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Figure 2. Schematic of virologic and serologic tests in West Nile virus encephalitis. Solid lines represent the more common results; broken lines represent reported ranges. The shaded box is an example of a typical patient. Incubation period is usually 5 to 14 days (median of 10 days in cases acquired after blood transfusion). Viremia has usually resolved by the time symptoms begin, but it may last up to 11 days in some immunocompromised patients. Results of polymerase chain reaction (PCR) in the cerebrospinal fluid are positive very early in the course of encephalitis and only for a short period. In most patients, the PCR test result is already negative during the symptomatic phase when viral specific antibodies are detected, although this may vary depending on the method used. In the patient described, the result was still positive 2 weeks into his illness. Serologic responses follow: IgM and IgG in the cerebrospinal fluid first and later in the serum, at approximately 1- to 2-day intervals each. Many patients will not show a positive result for IgG in the cerebrospinal fluid (lighter line). A typical patient (shaded box) will have results that are either positive for PCR (if sampled very early) or positive for IgM in the spinal fluid (occasionally), depending on the duration of symptoms. The serologic responses may persist for months.
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