The Topic of This Month Vol.21 No.5(No.243)
Enterohemorrhagic Escherichia coli (EHEC) [also called Verocytotoxin-producing E. coli (VTEC) or Shiga toxin-producing E. coli (STEC)] infection, having provoked over 7,000 cases in July 1996 in Japan (see IASR, Vol.18, No. 7), was listed as one of specially designated communicable diseases and notification of both symptomatic and asymptomatic new cases has become compulsory since August of the same year. The disease was classified into the category III infectious diseases under the Law Concerning the Prevention of Infectious Diseases and Medical Care of Patients of Infection (the new Infectious Diseases Control Law) enacted in April of 1999 (see IASR Vol. 20, No.4).
Table 1 shows the incidence of EHEC infection reported before and after enactment of the new Infectious Diseases Control Law. The notified cases from April to December 1999 totaled at 2,882 including 1,086 asymptomatic cases. Prefectural incidences tended to be slightly high in some parts of Tohoku, Kinki, Chugoku and Kyushu districts, showing some geographical differences from one district to another (Fig. 1). The age distribution of cases by sex during the same period is shown in Fig. 2. The ratio of symptomatic patients was high in the younger generation (75% of those under 19 years old) and asymptomatic carriers in about half of the adult patients (55% of those at ages over 20 years).
The reports of EHEC isolation sent from prefectural and municipal public health institutes (PHIs) to the Infectious Disease Surveillance Center numbered at about 100 per year during the period of 1991 through 1995 (see IASR, Vol. 17. No. 1), but increased abruptly to 3,021 in 1996, and thereafter kept on numbering at 2,020 in 1997, 2,053 in 1998 and 1,840 in 1999 (Fig. 3). Outbreaks mainly in primary schools frequently occurred in 1996. Nevertheless, since 1997, outbreaks have no longer occurred in primary schools and have been reduced in scale nationwide. This change may have been due to the intensified sanitary control of school lunches. Even so, outbreaks are still occurring in such facilities as nursery schools, where sanitary guidance or control might not spread satisfactorily (Table 2). In those outbreaks, the etiological agent was isolated from; salad at a nursery school in Kagoshima, salmon roe at a restaurant in Toyama, salad at a home for the aged in Yamaguchi, and tap water of a small water-supply system in Nagano. Examples of "diffuse outbreak" reported were; an outbreak of EHEC O157 infection in southern Kanto and Tokai districts in March 1997 (see IASR, Vol. 19, No. 6) and another one in seven prefectures including Toyama, Tokyo and the other five located mainly in the Metropolitan area in May 1998 (Table 2). In these outbreaks, pulsed-field gel electrophoresis performed after cleavage of DNA of isolates with a restriction enzyme Xba I demonstrated the identical genotype of the isolates from each episode, which worked a great deal to help find "diffuse outbreaks".
The serotypes and toxin types of EHEC isolates are shown in Table 3 (see IASR, Vol. 19, No. 6 concerning 1991-1996). The ratio of O157:H7, the serotype most often isolated, was 83% (436/525) during 1991-1995, 76% (2,307/3,021) in 1996, 67% (1,347/2,020) in 1997, 64% (1,320/2,053) in 1998, and 55% (1,003/1,840) in 1999, showing a gradual decrease. Yearly ratio of isolation of non-O157serotypes was 9.3% during 1991-1995, 11% in 1996, 25% in 1997 and 1998Aand 28% in 1999, showing a gradual increase. Of non-O157 EHEC, frequently isolated serotypes were O26:H11, O26:H-, and O111:H-. Such a tendency may reflect the serotypes of the etiological agents of the 1998 and 1999 outbreaks (Table 2). It seems necessary to seize the precise trend of non-O157 EHEC (refer to the Infectious Disease Surveillance Center homepage, http://idsc.nih.go.jp/prompt/vtec.html, concerning details of EHEC isolation and its quick reports). A fatal case from hemolytic uremic syndrome due to O86 infection was reported in 1999 (see IASR, Vol. 20, No. 11). The strain isolated from the patient was VT2-positive and eaeA-negative, possessing pCVD432 plasmid, a gene marker of enteroaggregative E. coli (EAEC). In 1996, 87% of EHEC O157:H7 isolates produced both VT1 and VT2, but the ratio decreased to 57% in 1999. On the other hand, EHEC O157:H7 isolates producing only VT2 accounted for 13% in 1996, while increased to 41% in 1999. More than 80% of isolates of other serotypes produced only VT1.
As of April 21, 2000, reports of patients diagnosed before March 31 were counted at 202 (Table 1). Precautions are necessary toward the coming summer season when an increase in EHEC infections is anticipated.