Japanese Encephalitis

How to Cite This Chapter: Wroczyńska A. Japanese Encephalitis. McMaster Textbook of Internal Medicine. Kraków: Medycyna Praktyczna. https://empendium.com/mcmtextbook/chapter/B31.II.18.1.21. Accessed July 15, 2024.
Last Updated: February 11, 2022
Last Reviewed: February 11, 2022
Chapter Information

Definition and PathogenesisTop

1. Etiologic agent: Japanese encephalitis virus (JEV) belonging to the genus Flavivirus, family Flaviviridae. Five JEV genotypes have been identified to date, with genotype I currently being the dominant one in Asia.

2. Pathogenesis: Initially the virus replicates in cells at the site of the mosquito bite and in local lymph nodes. This is followed by the viremic phase with transient inflammatory lesions within the heart, lungs, liver, and reticuloendothelial system. Some patients have neuroinfection; since JEV replicates in endothelial cells, a significant part of the central nervous system (CNS) may be involved, including the thalamus, basal ganglia, brainstem, cerebellum (with damage to Purkinje cells), hippocampus, and cerebral cortex. Apart from neurons the virus also attacks other cells in the CNS, including astrocytes and microglial cells, which may cause damage to the blood-brain barrier.

3. Reservoir and transmission: The reservoir of JEV is vertebrates, primarily pigs and wading birds. The vector is mosquitoes, mainly Culex spp, which are common in wet areas with rice fields and shallow freshwater reservoirs. The mosquitoes are typically active in the evenings and at night. Cases of JEV infection through transfusion of blood products from an infected donor have been reported.

4. Risk factors: Staying in countries with active JEV transmission, particularly in rural areas in the period of intense transmission and outdoors at the time of the highest activity of Culex mosquitoes. Factors that increase the risk of JEV infection in travelers according to the Centers for Disease Control and Prevention Advisory Committee on Immunization Practices (CDC ACIP): Table 1.

5. Incubation and contagious period: The incubation period is 5 to 10 days. As the viral load is small, patients usually are not contagious for contacts.


In Asia Japanese encephalitis (JE) is the most common neuroinfection that can be prevented through vaccination. Each year ~68,000 cases are reported in East and South Asia and on some islands of Australia and Oceania. JEV transmission occurs mostly in rural areas, often in places with intense irrigation of rice fields. In some regions of Asia, favorable environmental conditions also occur in suburban and urban areas.

In the temperate climate zone in Asia, infections in humans usually peak in summer and autumn. In the tropics transmission may occur all year round, and the majority of cases are observed during the wet season.

The risk of infection associated with travels to Asia is very low (<1/1,000,000 travelers) and depends on a number of factors that affect the exposure.

In endemic countries adults acquire immunity through natural infection, and the disease is mainly seen in children. Travel-associated disease may occur at any age. Information for travelers on JEV transmission in individual countries is published by the CDC and can be found at wwwnc.cdc.gov. It should be noted, however, that it is rather general and should be interpreted with caution, as JEV transmission may change from year to year.

Clinical Features and Natural HistoryTop

In most cases JEV infection is asymptomatic or mild with fever, weakness, headache, or diarrhea.

In <1% of infected individuals nonspecific presentations are observed for a few days, followed by encephalitis with qualitative and quantitative disorders of consciousness (up to and including coma), aphasia, focal deficits, spastic paresis, and other movement disorders. Seizures are very frequent, usually in the form of generalized tonic-clonic attacks. Less frequent manifestations include flaccid paralysis, parkinsonism, and isolated aseptic meningitis (see Meningitis). In some patients JE initially presents as behavioral aberrations or acute psychosis.

Full-blown JE usually manifests as a severe neuroinfection and is burdened with a high risk of death. Symptoms of increased intracranial pressure or brainstem involvement and prolonged seizures are associated with poor prognosis.


Diagnostic Tests

1. Identification of the etiologic agent:

1) Serologic studies: Detection of IgM antibodies in cerebrospinal fluid (CSF) or in serum (enzyme-linked immunosorbent assay [ELISA]). In the majority of patients, IgM are detectable in CSF after 4 days of symptom onset, and in serum, after 7 days (sensitivity >95% after 10 days from symptom onset). Because of the potential cross-reactions with antibodies against other flaviviruses (eg, dengue virus [DENV] or West Nile virus [WNV]), a plaque reduction neutralization test (PRNT) is performed: A ≥4-fold increase in the serum IgG titer detected through a PRNT in 2 samples collected ≥14 days apart confirms recent JEV infection.

2) Molecular studies (reverse transcriptase–polymerase chain reaction [RT-PCR], reverse transcription loop-mediated isothermal amplification [RT-LAMP]; material: CSF, blood, brain tissue) are not commonly used due to small viremic load in JE and sufficient diagnostic utility of serologic tests.

3) Detection of viral antigens in brain tissue (immunohistochemical or immunofluorescent method).

4) Virus isolation from blood or CSF.

2. Other tests:

1) General CSF examination: Lesions typical of viral encephalitis. CSF pressure is increased in ~50% of patients.

2) Laboratory blood tests: Findings may include leukocytosis (in most patients), hyponatremia, thrombocytopenia, mild anemia, elevated alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels.

3) Imaging studies: Magnetic resonance imaging (MRI) of the brain frequently shows lesions in the thalamus, basal ganglia, midbrain, pons, and medulla oblongata.

4) Electroencephalography (EEG): Altered background activity in patients in coma (theta coma and delta coma patterns, focal paroxysmal discharges, burst-suppression pattern).

Diagnostic Criteria

Suspect JE in patients with signs and symptoms of neuroinfection who have recently traveled to or are staying in endemic regions. A definitive diagnosis can be made on the basis of infection confirmed in laboratory testing.

Case definitions according to the World Health Organization (WHO):

1) Suspected case: Any person with sudden-onset fever and ≥1 of the following:

a) Altered mental status (including confusion, disorientation, coma, inability to talk).

b) New-onset epileptic seizures (other than febrile seizures).

2) Laboratory-confirmed case: Any person who meets the criteria of a suspected case, with laboratory-confirmed JEV infection (see above).

3) Probable case: Any person with suspected JE who is in a close geographical and temporal relationship to a laboratory-confirmed case of JE in the context of a JEV outbreak.

Differential Diagnosis

Encephalitis caused by other microorganisms, parainfectious encephalitis, postinfectious encephalitis, other noninfectious diseases of the CNS.


Antiviral Treatment

None available.

Symptomatic Treatment

Urgent initiation of intensive symptom management significantly reduces mortality. Maintaining normal intracranial pressure and cerebral perfusion pressure, seizure control, and preventing complications, including secondary bacterial infections, are of particular importance.


In patients with mild symptoms, the prognosis is good. In patients with symptoms of neuroinfection secondary to JEV infection, the prognosis is poor. Mortality among hospitalized patients is between 20% and 30%; 30% to 50% of convalescents have chronic neurologic complications. Damage to the upper and lower motor neurons (paresis and paralysis) as well as cerebellar and extrapyramidal signs are the most common observations. Severe cognitive dysfunction, mental disorders, and epileptic seizures may develop. Approximately 50% of patients after full-blown JE have no manifest complications, only discreet cognitive dysfunction and behavioral disturbances.


Specific Prevention

Vaccination: Immunization is usually recommended for long-term (≥1 month) and frequent travelers to JE-endemic areas, and should also be considered for travelers with an increased risk of infection based on travel duration, season, location, activities, and accommodations (Table 1). The inactivated Vero cell culture–derived vaccine is used for pretravel immunization in some European countries. Different inactivated and live attenuated JE vaccines are used in other countries, but they are not licensed in Europe or North America.

Nonspecific Prevention

1. Mosquito protection and avoiding mosquito bites (see: Nonspecific Insect and Tick Bite Precautions). The effectiveness of environmental interventions to ensure vector control and reduce JE incidence has not been proven.

2. Patient isolation: Not required.

3. Personal protective equipment (PPE) for health-care personnel: Standard.


Table 10.11-1. Factors increasing the risk of JEV in travelers according to ACIP

Duration of travel

– The incidence of JE is the highest among long-term travelers

– No specific duration of travel has been identified that would be associated with increased risk of JE, but long-term stay in endemic regions increases the likelihood of exposure to bites of an infected mosquito

– Long-term travels also include frequent shorter stays in regions endemic for JE

Season of the year

In some regions of Asia JEV transmission is seasonal whereas in others it occurs all year round


– The highest risk is associated with stays in rural (agricultural) areas

– The mosquitoes that transmit JE typically breed in freshwater reservoirs, flooded rice fields, and wet areas

– JE was also reported in travelers who stayed at the seaside, in the neighborhood of rural areas or rice fields

– Large, focal outbreaks of JE indicate intensive JEV transmission in the area

Traveler’s activities

– The mosquitoes that carry JE are typically active outdoors, from dusk to dawn, hence the increased risk of exposure in travelers planning activities such as camping, trekking, biking, rafting, fishing, hunting, or staying at farms

– Staying outdoors, particularly at night, increases the risk of JE


Accommodations in nonscreened rooms without air conditioning or bednets increase the risk of bites by infected mosquitoes

Adapted from CDC. Recommendations and Reports. 68(2);1-33.

ACIP, Advisory Committee on Immunization Practices; JE, Japanese encephalitis; JEV, Japanese encephalitis virus.

We use cookies to ensure you get the best browsing experience on our website. Refer to our Cookies Information and Privacy Policy for more details.