The Topic of This Month Vol.17 No.8 (No.198)
The bacilli of Escherichia coli producing Verotoxin (VT) are collectively called Verotoxin-producing Escherichia coli (VTEC), of which those causing hemorrhagic colitis (HC) are called enterohemorrhagic E. coli (EHEC). As the agent causative of HC often followed by hemolytic uremic syndrome (HUS), VT produced by EHEC is most important. The serotypes involved in EHEC are not only O157:H7 but also some others such as O26:H11, O91:H21, O111:H-, O113:H21, and O145:H-.
On May 28 ,1996, an outbreak of food poisoning due to EHEC O157:H7 arose in primary schools in Oku-cho, Okayama Prefecture, involving 468 patients, two of which accompanied HUS and died. After this one, similar outbreaks due to the same agent successively occurred in Hiroshima, Gifu, and Aichi Prefectures. These outbreaks made almost everyday headlines in mass media, becoming the focus of public attention. On July 13, when these outbreaks were somehow coming to an end, an extraordinarily huge-scale outbreak of food poisoning due to O157:H7 occurred in primary schools in Sakai City, Osaka Prefecture. As of July 22, the situation is so serious as patients of schoolchildren have numbered 6,176 and seriously ill cases 77, of which 17 are being in grave and other two in critical conditions.
EHEC O157:H7 is a diarrheagenic E. coli first found in obviously bloody diarrheal stool specimens in outbreaks of food poisoning arising in USA and Canada during 1982-1983. Since this period, more than 20 additional outbreaks have been confirmed in USA and Canada. In most of these outbreaks, hamburgers or ground beef was incriminated. In Japan, in October 1990, occurred an outbreak of diarrheal disease due to well water contaminated with EHEC O157:H7 in a kindergarten in Urawa City, Saitama Prefecture. Two children developed HUS and died. Since then, many incidents of EHEC O157:H7 infection including outbreaks and sporadic cases have been reported in IASR (see IASR Vol. 14, No. 10, 1993; Vol. 17, No. 1, 1996). We had better admit that EHEC O157:H7 infection is no longer a rare disease also in this country.
Outbreaks of EHEC O157:H7 infection after the one in Oku-cho, Okayama Prefecture, have been reported to the Ministry of Health and Welfare by 39 prefectural governments (Figure 1). Symptomatic patients have totaled 8,314, of which 565 are still being hospitalized and five died (as of July 22, 1996). The details of these outbreaks are as follows:
1) Those reported as food poisoning: The reports came from 16 prefectures. Symptomatic patients totaled 8,004, of which 496 are still being hospitalized and four died. Of these outbreaks, important 13 are summarized in Table 1. Food poisoning due to EHEC O157:H7, occurring first in Oku-cho, Okayama Prefecture on May 28, broke out afterward in other districts. The outbreak occurring in Sakai City on July 13 is in such extraordinarily huge scale, incompatible to any other previous outbreak in the world.
2) Those reported as suspects of food poisoning: Reports came from eight prefectures. Symptomatic patients totaled 69, of which one is still being in hospital.
3) Other incidents (sporadic cases): Reports came from 31 prefectures. Symptomatic patients totaled 241, of which 68 are still being in hospital and one died.
In the outbreak occurring in a primary school in Gifu City on June 10 (B in Table 1), EHEC O157:H7 was isolated from vegetable salad with a topping of dried bonito shavings served for school lunch. In the follow-up investigation of sporadic cases in Miura City, Kanagawa Prefecture occurring on June 18, EHEC O157:H7 was isolated from incriminated raw beef lever and the like. Unfortunately, there is no other incidents than these two in which causative foodstuff was identified.
Since the incubation period of EHEC O157:H7 infection, being four to nine days, seems to be considerably longer than that of other bacterial food poisoning, identification of the causative food is often difficult. From the epidemiological studies conducted so far in European and American countries, livestock, especially bovine feces, have been ascribed to the source of contamination. Meat is contaminated during the processes before reaching the meat processing plants. Other foodstuffs and potable water have been incriminated in some outbreaks, but secondary contamination from bovine feces or beef is being suspected.
Besides, patients suspected of secondary infection from person to person have been found also in Japan. EHEC O157:H7 infection with a feature considerably different from any other food poisoning may be transmitted with a small amount of the organisms as is the case in shigellosis. Therefore, preventive measures against infection with this organism should be taken by paying careful attention to this point.