Clinical and Laboratory Evaluation

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.

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.

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.

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.