe disease manifests as rapid high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis, and can be fatal, with up to 30% case fatality among symptomatic cases and 20–30% of survivors left with permanent neurological or cognitive sequelae ^[1][2].

Japanese encephalitis is an important cause of viral encephalitis in Asia, with the first documented case reported in Japan in 1871 ^[1]. Annual incidence varies within and across endemic countries, reaching 10 per 100,000 or higher during outbreaks, and modeling estimates about 100,000 clinical cases (95% CI: 61,720–157,522) and 25,000 deaths (95% CI: 14,550–46,031) worldwide each year ^[1]. Twenty-four countries in the WHO South-East Asia and Western Pacific Regions carry transmission risk affecting more than 3 billion people, and the disease primarily affects children although any age group may be involved ^[1].

Japanese encephalitis virus is spread to humans by bites of infected Culex mosquitoes, principally Culex tritaeniorhynchus ^[1][2]. Secondary amplification in domesticated animals contributes to its emergence dynamics as an arbovirus ^[3].

An inactivated Japanese encephalitis vaccine (Ixiaro) is available and highly protective, with recent ACIP guidelines expanding its recommended use for travelers to endemic Asian countries ^[4]. Travelers to the 24 endemic nations may be at risk and should receive counseling in travel clinics regarding vaccination decision-making ^[4]. No specific antiviral treatment is currently approved for established disease ^[5].

1. 1 World Health Organization. Japanese encephalitis [Internet]. cited 23 Oct 2026. Available from: https://www.who.int/news-room/fact-sheets/detail/japanese-encephalitis
2. 2 Sharma KB et al. Pathobiology of Japanese encephalitis virus infection. Mol Aspects Med. 2021 Oct. PMID: 34274157. doi: 10.1016/j.mam.2021.100994. PubMed: https://pubmed.ncbi.nlm.nih.gov/34274157/
3. 3 Weaver SC et al. Zika, Chikungunya, and Other Emerging Vector-Borne Viral Diseases. Annu Rev Med. 2018 Jan 29. PMID: 28846489. doi: 10.1146/annurev-med-050715-105122. PubMed: https://pubmed.ncbi.nlm.nih.gov/28846489/
4. 4 Mileno MD et al. Japanese Encephalitis Vaccine. R I Med J (2013). 2020 Aug 3. PMID: 32752568. PubMed: https://pubmed.ncbi.nlm.nih.gov/32752568/
5. 5 Ashraf U et al. Pathogenicity and virulence of Japanese encephalitis virus: Neuroinflammation and neuronal cell damage. Virulence. 2021 Dec. PMID: 33724154. doi: 10.1080/21505594.2021.1899674. PubMed: https://pubmed.ncbi.nlm.nih.gov/33724154/