Influenza surveillance system of Japan and acute encephalitis and encephalopathy in the influenza season

Aim: To understand the situation of influenza epidemics and neurologic comp lications during the influenza season in Japan. Methods: The influenza surveillance system in Japan was reviewed and the da ta on influenza/influenza-like illness and neurologic complications, such a s acute encephalitis/encephalopathy, were evaluated. Results: From the data of the National Epidemiological Surveillance of Infe ctious Diseases (NESID), an increase in clinically diagnosed encephalitis/e ncephalopathy was clearly noted recently. Most cases reported were acute en cephalitis/encephalitis. The number of cases of Reye's syndrome reported wa s stable and low in number, since the time of the recommendation of not usi ng aspirin for pediatric febrile disease, particularly for influenza-like i llness, in Japan. These acute encephalitis/encephalopathy cases were not co rrelated clearly with clinically diagnosed influenza patients before the 19 97/98 influenza season from the surveillance data, although several case re ports have shown a relationship. Conclusions: It is clarified from the NESID that neurologic complications s uch as acute encephalitis/ encephalopathy, apart from Reye's syndrome, have occurred associated with influenza virus infection, especially among young children. Further investigation should be necessary to find the pathogenes is of this serious complication of influenza virus infection and whether th is is unique in Japan, due to unclarified factor/factors, or whether it has not been recognized in other countries.