EHEC infection is a target disease of pathogen surveillance under the NESID. Prefectural and municipal public health institutes (PHIs) conduct isolation of EHEC, serotyping, and VT typing, and the Department of Bacteriology, National Institute of Infectious Diseases (NIID), conducts molecular epidemiological analysis of the isolates and provides the data through the PulseNet Japan (see p. 124 of this issue).
Cases notified under NESID: In 2008, EHEC infections, including both symptomatic patients and asymptomatic carriers were 4,330, exceeding 4,000 for the second consecutive year (Table 1 and IASR 29: 117-118, 2008). There is a regular seasonal variation with a large peak in summer seasons (see the weekly incidence in Fig. 1). In 2008, incidence per 100,000 population was highest in Saga Prefecture (19.97) followed by Iwate (11.91), Fukui (9.49) and Nagasaki (9.40). There was wide regional variation in the incidence of EHEC infection (Fig. 2, left). The prefectures with higher incidence in 2005-2007 tended to report higher incidence of EHEC infection in 2008 too. As in preceding years, incidence of EHEC infection was highest among the age group of 0-4 years followed by 5-9 year age group (Fig. 3). When the incidence among 0-4 year age group was compared for different prefectures, highest incidence was reported from Saga and Iwate Prefectures, which experienced EHEC outbreaks in nursery schools and kindergartens (Fig. 2, right). Among EHEC infected population younger than 14 years, there were more males than females, while among the population older than 15 years there were more females than males. Percentage of symptomatic patients among the infected was high for young (≤14 years) and advanced ages (≥70 years), 73% for the both, and relatively low, less than 43%, for those in their thirties and forties (Fig. 3). Eight deceased cases of HUS or acute renal failure were reported in 2008.
Isolation of EHEC: In 2008, PHIs reported to the Infectious Disease Surveillance Center (IDSC), NIID 2,471 isolations, which number was about half of the number of reported EHEC infection cases, 4,330 (Table 1). The discrepancy is due to the present situation where a substantial amount of the laboratory data obtained outside of PHIs does not reach NIID. O157 that increased to 75% in 2007 decreased to 65% in 2008. O26 increased from 13% in 2007 to 24% in 2008. O111 that was 6% in 2007 slightly decreased to 4% in 2008 (http://idsc.nih.go.jp/iasr/virus/bacteria-e.html). Varieties of other serotypes have been obtained in addition.
As there are strains that cannot be identified serotype by the commercially available anti-sera (IASR 25: 141-143, 2004), confirmation of EHEC by detection of VT is indispensable for its identification. In 2008, VT1&2 occupied 61% of O157 (53-68% in 1997-2007), VT1 alone 96% of O26, and VT1 alone 36% of O111 (less than in preceding years).
Among 2,471 cases in 2008 from which EHEC was isolated, 1,611 cases were O157 infections. Of 1,541 data-available O157 cases, the main symptoms were diarrhea (57%), abdominal pain (53%), bloody diarrhea (39%) and fever (21%) (see p. 121 of this issue). There were 26 HUS cases (8 cases of VT2 and 18 cases of VT1&2). One HUS case (VT2) was found among 34 O145 cases. The incidence of HUS cases was 1.9% of all the symptomatic cases and was less than that in NESID, 3.3% (see p. 122 of this issue). It is because, as indicated above, in NESID, the reported HUS cases include those positive only for VT in stool specimens or for anti-O-antigen agglutinating antibody or anti-VT antibody in serum (without bacterial isolation). The percentage of asymptomatic carriers was 52% in O26 cases (see p. 121 of this issue).
Outbreaks: In 2008, PHIs reported 37 EHEC outbreaks to IDSC, 21 incidents of which were caused by O157. There were 20 outbreaks each involving 10 or more EHEC-positive cases (Table 2). Among them, 4 incidents were suspected to be foodborne, and 12 incidents person-to-person infections. In 2008, 17 EHEC incidents involving 115 patients were reported by prefectures in compliance with the Food Sanitation Law (25 incidents and 928 patients in 2007). The number of EHEC patients reported in compliance with the Food Sanitation Law is far lower than that reported according to the Infectious Diseases Control Law. This is because many cases lacked direct evidence of implication of food(s) as the causative material, and also because in reality EHEC infections involving only one person are rarely reported as food poisoning.
In 2008, there were 13 outbreaks in nursery schools and kindergartens, which was almost as frequent as in previous years. As small numbers of bacteria are sufficient for establishing infections, EHEC can easily spread from person to person. For preventing outbreaks in nursery schools or kindergartens, proper sanitation measures, such as routine hand washing by children and staff and keeping padding pools for children in sanitary condition during summer, are necessary. Spread of infection within a family is not infrequent. Once a patient has appeared in a family, the health center should provide the family with thorough instructions necessary for preventing the secondary infections.
Foods contaminated with small number of EHEC can cause foodborne infection. Therefore, it is important to keep basic practice for preventing food poisoning. It is also important to avoid feeding those with weak immunity including younger children and the elderly with raw or undercooked meat (http://www.mhlw.go.jp/topics/syokuchu/03.html).
Update 2009: During weeks 1-15 of this year, 247 EHEC cases were reported (Table 1). EHEC O121 was isolated from 31 cases in an outbreak at a nursery school in Oita Prefecture during weeks 5-6. As EHEC infection increases in every summer, increased vigilance on this infection is requested from now.