Acute undifferentiated fever caused by infection with Japanese encephalitis virus.

To determine the proportion of acute undifferentiated fevers without neurologic deficits related to infection with Japanese encephalitis (JE) virus, flavivirus serology (dengue and JE) was performed in a cohort of 156 adults presenting to a hospital in Chiangrai, Thailand. Recent flavivirus infection was diagnosed for any individual with an IgM result > 40 units. A ratio of dengue virus IgM to JE virus IgM < 0.91 defined a JE virus infection. Diagnostic criteria for Japanese encephalitis were met in 22 individuals (14%), and were unequivocal in 8 patients. The admission findings in these eight subjects were similar to those described for other flavivirus infections. Thrombocytopenia was the most striking laboratory abnormality (median platelet count = 119,000/mm3, range = 44,000-236,000/mm3). Headache (75%), nausea (50%), myalgia (38%), rash (38%), and diarrhea (25%) were the most frequently encountered signs and symptoms. Infection with Japanese encephalitis virus is an underappreciated cause of acute undifferentiated fever in Asia.