Epidemiology of Pediatric Acute Encephalitis/Encephalopathy in Japan

We studied the etiology of pediatric acute encephalitis/encephalopathy (pAEE) using epidemiological data obtained from a nationwide survey in Japan. Two-step questionnaires were sent to the pediatric departments of hospitals throughout the country in 2007, querying the number of the cases during 2005-2006 as the first step, and asking for the details of clinical information as the second step. In all, 636 children with pAEE (age ≤ 15 years) were enrolled. For the known etiology of pAEE (63.5% of the total cases), 26 microbes and 2 clinical entities were listed, but the etiology of 36.5% remained unknown. Influenza virus (26.7%), exanthem subitum (12.3%), and rotavirus (4.1%) were the most common, and the incidence of pAEE peaked at the age of 1 year. This trend was common among all etiologies. Among the neurological symptoms observed at the onset of pAEE, seizures were observed more often in patients aged ≤ 3 years, although abnormal speech and behavior were also common in older children. Undesirable outcomes (death and neurological sequelae) occurred at high rates in patients with any known etiology other than mycoplasma. In conclusion, these findings provide comprehensive insight into pAEE in Japan.