The Topic of This Month Vol.19 No.6(No.220)
Enterohemorrhagic Escherichia coli: EHEC (verocytotoxin-producing E. coli: VTEC, or Shiga toxin-producing E. coli: STEC) infection has been drawing attention since the 1990 incident (see IASR, Vol. 13, No. 10), in which two kindergartners among 319 patients in an outbreak of diarrheal disease died from hemolytic uremic syndrome (HUS). Reports of EHEC isolation from prefectural and municipal public health institutes (PHIs) to IASR have been collected since 1991. Until 1995, such reports of isolation counted about 100 a year, but after May 1996, those of mainly serotype O157 increased abruptly.
According to the Food Sanitation Division, the Ministry of Health and Welfare, total reported patients presumably infected with EHEC O157 numbered 17,877 in 1996, 1,576 in 1997, and 100 in 1998 as of May 8. Food poisoning outbreaks due to serotype O157:H7 involving 10 or more symptomatic patients after 1996 are shown in Table 1 (see IASR, Vol. 18, No. 7). Outbreaks occurring frequently in primary and nursery schools, and nursery homes for the aged were caused presumably by the meals supplied. EHEC was isolated from specimens of incriminated salad in Gifu City, salad and seafood sauce in Morioka City, and salad in Obihiro City. The contaminated foods causing infection were thus identified. In the outbreak occurring in Sakai City, white radish sprouts were suspected of being the source of infection by epidemiological investigations, although no EHEC was isolated. Among the outbreaks occurring in 1997, EHEC was isolated from melon in Kashiwa City (see IASR, Vol. 18, No. 12), sliced raw tuna in Isezaki City (see IASR, Vol. 19, No. 3), and buckwheat noodles with topping in Okayama City. The VT type of serotype O157:H7 strains isolated in 19 incidents was VT1 & VT2 in 17 and that of the other two was VT2 only.
The genotype analysis by pulsed-field gel electrophoresis (PFGE) after cleavage with restriction enzyme XbaI revealed that such organisms having the PFGE pattern identical to that of the isolates in 1996 were also involved in the 1997 incidents (Table 1). In an incident occurring in a hospital in Okayama City late in June 1997, the isolates from patients and the meals supplied had the PFGE Ia type, being identical to that of the isolates in an incident occurring in Tojo-cho, Hiroshima Prefecture, in 1996. EHEC strains isolated from sporadic cases in a diffuse outbreak occurring in a wide area from south Kanto to Tokai districts in March 1997 showed PFGE IIa type as well as phage type 32,being identical to those of the isolates from the outbreak in Sakai City in 1996 (see p. 124 of this issue).
The reports of EHEC isolation to IASR numbered about 100 a year during the period from 1991 to 1995 (see IASR, Vol. 17, No. 1), but increased to 3,022 in 1996 and 1,959 in 1997 (Fig. 1). The abrupt increase in 1996 reflects such a large number of outbreaks occurring in primary schools, but in 1997, no outbreak occurred in primary schools, and the total outbreaks also decreased in number (Table 1); nevertheless there were a large number of reports of isolation. This was due to the increased reports of isolation from sporadic cases occurring at homes and from familial infections. Incidents of familial infection reported to IASR numbered only several per year during the period from 1991 to 1995, but increased to 106 in 1996 and 199 in 1997.
The serotypes and VT types of EHEC isolates are shown in Table 2. The most predominant serotype was O157:H7, which accounted for 76% (2,309/3,022) in 1996, 67% (1,319/1,959) in 1997, and 79% (67/85) in 1998 as of last April. The next most predominant serotype was O26:H11, which accounted for 1.5% during 1991-1995, 3.4% in 1996, increased to 13% in 1997, but none has been reported in 1998 as of last April. Other frequently isolated serotypes than O157 has been O26:H-, O26: HNT, O111: H-, O111: HNT, etc. O118: H2 increased in 1996 (see IASR, Vol. 17, No. 10), but it was not isolated in 1997. Production of both VT1 and VT2 was demonstrated with 87% of EHEC O157: H7 strains in 1996, but it decreased to 67% in 1997 and to 61% as of April 1998. The ratio of strains producing VT2 only increased from 13% in 1996 to 31% in 1997 and further to 39% in 1998. Strains of other serotypes showed a tendency of producing VT1 only.
The age distribution of cases yielding EHEC showed that those under 15 years accounted for 86% before 1995 (see IASR, Vol. 17, No. 1), which decreased to 76% in 1996 (see IASR, Vol. 18, No. 7) and further to 54% in 1997 (Table 3). On the other hand, asymptomatic cases under 15 years accounted for 23% and those over 16 years 33% in 1996 (see IASR, Vol. 18, No. 7), but the corresponding age groups accounted for 23% and 53%, respectively, in 1997 (Table 3). Therefore, it has been the case that EHEC infection is not restricted to the younger generation, but the rate of symptomatic cases in that generation tends to be high.
EHEC O157 strains having the same VT types as those from the patients were isolated from houseflies (Musca domestica vicina) caught inside and outside the facilities in which infection broke out in October 1996 in Saga Prefecture. From this finding, a nationwide survey of houseflies for carrying EHEC O157 was undertaken by the National Institute of Infectious Diseases in collaboration with PHIs and other institutions in 1997. In the eight prefectures situated from north to south of 15 prefectures in which the survey was undertaken, O157-carrying houseflies were detected. The sites of specimen collection in the 15 prefectures numbered 217, of which 15 (cowsheds and slaughterhouses) gave EHEC O157-carrying houseflies. The rate of O157-carrying houseflies to the total ones collected (the O157-carrying rate) at these 15 sites was 7.2% (see p. 125 of this issue).
In this year, there have been relatively few reports on isolation of EHEC, but the reports slightly increased in May. As described above, transmission of EHEC O157 by flies has been demonstrated and further precaution seems warranted for the forthcoming summer season.