The Topic of This Month Vol.22 No.6(No.256)
Infections with enterohemorrhagic Escherichia coli (EHEC), Verocytotoxin-producing E. coli (VTEC) or Shiga toxin-producing E. coli (STEC), have been placed in the category III infectious diseases under the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections (the Infectious Diseases Control Law) enacted in April 1999. All physicians having diagnosed such infections are obliged to notify health centers of symptomatic and asymptomatic new cases.
Table 1 shows symptomatic and asymptomatic new cases of EHEC infection (hereafter referred to as cases of EHEC infection) based on the Statistics on Communicable Diseases in Japan (former Ministry of Health and Welfare) and the National Epidemiological Surveillance of Infectious Diseases (NESID). These cases numbered at 3,622 in 2000, outnumbering those in 1999, 2,957. The weekly reports were on the increase in summer as was the case in the preceding years (Fig. 1). The prefectural incidence per 100,000 of the population was slightly high in some areas of Tohoku, Hokuriku, Kinki, Chugoku, and Kyushu Districts (Fig. 2). The incidence in Akita, Iwate, Tottori, Saga, and Miyazaki Prefectures was repeatedly high in both 1999 and 2000. The age distribution of cases of EHEC infection in 2000 is shown in Fig. 3. The incidence was the highest in the age group of 0-4 years, followed by that of 5-9 years. The ratio of symptomatic patients was high among younger groups (74% of those aged less than 19 years), and half of adult cases were asymptomatic carriers (53% of those aged more than 20 years). The ratios were similar to those in the previous year (see IASR, Vol. 21, No. 5). The age groups of less than 19 years stood at 1,048 males and 942 females. Those of more than 20 years stood at 570 males and 1,054 females.
The reports of EHEC isolation by prefectural and municipal public health institutes (PHIs) (http://idsc.nih.go.jp/prompt/graph-l.html) numbered at about 100 constantly during 1991-1995 (see IASR, Vol. 17, No. 1). They increased abruptly to 3,022 in 1996 and, thereafter, kept similar levels, 1,968 in 1997, 2,054 in 1998, 1,933 in 1999, and 1,656 in 2000. Although reports of EHEC infection cases, according to NESID, increased in 1999 and 2000 (Table 1), those of microbial isolation decreased. This might indicate that not all information of microbial isolation reached PHIs.
Although a large number of foodborne outbreaks occurred mainly in elementary schools in 1996 (see IASR, Vol. 19, No. 6), large outbreaks have no longer occurred there after 1997. However, outbreaks could still be seen in facilities; four outbreaks at nursery schools and two outbreaks at hospitals were reported in 2000 (Table 2). Such might be explained by the fact that EHEC infection is brought about with a very small amount of the organisms and it easily accompanies secondary infection through person to person transmission (see IASR, Vol. 17, No.8). It seems necessary to intensify the control against foodborne and nosocomial incidents to prevent EHEC infection outbreaks. Among the outbreaks occurring in 2000, food involved was identified in only the Chiba outbreak (No. 9) (barbecued beef offered at an outdoor event; see p. 139 of this issue). In the outbreak at a hospital in Toyama Prefecture (No. 1), EHEC was isolated from the served lettuce, but it was not identified as the source of infection (see IASR, Vol. 21, No. 9).
Cases of EHEC infection developing hemolytic uremic syndrome (HUS) might be critical. EHEC O157 infection cases in children sometimes accompany HUS (see IASR, Vol. 17, No. 1). In 2000, HUS cases, from which the agent was isolated, were mostly children (four cases were at the age of 0-1 year, 18 at 2-5 years, eight at 6-15 years, and one over 40 years). EHEC O157 was isolated from 30 of the 31 patients.
The serotypes and the toxin types of EHEC isolated in 1999-2000 are tabulated in Table 3. The ratio of EHEC O157:H7 was high during 1991 through 1995, being 83% (436/525), which was on the gradual decrease to 55% in 1999. However, the ratio in 2000, being 56%, stood at the same level as that in the preceding year. The ratio of such non-O157 serotypes as O26 and O111 was low during 1991-1995, being 9.3%. After 1996, however, the ratio kept on increasing up to 30% in 2000, particularly the ratio of EHEC O26:H11 increased. A similar trend was seen in the serotypes of the agents of outbreaks in 2000 (Table 2). A fatal case of HUS caused by EHEC O86 infection was reported in 1999 (see IASR, Vol. 20, No. 11) and an HUS case caused by O121 infection was also reported in 2000 (see p. 141 of this issue). It seems necessary to continuously seize the precise trend of O157 and also non-O157 EHEC.
In 1996, 87% of EHEC O157:H7 isolates produced both VT1 and VT2, but the ratio decreased to 54% in 2000. On the other hand, EHEC O157:H7 isolates producing only VT2 accounted for 13% in 1996, while such isolates increased to 42% in 2000. More than 90% of isolates of O26 and O111 produced only VT1.
Flash report for 2001: The notified cases of EHEC infection diagnosed before May 27 of this year totaled at 668 as of June 5 (Table 1), largely outnumbering those in the corresponding period of 2000 (Fig. 1). From the end of February to March, six cases of EHEC O157:H7 infection occurred due to tenderized cubic beef products in Shiga, Toyama, and Nara Prefectures (see p. 137&138 of this issue). From the middle of March to April, 193 persons in Chiba, Saitama, Tokyo, Kanagawa, Gunma, Ibaraki, and Yamagata Prefectures were infected with O157 from consuming sliced rare roast beef (53 of them were asymptomatic) and 13 of them developed HUS (see p. 137 of this issue). In all incidents, the imported frozen beef, incriminated as the source of infection, was distributed widely to many districts causing so-called "diffuse outbreaks".
When EHEC infection due to food and food materials distributed in wide areas occurs, rapid detection and countermeasures taken by cooperation among organizations and efforts to prevent spreading infections are must. For this purpose, construction of the PulseNet Japan is under way (see p. 142 of this issue) to analyze and compare digitized image data of DNA patterns obtained by pulsed-field gel electrophoresis (PFGE) of EHEC strains isolated at PHIs. It is required to arouse more attention to EHEC infection, which might be on the increase toward summer.