The Topic of This Month Vol.18 No.7(No.209)
Verocytotoxin-producing Escherichia coli (VTEC) is also called Shiga toxin-producing E. coli (STEC) since verocytotoxin it produces is very similar to Shiga toxin produced by Shigella dysenteriae 1, or enterohemorrhagic E. coli (EHEC) since it is a cause of hemorrhagic colitis. VTEC infection has now become a worldwide problem. In this country, outbreaks and sporadic cases have frequently occurred since May 1996. According to the report by the Food Sanitation Division, the Ministry of Health and Welfare, total reported patients of VTEC infection numbered 9,451, hospitalized ones 1,808, and deaths 12 in 1996.
Sixteen outbreaks involving 10 or more patients occurred in 1996 and are shown in Table 1 (see IASR, Vol. 17, No. 8). They broke out in such facilities as elementary and nursery schools and nursing homes, and meals supplied at these facilities were often incriminated as the source of infection. VTEC was isolated from specimens of salad in Gifu City, salad and seafood sauce in Morioka City, and salad in Obihiro City, and these were determined as the contaminated foodstuffs. In the extraordinarily large-scale outbreak in Sakai City, epidemiological investigations incriminated white radish sprouts as the contaminated food. The organism was not necessarily isolated from vegetables nor other food materials in these outbreaks. The isolates from these outbreaks were all E. coli serotype O157:H7, and their VT types were VT1+VT2 in 14 outbreaks and VT2 in the other two outbreaks.
The genotypes of the O157:H7 isolates were analyzed by pulsed-field gel electrophoresis (PFGE) after cleavage with XbaI, a restriction enzyme; the genotypes related to outbreaks in 1996 were grouped largely into six, from type I to VI (the variations in each type are shown in Roman alphabet, as a, b....). About 1,700 strains of O157:H7 isolated from outbreaks of infections including familial ones and sporadic cases were analyzed by PFGE, which gave more than 200 patterns, indicating that contamination is being due to a variety of genotypes of VTEC O157:H7 (J. Clin. Microbiol., Vol. 35, p. 1675, 1997 and p. 3 of this issue).
The trend of VTEC isolation reported to IDSC from prefectural and municipal public health institutes is shown in Fig. 1. The reports of isolation during the period from 1991 through 1995 counted about 100 a year (see IASR Vol. 17, No. 1), but as many as 3,021 in 1996 and 219 in 1997 (as of June 5). Of the VTEC strains isolated in 1996, serotype O157 numbered 2,687 and non-O157 334; of those in 1997, they numbered 201 and 18, respectively. The peak seen in July in 1996 reflects the outbreaks having occurred in Sakai City in Osaka Prefecture and in the vicinities (Wakayama Prefecture, Kyoto City in Kyoto Prefecture, and Habikino City in Osaka Prefecture) and other sporadic ones at the same period in the same districts. Most of these outbreaks were due to the organisms of PFGE type IIa (Table 1).
The serotypes and VT types of VTEC isolated are shown in Table 2. The serotype most often isolated in 1996 and 1997 was O157:H7, accounting for 76% (2,307/3,021) and 79% (173/219), respectively, showing the same tendency as that during 1991 through 1995 (see IASR, Vol. 17, No. 1). Non-O157 isolates included serotypes of O26:H11, O26:H-, O26:HNT, O111:H-, and O118:H2. The increased isolation of O118:H2 is characteristic of 1996 (see IASR, Vol. 17, No. 10). In 1997, however, no O118:H2 has so far been isolated. The proportion of O157:H7 strains producing VT1+VT2 is very high (accounting for about 90%); those of other serotypes tended to produce VT1 alone. All isolates of O118:H2 (134 strains) were VT1 alone producers.
The age distribution of 3,021 VTEC-yielding cases in 1996 shows that cases under 15 years old accounted for 76% (1,920/2,522, excluding age-unknown cases)(see Table 3A), being lower than the corresponding figure (86%) before 1995 (see IASR, Vol. 17, No. 1). Such a tendency is also being the case in 1997 (as of June 5) (47%) (Table 3B). Thus, it seems that VTEC infection is no longer restricted to the younger generation.
The most infections occurred at home in 1997. In mid March, many sporadic cases arose covering a wide area from the Kanto district to the Tokai district. In this "diffuse outbreak", O157 of the same PFGE genotype as that from patients was isolated from remnant white radish sprouts in Aichi Prefecture and Yokohama City in Kanagawa Prefecture, and they have been suspected of being the source of infection.
In late June, an outbreak occurred in a hospital in Okayama City, in which hospital meals were incriminated as the source of infection. Further precautions to prevent VTEC infection must be taken during the forthcoming summer season.
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