Japanese encephalitis

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Japanese Encephalitis
Japanese Encephalitis

Japanese encephalitis (JE) virus is the leading cause of vaccine-preventable]] encephalitis in Asia and the western Pacific. For most travelers to Asia, the risk for JE is very low but varies based on destination, length of travel, season, and activities.  

Most people infected with JE do not have symptoms or have only mild symptoms. However, a small percentage of infected people develop inflammation of the brain (encephalitis), with symptoms including sudden onset of headache, high fever, disorientation, coma, tremors and convulsions. About 1 in 4 cases are fatal. To prevent getting sick from JE, use an EPA-registered insect repellent, wear long-sleeved shirts and long pants, and get vaccinated.


Most human infections with Japanese encephalitis (JE) virus are asymptomatic; <1% of people infected develop clinical disease. Acute encephalitis is the most commonly recognized clinical manifestation. Milder forms of disease, such as aseptic meningitis or undifferentiated febrile illness, can also occur. Illness usually begins with sudden onset of fever, headache, and vomiting. Mental status changes, focal neurologic deficits, generalized weakness, and movement disorders may develop over the next few days.

Clinical features

The classical description of JE includes a Parkinsonian syndrome with masklike facies, tremor, cogwheel rigidity, and choreoathetoid movements. Acute flaccid paralysis, with clinical and pathological features similar to those of poliomyelitis, has also been associated with JE. Seizures are common, especially among children. The case-fatality ratio is approximately 20%–30%. Among survivors, 30%–50% have significant neurologic, cognitive, or psychiatric sequelae.

Lab findings

Clinical laboratory findings might include a moderate leukocytosis, mild anemia, and hyponatremia. Cerebrospinal fluid (CSF) typically has a mild to moderate pleocytosis with a lymphocytic predominance, slightly elevated protein, and normal ratio of CSF to plasma glucose. Magnetic resonance imaging (MRI) of the brain is better than computed tomography (CT) for detecting JE virus-associated abnormalities such as changes in the thalamus, basal ganglia, midbrain, pons, and medulla. Thalamic lesions are the most commonly described abnormality; although these can be highly specific for JE in the appropriate clinical context, they are not a very sensitive marker of JE. EEG abnormalities may include theta and delta coma, burst suppression, epileptiform activity, and occasionally alpha coma.

Japanese Encephalitis vaccine
Japanese Encephalitis vaccine

Antibody testing

JE virus infections are confirmed most frequently by detection of virus-specific antibody in CSF or serum. Because humans have low or undetectable levels of viremia by the time distinctive clinical symptoms are recognized, virus isolation and nucleic acid amplification tests are insensitive and should not be used for ruling out a diagnosis of JE. Click here for more information about diagnostic testing.


Japanese encephalitis (JE) should be considered in a patient with evidence of a neurologic infection (e.g., meningitis, encephalitis, or acute flaccid paralysis) who has recently traveled to or resided in an endemic country in Asia or the western Pacific.

Laboratory diagnosis of JE is generally accomplished by testing of serum or cerebrospinal fluid (CSF) to detect virus-specific IgM antibodies. JE virus IgM antibodies are usually detectable 3 to 8 days after onset of illness and persist for 30 to 90 days, but longer persistence has been documented. Therefore, positive IgM antibodies occasionally may reflect a past infection or vaccination. Serum collected within 10 days of illness onset may not have detectable IgM, and the test should be repeated on a convalescent sample. For patients with JE virus IgM antibodies, confirmatory neutralizing antibody testing should be performed. In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry, and virus culture of autopsy tissues can also be useful.

Diagnostic testing for JE virus IgM antibodies is commercially-available. Confirmatory testing is only available at CDC and a few specialized reference laboratories.

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