Notified cases under the NESID: In 2004, 3,711 new symptomatic and asymptomatic cases of EHEC infection (hereafter referred to as cases of EHEC infection) were reported (Table 1). This figure is the second largest following that in 2001 after enactment of the Infectious Diseases Control Law. Weekly reports in 2004 increased in the summer as usual, with a peak occurring in the 29th week (July 12-18) due to an outbreak among high school students in Ishikawa Prefecture returning from a school excursion to Korea (see Table 2 and p. 141 of this issue) (Fig. 1). Incidence by prefecture in 2004 ranged from 0.90 to 14.8 per 100,000 population, showing considerable local differences (Fig. 2). The highest incidence was seen in Ishikawa Prefecture (14.8), followed by Okayama (9.83) and Tottori Prefectures (8.66). Districts experiencing high incidence during l999-2003 also tended to demonstrate high incidence in 2004. The number of cases that acquired infection in foreign countries, which until 2002 did not exceed 20 to 30, increased to 66 in 2003 and 151 in 2004. In 2004, the largest number of cases occurred in those aged 0-4 years, followed by those aged 5-9 years. Males predominated among cases 0-14 years of age, with females predominating among those aged 15 years or over. The proportion of symptomatic patients was high in the younger and elderly age groups, as is typically seen yearly (<=19 years - 79%, >= 65 years - 66%), and less than 45% in those aged 30-50s (Fig. 3).
EHEC isolation: In 2002, 1,800 EHEC isolations were reported from prefectural and municipal public health institutes (PHIs) to the Infectious Disease Surveillance Center (IDSC) of the National Institute of Infectious Diseases (NIID); this number decreased to 1,400 in 2003, followed by an increase to 1,800 in 2004. These figures differ from those of reported cases in Table 1. These discrepancies can be explained by the fact, under the present system, a portion of the information of strains detected in laboratories other than PHIs are not reported to PHIs. During 1991-1995, more than 80% of isolates were O157:H7, while serotypes other than O157, such as O26 and O111, increased. In 2004, O157:H7 isolations decreased by about 50%, while O26 and O111 increased by 24% and 8.2%, respectively (see p. 139 of this issue). In addition, various other serotypes were detected, including some Verocytotoxin (VT)-producing isolates that are untypable using commercially available antisera (see IASR 25:141-143, 2004). For identification of EHEC, confirmation of VT is important. Looking at VT types (or VT gene types) of EHEC isolates (see p. 139 of this issue), VT1 & VT2 comprised 63% of O157 isolates in 2004, similar to previous years (53-68% during 1997-2003). Of the O26 isolates, VT1 alone has accounted for more than 90% every year, reaching 97% in 2004. Among O111 isolates, VT1 alone comprised more than 60% of the isolates, but in 2004, VT1 & 2 accounted for 86%, reflecting the occurrence of a large-scale outbreak (Table 2).
In 2004, 14 cases of hemolytic uremic syndrome (HUS) were reported among the 1,809 EHEC-positive cases, of which O157 was found in 13 cases (VT1 & 2 - 8 cases, VT2 alone - 5 cases) and O165 (VT2) in one case. Reported symptoms of 1,114 cases from which O157 was detected were bloody diarrhea (31%), diarrhea (47%), abdominal pain (41%), and fever (17%). Of the 124 cases of HUS reported during 2000-2004, 12 were aged one year or under (1.5% of 796 cases), 61 were aged 2-5 years (3.2% of 1,902 cases), 32 were aged 6-15 years (1.9% of 1,672 cases), 7 were aged 16-39 years (0.3% of 2,379 cases), and 12 were aged 40 years or over (0.6% of 1,949 cases). The number of cases and incidence rate of HUS were high in the younger ages.
Outbreaks: Among outbreaks involving 10 or more EHEC-positive cases reported to IDSC in 2004, one was thought to be due to food-borne transmission and six due to person-to-person transmission (Table 2). In 2004, there were 18 incidents of EHEC food poisoning (excluding foreign-acquired incidents) involving 70 cases, reported from prefectural governments in compliance with the Food Sanitation Law (note: the number of cases was much lower than that reported under the Infectious Diseases Control Law, due to the fact that incidents in which food was incriminated as the source of infection were few, and also that incidents involving only a single case are not always reported as food poisoning).
In 2004, outbreaks in nursery schools and kindergartens remained high, with 11 events reported. The etiologic serotype was identified as O26 in more events than O157. To prevent outbreaks due to person-to-person transmission in nursery schools, it is necessary to take proper precautions, including hand washing by children and staff members and sanitary control of paddling pools for children (see p. 142 of this issue). Furthermore, a characteristic feature of EHEC infections is the frequent occurrence of secondary infections among family members (Table 2). Thorough instruction of family members is required to prevent secondary infections.
There are many incidents in which the route of infection is unknown (neither person-to-person infection nor food-borne), and a fatal case has been reported in a home for the aged (see p. 144 of this issue).
After a report from Fukuoka Prefecture in 2003 (see IASR 25:147-148, 2004) of an outbreak during a school excursion to Australia, another outbreak among participants of a school excursion to Korea was reported from Ishikawa Prefecture in 2004 (Table 2). One hundred and ten cases were examined and treated at more than 25 different medical clinics, with health centers deluged with outbreak response activities such as the provision of information. It has been pointed out that health risk management is necessary when planning overseas school excursions (see p. 141 of this issue).
Pulse-Net Japan: In 2004, seven clusters of O157 strains and one cluster of O26 strains were found; each cluster consists of strains showing the indistinguishable PFGE pattern and was isolated in more than five prefectures (see p. 140 of this issue). Construction of a system that can respond to outbreaks and events on an international scale is in progress, led by the US CDC. In 2004, PFGE patterns of strains originating from an O157 incident on a US Air Force Base in Okinawa were shared by the US and Japan, resulting in the recall of approximately 40,000 tons of US beef suspected of being the source of infection (MMWR 54:40-42, 2005).
Update 2005: A total of 390 cases of EHEC infection have been reported over the first 21 weeks of this year (Table 1). Small peaks were seen in the 3rd, 10th and 13th weeks (see Fig. 1 and p. 147-150 of this issue). This coming summer, further increases in cases of EHEC infection are anticipated, thereby necessitating further attention to infection control and prevention.