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

AIM: To understand the situation of influenza epidemics and neurologic complications during the influenza season in Japan. METHODS: The influenza surveillance system in Japan was reviewed and the data on influenza/influenza-like illness and neurologic complications, such as acute encephalitis/encephalopathy, were evaluated. RESULTS: From the data of the National Epidemiological Surveillance of Infectious Diseases (NESID), an increase in clinically diagnosed encephalitis/encephalopathy was clearly noted recently. Most cases reported were acute encephalitis/encephalitis. The number of cases of Reye's syndrome reported was stable and low in number, since the time of the recommendation of not using aspirin for pediatric febrile disease, particularly for influenza-like illness, in Japan. These acute encephalitis/encephalopathy cases were not correlated clearly with clinically diagnosed influenza patients before the 1997/98 influenza season from the surveillance data, although several case reports have shown a relationship. CONCLUSIONS: It is clarified from the NESID that neurologic complications such 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 pathogenesis of this serious complication of influenza virus infection and whether this is unique in Japan, due to unclarified factor/factors, or whether it has not been recognized in other countries.