Annual and monthly incidence: Notified cases of acute encephalitis in NESID were 166 in 2004, 188 in 2005, 167 in 2006, and 169 in 2007 (week 1 -35), totaling 690 (as of October 4, 2007). By the month of disease onset (Fig. 1), reports were few in the first half of 2004, soon after the start of notification of acute encephalitis, while in 2005-2007, large peaks were shown during January-March due to the increase of influenza encephalopathy in winter. In 2007, as compared with 2005-2006, a larger number of cases reported during April-August. On the other hand, during September-November 2004, acute encephalopathy related to Angel's wing mushroom, as mentioned below, increased (see p. 346 of this issue).
Gender and age: Of 690 cases reported during 2004-2007, 370 were males and 320 females. In each year, there were many cases of infants and young children (Fig. 2); 0-9 years accounted for half (344 cases), including 1 year (78 cases), 0 year (53 cases) and 2 years (48 cases). In 2004, the accumulation of cases was seen among ages of 50-80; more than half of cases aged over 50 years were associated with Angel's wing mushroom. Also in 2005, peaks were seen among ages of 50s and 60s, although fewer than in 2004.
Incidence by prefecture (Fig. 3): In 2004, many reports came from prefectures, in which Angel's wing mushroom-related cases occurred; 26 cases in Akita, 17 cases in Niigata, and 14 cases in Yamagata prefectures. In 2005, there were 22 cases in Osaka, 16 cases in Tokyo, 16 cases in Fukuoka, 13 cases in Chiba, 12 cases in Akita, and 12 cases in Yamagata. In 2006, there were 19 cases in Chiba, 19 cases in Osaka, and 12 cases in Hiroshima. In 2007, there were 22 cases in Osaka, 16 cases in Tokyo, 16 cases in Miyazaki, and 14 cases in Chiba. Many cases tended to occur in large cities. In Miyazaki Prefecture, influenza encephalopathy started to be reported in 2007 and the number of reports became conspicuous. There has been no report until now in Tokushima Prefecture.
Incidence by pathogen: As for the pathogenic agents estimated to be the cause of 381 cases aged 0-14 years (Fig. 4), excluding 155 cases in which pathogenic agent was unknown, reported were mostly viruses; influenza virus in 142 cases (37), HHV-6 in 19 cases, rotavirus in 11 cases, herpes simplex virus (HSV) in 8 cases, adenovirus in 6 cases and coxsackievirus in 6 cases. Bacteria were ascribed in 11 cases; Salmonella spp. and Mycoplasma spp. were each 4 and Enterococcus sp., Streptococcus bovis , and Streptococcus pneumoniae +Legionella sp. were each 1.
Of 309 cases aged over 15 years, pathogenic agent was unknown in 221 cases (72), and for viruses, herpes virus group such as HSV was reported in 40 cases and influenza virus in 14 cases. Bacterial agents were reported in 15 cases; S. pneumoniae in 10 cases, some of which were recorded as PRSP or PISP, Mycobacterium tuberculosis in 3 cases and Mycoplasma sp. in 1 case.
Measles encephalitis was reported in 1 case in 2004, in 1 case in 2006, and in 8 cases in 2007. These cases were affected by measles epidemics in 2007 mainly involving late 10s and 20s (see IASR 28: 239-240, 2007).
Fatal cases: Reports for fatal cases counted at 78 (29 in 2004, 27 in 2005, 14 in 2006 and 8 in 2007), corresponding to 11% of the total case reports during 2004-2007. Of these, 40 cases were aged 0-14 years, including 13 males and 27 females; 0-4 years 26 cases, 5-9 years 9 cases and 10-14 years 5 cases. The pathogenic agents were influenza virus in 16 cases (type A 8 cases, type B 6 cases, and type unknown 2 cases), rotavirus and respiratory syncytial virus in 2 cases each, adenovirus type 3, adenovirus type 42, group A coxsackievirus (CA) type 6, CA7, HSV, Salmonella Enteritidis, and S. bovis each 1 and unknown in 13 cases. Of 38 cases aged over 15 years, 20 were males and 18 females; 15-19 years 1 case, 20s 3 cases, 30s 2 cases, 40s 3 cases, 50s 6 cases, 60s 9 cases, 70s 10 cases, and 80s 4 cases. The pathogenic agents were influenza virus in 4 cases (type A 1 caseAtype B 1 case and type unknown 2 cases), measles virus, HSV, and mumps virus each 1, and unknown in 31 cases. Reports of death were received mostly at the time of notification, although they include those supplementarily reported after notification. It is estimated that there were actually more deaths. We do not know complications or their degrees from the information notified.
Influenza encephalopathy: It is estimated that annually 100-300 cases of acute encephalopathy accompanying influenza occur according to the study group of MHLW on influenza encephalopathy headed by Dr. T. Morishima. According to NESID, 53 cases (type A 19 cases, type B 29 cases, types A+B 3 cases, and type unknown 2 cases) in 2004/05 season, 53 cases (type A 48 cases, type B 4 cases, and type A+rotavirus 1 case) in 2005/06 season, and, 42 cases (type A 30 cases, type B 7 cases and type unknown 5 cases) in 2006/07season were reported, reflecting the prevalent type of each season.
Conclusion: We should all the more make it known to the public that acute encephalitis/encephalopathy is a severe disease, sometimes accompanying death or sequelae and its notification is duty of a doctor. Prompt notification based on clinical diagnosis in the course of searching pathogen is important for rapid understanding of accumulation of cases, and it is useful to monitor unknown severe diseases. On the other hand, identification of pathogenic agents is very important for planning early diagnosis and treatment, preventive measures, and immunization. For this purpose, it is demanded that investigation should be performed as much as possible and additional reports of the pathogenic agent should be made even after notification. Against outbreaks or local epidemics, if considered to be important in public health, more active pathogen surveillances in cooperation with medical institutions and local administrative organizations are desired. The pathogen detection of this disease is performed basically by the clinical examination at medical institutions. Laboratory tests are performed at a number of public health institutes (see p. 342 of this issue) and the National Institute of Infectious Diseases actively cooperates in pathogen detection (see p. 341, 344, 345 & 350 of this issue).