Japanese encephalitis virus

Japanese encephalitis virus is the leading cause of vaccine-preventable viral encephalitis in Asia, an Orthoflavivirus of the same serocomplex as West Nile virus and, like it, neurotropic. It is maintained in an agricultural cycle between Culex mosquitoes, pigs and wading birds, and humans are incidental dead-end hosts who acquire it near rice cultivation and pig rearing. The great majority of infections are silent, but the small fraction that reach the brain cause a severe encephalitis that kills a fifth to a third of those affected and leaves many survivors with permanent neurological damage, falling most heavily on children in endemic areas. Unlike most arboviral encephalitides, Japanese encephalitis is preventable: effective vaccines underpin routine childhood immunisation across much of endemic Asia and are recommended for at-risk travellers.

Discovery and historical significance

Large summer epidemics of encephalitis had been recorded in Japan since the nineteenth century, and in 1935 the prototype Nakayama strain was isolated from a fatal human case, establishing the virus as the type member of the Japanese encephalitis serocomplex. Through the twentieth century the virus was recognised across a widening area of Asia, and the development of effective vaccines from the mid-twentieth century onward transformed it from a major cause of childhood epidemic encephalitis in countries such as Japan and South Korea into a disease largely of areas with incomplete vaccine coverage.

Classification, structure, and genome

Classification

Japanese encephalitis virus belongs to the genus Orthoflavivirus (family Flaviviridae), current binomial Orthoflavivirus japonicum, and is the prototype of the Japanese encephalitis serocomplex that also contains West Nile, St Louis encephalitis and Murray Valley encephalitis viruses. It exists as a single serotype but resolves into five genotypes (I to V): genotype III was historically dominant across Asia and was the basis of most vaccines, genotype I has since become dominant for reasons that are not understood, and the rare genotype V has re-emerged after decades. The single-serotype structure means the vaccines protect across genotypes.

Virion structure

The virion is a small enveloped particle about 50 nm across, its surface carrying the envelope (E) glycoprotein, the receptor-binding and fusion protein and dominant neutralising target, and the membrane (M) protein. Mature particles show the flat herringbone lattice of 90 E dimers; immature particles are spiky and carry the uncleaved precursor membrane (prM) protein.

Genome organisation

The genome is a positive-sense single-stranded RNA of about 11 kb with a single open reading frame, translated as one polyprotein and cleaved into three structural proteins (capsid C, prM/M, envelope E) and seven nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5). NS3 is the protease and helicase, NS5 the polymerase and methyltransferase, and NS1 the secreted diagnostic antigen.

Replication cycle

Japanese encephalitis virus follows the canonical flavivirus cycle: the E protein binds attachment factors and the particle enters by clathrin-mediated endocytosis, endosomal acidification drives E to a fusogenic trimer that releases the genome, translation and replication proceed on endoplasmic-reticulum-derived vesicles with NS5 as the polymerase, and immature particles mature when furin cleaves prM to M in the trans-Golgi network before egress.

Pathogenesis

After a mosquito bite the virus replicates in skin and lymphoid tissue and produces a viraemia; in most people the infection is controlled before it reaches the brain. In the minority who develop encephalitis, the virus crosses into the central nervous system and infects neurons, with a striking predilection for the thalamus, basal ganglia and substantia nigra. This deep grey-matter tropism explains the movement disorders and the Parkinson-like syndrome that characterise the disease, while widespread neuronal infection and the accompanying inflammatory response produce the encephalitis and its high mortality. The very high ratio of silent to overt infection indicates that host factors and the efficiency of immune control largely determine who progresses to neuroinvasive disease.

Epidemiology

Japanese encephalitis is maintained in an enzootic cycle in which Culex tritaeniorhynchus, a mosquito that breeds in rice paddies and other ground pools, transmits the virus between amplifying hosts. Pigs are the key amplifying host, developing a high viraemia and living in close contact with people in rural Asia, while ardeid wading birds such as herons and egrets are natural maintenance hosts. Humans are dead-end hosts. The disease occurs across Asia from far-eastern Russia to Pakistan and into parts of the western Pacific, with an estimated 68,000 cases a year. In temperate zones it is epidemic in summer, and in the tropics endemic year-round, with transmission rising after rice planting. The ratio of subclinical to clinical infection is very high, on the order of 250 to 1, so by adulthood most people in endemic areas are immune and clinical disease falls mainly on children.

Natural history

After an incubation of about 4 to 14 days, the great majority of infections are subclinical or a mild febrile illness. When encephalitis develops it is preceded by a prodrome of fever, headache and vomiting over several days before central nervous system signs appear. Only around 1 in 250 infections becomes neuroinvasive, but that illness is grave: case-fatality is roughly 20% to 30%, falling toward 5% where intensive care is available, and up to half of survivors are left with lasting neurological or psychiatric sequelae. Children under ten are more likely to die and to be left disabled.

Clinical presentations and complications

The hallmark of Japanese encephalitis is altered consciousness, ranging from mild clouding to deep coma, often with a change in personality and, in children, mutism. Seizures are common, occurring in over half of affected children, and subtle continuous seizure activity may represent status epilepticus. Deep grey-matter involvement produces extrapyramidal features, including a masked facies, rigidity, tremor and other involuntary movements, and a Parkinson-like syndrome that typically appears during convalescence and usually recovers. Weakness may be spastic or flaccid, and an acute flaccid paralysis resembling poliomyelitis can occur. Hyponatraemia from inappropriate antidiuretic hormone secretion is frequent, and the cerebrospinal fluid shows a modest pleocytosis with a normal glucose. Neuroimaging characteristically shows bilateral thalamic lesions.

Diagnosis

The standard diagnostic test is the IgM-capture ELISA performed on both serum and cerebrospinal fluid, with a sensitivity approaching 100% when paired samples are taken one to two weeks after onset; IgM in cerebrospinal fluid indicates central nervous system infection. As with the other flaviviruses, cross-reactivity with co-circulating dengue and West Nile viruses and with prior flavivirus vaccination limits serology, so results are interpreted against the local epidemiology and, where needed, confirmed by neutralisation. Virus is rarely isolated from blood, which is cleared early, or from cerebrospinal fluid except in fatal cases, so molecular and culture methods play a secondary role. The differential in a child with acute encephalitis in Asia is broad and includes bacterial and tuberculous meningitis, cerebral malaria, dengue and other viral encephalitides.

Management

There is no specific antiviral, and controlled trials of interferon and corticosteroids showed no benefit. Management is supportive, but good intensive care matters: ventilation, control of seizures and raised intracranial pressure, and correction of hyponatraemia have reduced mortality from over 30% toward 5% where such care is available. Rehabilitation addresses the frequent motor, cognitive and behavioural sequelae.

Prevention and public health

Vector control

Reducing exposure to Culex tritaeniorhynchus through personal protection, avoiding outdoor evening activity in rural areas, and larval control in irrigated fields supplements vaccination, but vector control alone is difficult given the vast breeding habitat of rice agriculture. Siting pig pens away from dwellings can reduce human exposure to the amplifying host.

Vaccination

Vaccination is the mainstay of control, and several effective vaccines are in use.

Vaccine	Type	Notes
Vero-cell inactivated (for example IXIARO)	Inactivated, based on strain SA14-14-2	The standard traveller vaccine; two-dose primary series
SA14-14-2 live-attenuated	Live-attenuated	Single dose; very widely used in endemic Asia, WHO prequalified
Live chimeric (for example IMOJEV)	Live chimeric on a yellow fever 17D backbone	Single-dose schedule in several countries
Mouse-brain inactivated	Inactivated	The original vaccine, now largely phased out

Routine childhood immunisation has driven the near-disappearance of the disease in several formerly high-burden countries, and vaccination is recommended for travellers with significant rural exposure during the transmission season.

Surveillance and notification

Surveillance combines acute-encephalitis-syndrome case detection with laboratory confirmation, and monitoring of pig infection and vector activity gives early warning of seasonal transmission, informing the timing of vaccination and vector control.

South African context

Japanese encephalitis does not occur in South Africa or elsewhere in Africa: the virus is confined to Asia and the western Pacific, and neither the disease nor its enzootic cycle is present locally. Its relevance to South African practice is therefore in pre-travel medicine. Travellers to rural, agricultural parts of endemic Asia, particularly for prolonged stays or during the transmission season, should be assessed for Japanese encephalitis vaccination, which is available through travel clinics rather than the routine immunisation schedule, alongside standard mosquito-bite precautions. A returning traveller with encephalitis and a compatible exposure would be investigated with reference-laboratory serology through the National Institute for Communicable Diseases, bearing in mind cross-reactivity with the flaviviruses that do circulate locally.

References and recommended reading

• Pierson TC, Lazear HM, Diamond MS. Flaviviruses: Dengue, Zika, West Nile, Yellow Fever and Other Flaviviruses. In: Fields Virology, 7th edition, Chapter 9. Philadelphia: Wolters Kluwer; 2023. The principal source for Japanese encephalitis pathogenesis and clinical disease.
• Lindenbach BD, Randall G, Bartenschlager R, Rice CM. Flaviviridae: The Viruses and Their Replication. In: Fields Virology, 7th edition, Chapter 7. Philadelphia: Wolters Kluwer; 2023. The source for virion structure, genome organisation and the replication cycle.
• Petersen LR, Barrett ADT. Arthropod-Borne Flaviviruses. In: Richman DD, Whitley RJ, Hayden FG (eds.), Clinical Virology, 4th edition, Chapter 53. Washington: ASM Press; 2016. The foundational account of Japanese encephalitis transmission, clinical spectrum and vaccines.
• World Health Organization. Japanese encephalitis. WHO fact sheet; 2025. The source for current burden, vaccine recommendations and control.
• Centers for Disease Control and Prevention. Japanese Encephalitis. CDC Yellow Book: Health Information for International Travel; 2026. The source for traveller vaccination and risk assessment.