Diagnosis

During the first phase of the disease, the most common laboratory abnormalities are a low white blood cell count (leukopenia) and a low platelet count (thrombocytopenia). Liver enzymes in the serum may also be mildly elevated. After the onset of neurologic disease during the second phase, an increase in the number of white blood cells in the blood and the cerebrospinal fluid (CSF) is usually found. Virus can be isolated from the blood during the first phase of the disease. Laboratory diagnosis usually depends on detection of specific IgM in either blood or CSF, usually appearing later, during the second phase of the disease.