Meanwhile, EHEC infection is a target disease of pathogen surveillance under the NESID and prefectural and municipal public health institutes (PHIs) undertake isolation, serotyping and VT typing of EHEC. In addition, the Department of Bacteriology I, the National Institute of Infectious Diseases (NIID) conducts molecular epidemiological analysis of the isolates and provides information by Pulse-Net Japan (see p. 119 of this issue).
Cases notified under the NESID: In 2007, 4,606 new symptomatic and asymptomatic cases of EHEC infection (hereafter referred to as cases of EHEC infection) were reported (Table 1), largely exceeding 3,922 cases in 2006. Seasonal variance was large in the weekly reports and the peak of epidemic in 2007 was seen in summer as usual (Fig. 1). Incidence per 100,000 population by prefecture was the highest in Ishikawa Prefecture (11.26), followed by Miyazaki (11.24) and Miyagi Prefectures (10.66), where a large foodborne outbreak occurred. A considerable regional difference was seen as usual (Fig. 2). In the regions where many cases occurred during 2002-2006, many cases tended to occur also in 2007. A largest number of cases of EHEC infection in 2007 were seen in children of 0-4 years, followed by 5-9 years old, as was the case in preceding years. Of cases of 0-14 years old, there were more males and of those of 15 years or older, there were more females. The rate of symptomatic cases was high among younger and the aged generations (77% under 14 years old and 71% above 70 years old) and lower than 41% among thirties and forties (Fig. 3).
Isolation of EHEC: Reports of isolation of EHEC from PHIs counted at 2,531 (IASR as of April 16, 2008), which was smaller than cases of EHEC infection (Table 1). This is due to the fact that not all reports of isolation outside of PHI reached under the present system. The ratio of O157:H7, which had been on a gradual decrease in recent years, slightly increased to 62%, and that of O26 was 11% and that of O111 6% in 2007 (http://idsc.nih.go.jp/iasr/virus/graph/vtec0007.gif).
In addition, various other serotypes were detected. Serotypes of some strains producing VT were untypable with commercial anti-sera (IASR 25: 141-143, 2004). For identification of EHEC, therefore, it is important first to confirm VT. The types of VT produced by isolates (or the toxin gene possessed) were VT1 & 2 accounting for 68% among O157 in 2007 as usual (53-68% during 1997-2006), among O26, VT1 alone accounted for 97%, and among O111, VT1 alone accounted for 69%.
Of 2,531 cases in which EHEC was isolated in 2007, O157 was detected from 1,930 cases, and their main symptoms were diarrhea in 53%, abdominal pain in 51%, bloody diarrhea in 38%, fever in 20%, and HUS in 29 cases (VT2 alone in 16 cases, VT1 & 2 in 13 cases) (see p. 119 of this issue). In addition, HUS was reported in 3 of 21 cases of O165 (VT 2 alone) and in one of 41 case of O121 (VT2 alone). Besides, three HUS cases were reported from which EHEC was not isolated but serodiagnosed by O157-antibody.
Outbreaks: Among 45 outbreaks of EHEC infection reported by PHIs in 2007, 34 were due to O157. In 18 outbreaks involving 10 or more EHEC-positive cases (Table 2), the suspected route of infection in five were thought to be due to foodborne transmission and nine due to person-to-person transmission. In 2007, there were 25 incidents of EHEC food poisoning involving 928 cases reported from prefectural governments in compliance with the Food Sanitation Law. The number of cases largely increased due to two incidents by food served at a refectory in Tokyo (No. 2 in Table 2 and see p.120 of this issue) and by boxed meals served by a restaurant in Miyagi Prefecture (No. 16 in Table 2 and see p. 122 of this issue) to 928 (24 incidents in 2006 involving 179 cases). (Note: the number of cases was much smaller than that reported under the Infectious Diseases Control Law, due to the fact that incidents in which food was incriminated as the source of infection were few, and also that incidents involving only a single case are not necessarily reported as food poisoning).
In 2007, as many as 11 outbreaks still occurred in nursery schools and kindergartens as in previous years (Table 2 and see p. 123, 124, 125 & 126 of this issue). Since EHEC, as is the case with Shigella , causes infection with a minute quantity of the organisms, infection is liable to expand by person-to-person transmission, and it is necessary to take proper precautions, including daily hand washing by children and staff members and sanitary control of paddling pools for children in summer. When a case occurs, familial infection may often follows; therefore thorough instruction to family members is required to prevent secondary infections.
Since food contaminated with a minute quantity of the EHEC organisms may cause infection, it is important to keep basic precautions against food poisoning and especially to avoid feeding such persons with weak immunity including younger children and the elderly with raw or undercooked meat (http://www.mhlw.go.jp/topics/syokuchu/index.html).
Update 2008: During weeks 1-17 in this year, reports of cases of EHEC infection counted at 311 (Table 1). In Saga Prefecture, O26:H11 (VT1) was isolated from a total of 76 of the group that had visited Australia on school excursion and their family members during weeks 10-12 (http://idsc.nih.go.jp/iasr/prompt/graph-le.html). Increase in EHEC infection in summer is anticipated, so it seems necessary to call further attention from now on.