Trend of notified cases: Table 1 shows reported symptomatic and asymptomatic new cases of EHEC infection (hereafter referred to as cases of EHEC infection) in the Statistics on Communicable Diseases in Japan (the former Ministry of Health and Welfare) and NESID. The notified cases numbered at 4,319 in 2001, largely outnumbering 3,649 in 2000. Weekly reports were in the increase in the spring due to a diffuse outbreak (Table 3), largely increased in the summer with a peak in the 31st week, and suddenly decreased after the 41st week (Fig. 1). The incidence in 2001 differed from one prefecture to another, giving 1.04-12.34 cases per 100,000 population (Fig. 2). The three prefectures; Shimane, Saga, and Toyama counted the largest number of cases for the second year in a row, while Fukui, Chiba, and Nagasaki prefectures showed a large increase compared with those in 2000. Fig. 3 shows the age distribution of cases of EHEC infection. Cases aged 0-4 years counted at the largest number, followed by those aged 5-9 years. The age group of 0-14 years involved a larger number of males, while that over 20 years of females. The ratio of symptomatic patients was high in young and aged groups both for males and females (78% of those under 19 years and 70% of those over 65 years), and was less than 50% of those aged 30s and 40s. Of the cases aged 35-39 years, 69% were asymptomatic.
In 2001, four cases occurring in the 31st week corresponding to the peak in incidence and another case occurring in the 33rd week were already dead when reported, and four of them were of advanced ages (73, 81, 89, and 97 years old). The other one was at the age of 5 years and died of hemolytic uremic syndrome (HUS) associated with O157 infection. Other symptomatic cases counted at 2,862, of which 47 showed symptoms of HUS and renal failure, including 24 cases at the age of 0-4 years, 12 cases at 5-9 years, and 11 cases over 10 years.
Reports of EHEC isolation: Reports of EHEC isolation by prefectural and municipal public health institutes (PHIs) counted at about 100 annually during 1991-1995 (see IASR Vol. 17, No. 1). The reports increased abruptly to 3,022 in 1996, and have been kept at about 2,000 per year since then (see http://idsc.nih.go.jp/iasr/prompt/graph-l.html). The reports of EHEC isolation show some differences in number from the reported cases of EHEC infection (Table 1). This is due to the fact that some information on the isolates obtained at other places than PHIs may not reach PHIs by the current system. During 1991-1995, the serotype of 83% of the isolates (436/525) was O157:H7. Thereafter, O26 and O111 have increased. In 2001, O157:H7 accounted for 65% of the isolates (Table 2 and see http://idsc.nih.go.jp/iasr/23/268/graph/t2682.gif). In 1996, 87% of EHEC O157:H7 isolates produced both VT1 and VT2 (VT1&2), subsequently the ratio of VT2 was on the increase and accounted for 43% in 2000, but in 2001 VT1&2 markedly increased. Among O26 and O111 isolates, VT1 predominated. In 2001, HUS associated with EHEC isolation was diagnosed in 29 cases (three at the age of 0-1 year, 14 at 2-5 years, six at 6-15 years, one at 16-39 years, and five over 40 years). O157 was isolated from 26 of these cases (13 were VT2 and the other 13 VT1&2), O111 (VT1&2) from two, and O165 (VT2) from one case.
Outbreaks: Although a large number of outbreaks occurred in 1996 (see IASR Vol. 19, No. 6), no additional large-scale outbreak has since occurred in any primary school. Nevertheless, outbreaks still occur in some other facilities after 1997. In 2001, outbreaks occurred in seven nursery schools and other welfare/nursing facilities, hospitals, and dormitories (Table 3). In many of these incidents, secondary infection was reported. To prevent prolongation and further the spread of the disease due to secondary infection, it is vital to detect all asymptomatic carriers as well as symptomatic cases in an early stage (see IASR Vol. 22, No. 11). EHEC-positive individuals should be given accurate guidance to prevent secondary infection. Repeated tests to confirm the elimination and absence of the organisms should also be performed (see IASR Vol. 23, No. 1).
The incidents due to such foodstuffs as sliced rare roast beef (#2 in Table 3 and see IASR Vol. 22, No. 6), tenderized cubic beef products (see IASR Vol. 22, No. 6), and salted vegetables with hot peppers (#15 in Table 3 and see IASR Vol. 22, No. 11 and p. 139 of this issue) were diffuse outbreaks occurring in wide areas due to wide distribution of the foodstuffs. In each incident, the genotypes of the isolates from the patients and the incriminated foodstuffs detected by pulsed-field gel electrophoresis (PFGE) were identical to each other (see p. 139 of this issue). When an outbreak of EHEC infection in such a large area like the current one occurs, it is required to minimize the damages by rapid exchange of information through the Pulse-Net Japan (see IASR Vol. 22, No. 6) and cooperation of hospitals, health centers, PHIs and the National Institute of Infectious Diseases.
An outbreak of O121:H19 infection occurring at a nursery school in Saga Prefecture was reported in April of this year (see p. 143 of this issue). More public attention to the increase in EHEC infection anticipated for the forthcoming summer should be recommended.