At present, a molecular EHEC surveillance system (Pulse-Net Japan) which combines genotypes of the isolates determined by pulsed-field gel electrophoresis (PFGE) with epidemiological data is on trial as one of the methods for rapid detection of diffuse outbreaks. Also, a project to expand the Pulse-Net is in progress under the collaboration with U.S.A. (CDC) and other countries toward the establishment of an international rapid detection system of outbreaks including EHEC infections.
Trend of notified cases: A total of 3,185 symptomatic and asymptomatic cases of EHEC infection (hereafter referred to as cases of EHEC infection) were notified in 2002 (Table 1). Weekly case reports increased with small peaks at the 16th (April 15-21), the 21st (May 20-26) and the 26th (June 24-30) weeks due to several outbreaks and a diffuse outbreak (Table 3), largely increased in the summer with a peak at the 33rd week (August 12-18), and decreased thereafter (Fig. 1). The incidence in 2002 varied from one prefecture to another, giving 0.60-19.68 cases per 100,000 population (Fig. 2). The highest incidence was seen in Saga and Ishikawa Prefectures where several outbreaks occurred, and Tochigi Prefecture where a large outbreak occurred. Cases of EHEC infection aged 0-4 years counted at the largest number, followed by those aged 5-9 years. As to the gender, a slightly larger number of males were reported in the age group of 0-19 years, while more females over 20 years (Fig. 3). The ratio of symptomatic patients was high in young and aged groups both for males and females (72% of those under 19 years and 67% of those over 65 years), and was less than 50% of in ages of 30s, 40s, and 50s.
Reports of EHEC isolation: Reports of EHEC isolation by prefectural and municipal public health institutes (PHIs) to the Infectious Disease Surveillance Center, the National Institute of Infectious Diseases, increased abruptly to 3,022 in 1996, and have been kept at about 2,000 per year since then (see http://idsc.nih.go.jp/iasr/prompt/graph-l.html). The reports of EHEC isolation showed some differences in number from the reported cases of EHEC infection (Table 1). This is due to the fact that some information on EHEC isolation at other places than PHIs may not reach PHIs under the current system. During 1991-1995, 83% of the isolates (436/525) were demonstrated as serotype O157:H7. Thereafter, O26 and O111 have increased. In 2002, O157:H7 decreased to 53% of the isolates (see page 131 of this issue). As to Verocytotoxin (VT) types (or VT gene types) among serotypes, EHEC O157:H7 with both VT1 and VT2 (VT1&2) largely increased in 2001 (65%), and was also found frequently in 2002 (62%). In contrast, VT1 was predominant among O26 and O111 isolates.
Of 1,601 cases with EHEC isolation in 2002, hemolytic uremic syndrome (HUS) was reported in 19 cases (Table 2). O157 was isolated from 15 of these cases (nine were VT1&2 and the other six VT2), O111 (VT1&2) from two, O26 (VT1&2) from one and OUT (VT2) from one case. It is noteworthy that all isolates from these HUS cases were VT2-positive.
Symptoms of 1,058 cases yielding O157 include bloody diarrhea (33%), blood-free diarrhea (50%), abdominal pain (39%) and fever (13%).
Diffuse outbreak: In 2001, large-scale diffuse outbreaks in wide areas were reported due to wide distribution of the foodstuffs (see IASR, Vol. 23, No. 6).
Also in 2002, a diffuse outbreak due to EHEC O157:H7 at grilled-meat chain restaurants in Kansai area occurred during April-May and the patients were reported from six prefectures. The PFGE patterns of the isolates from the patients and the stocked beef were indistinguishable each other (Table 3, No.3).
Outbreaks: Among outbreaks with 10 or more EHEC-positive cases (including carriers) reported in 2002 (Table 3), foodborne infection was suspected in three incidents. In an outbreak in August at a hospital and the neighboring care facility for the aged (No. 11), nine of 123 patients died. The PFGE genotype of the O157 isolates from the patients and the incriminated foodstuffs were identical to one of the genotypes prevalent in wide areas in 2001 (see p. 132 of this issue).
Although many large-scale outbreaks at primary schools occurred in 1996 (see IASR, Vol. 19, No. 6), no additional such incident has since occurred. In 2002, however, three small- or medium-scale outbreaks at primary schools were reported. In an incident in July (No. 8), more than 10,000 people were examined for the detection of EHEC from stools to find out all asymptomatic as well as symptomatic cases at an early stage.
Furthermore, outbreaks occurred in nine nursery schools and a kindergarten in 2002. In three of these incidents, exposure to a paddling pool was suspected to be a cause of infection. To prevent outbreaks due to person-to-person transmission at nursery schools or kindergartens, the personnel must practice daily hand-washing (special attention is necessary after a change of a diaper) and educate children in habits of hand-washing after defecation and before the meal. In addition, adequate disinfection and maintenance of pools are important in the summer (see p. 133 of this issue and PHLS CDR Vol. 6, Review No.2, 1996).
In outbreaks at nursery schools, kindergartens and primary schools, infection tends to spread outside these facilities, resulting in the secondary infections among the family members (Table 3). In order to prevent prolongation and further spread of disease outbreaks due to familial infection, it is vital for guardians to receive accurate guidance for the prevention of secondary infection.
Update 2003: As of May 29, 2003, cases of EHEC infection diagnosed before May 25 counted at 279 (Table 1). The incidence before April remained at a low level, but is gradually increasing at the 20th and the 21st weeks in May and outbreaks are reported (Fig. 1). More public attention to the increase in EHEC infection anticipated for the forthcoming summer should be recommended.