Japanese encephalitis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Diagnosis of Japanese encephalitis is based on a combination of clinical signs and symptoms and specialized laboratory tests of blood or cerebrospinal fluid. It is diagnosed by detection of antibodies in serum and CSF (cerebrospinal fluid) by IgM capture ELISA. 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. 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.

Laboratory Findings

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.

Other cerebrospinal fluid (CSF) findings are:

• Mild to moderate pleocytosis with a lymphocytic predominance
• Slightly elevated protein
• Normal ratio of CSF to plasma glucose.

Other laboratory findings include:

• Leukocytosis
• Mild anemia
• Hyponatremia.

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 may 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.