When food is suspected to be the source of infection, and notification of food poisoning is made by a physician or food poisoning is recognized by the director of a health center, investigation and reporting to the national government are conducted by each local municipality under the Food Sanitation Law in a coordinated manner.
In pathogen surveillance, prefectural and municipal public health institutes (PHIs) undertake EHEC isolation, serotyping and VT typing and report to the Infectious Disease Surveillance Center (IDSC), the National Institute of Infectious Diseases (NIID). The Department of Bacteriology I, NIID conducts molecular epidemiological analysis of the isolates and provides information by Pulse-Net Japan (see p. 144 of this issue).
Notified cases under the NESID: In 2005, 3,577 new symptomatic and asymptomatic cases of EHEC infection (hereafter referred to as cases of EHEC infection) were reported (Table 1). Slightly decreased reports can be seen when compared with the 2004 reports, but remain on more or less the same level. Large seasonal variations were seen as usual in the weekly reports of 2005, with epidemic peaks in the summer season (Fig. 1). Incidence by prefecture in 2005 was 0.87-9.1 per 100,000 population, with a considerable regional difference (Fig. 2). The largest number of cases occurred in Miyazaki Prefecture (9.1), followed by Oita (8.7) and Shimane (8.6) Prefectures. Regions where there were many cases in 1999-2004 (see IASR 26:137-138, 2005), many cases tended to occur also in 2005. Cases that acquired infection in foreign countries used to be only 20-30 until 2002, increased largely to 66 in 2003 and to 151 in 2004, whereas decreased to 27 in 2005. EHEC infection was most prevalent in those aged 0-4 years, followed by those aged 5-9 years. There were more males among those aged 0-14 years, and more females among those over 15 years. The proportion of symptomatic cases was high in young and aged persons (80% in those under 19 years and 70% in those over 65 years). The symptomatic cases were less than 36% among those aged 30s and 40s (Fig. 3).
EHEC isolationFReports of EHEC isolation from PHIs to IDSC counted about 1,400 in 2003, 1,800 in 2004 and 1,600 in 2005. These figures differ from those of reported cases as shown in Table 1. Such discrepancies can be explained by the fact that a portion of the strains detected in laboratories other than PHIs are not sent to PHIs under the present system.
During 1991-1995, O157:H7 accounted for 80% of all isolates. In later years, serotypes other than O157, such as O26 and O111, have increased. In 2005, O157:H7 accounted for 59%, O26 for 22% and O111 for 4.6% (see p. 143 of this issue). In addition, various other serotypes were detected, including some Verocytotoxin (VT)-producing isolates that are untypable with commercially available antisera (see IASR 25:141-143, 2004). For identification of EHEC, confirmation of VT is important. The VT type produced (or the toxin gene possessed) by the isolates were VT1&2 in 68% of O157 in 2005 as usual (53-68% in 1997-2004). Of O26, more than 90% produced VT1 alone and 97% did so in 2005. Of O111, more than 60% produced VT1 alone every year, but VT1&2 accounted for 86% in 2004 and for 59% in 2005.
Of 1,574 cases in which EHEC was detected in 2005, the symptoms of 1,076 cases from which O157 was isolated were bloody diarrhea in 34%, diarrhea in 56%, abdominal pain in 44%, fever in 16%, HUS in 18 cases (VT1&2 in 10 cases and VT2 alone in 8 cases). In addition, HUS was reported in 5 cases of O111 (VT1&2 in 4 cases, VT2 alone in one case) and in one case of OUT (VT2).
Of 148 cases in which HUS was reported in 2000-2005, those younger than one year were 17 cases (1.5% of 977 cases from which EHEC was isolated), 2-5 years 75 cases (3.3% of 2,269 cases), 6-15 years 32 cases (1.7% of 1,936 cases), 16-39 years 7 cases (0.3% of 2,782 cases), and over 40 years 17 cases (0.7% of 2,268 cases). Among younger generations, the number of cases was large and the incidence rate of HUS high.
Outbreaks: Among 28 outbreaks of EHEC infection reported by PHIs to IDSC in 2005, more than 50% were due to O157. In 12 outbreaks involving 10 or more EHEC-positive cases (Table 2), four were thought to be due to foodborne transmission and three due to person-to-person transmission. In 2005, there were 24 incidents of EHEC food poisoning involving 105 cases, reported from prefectural governments in compliance with the Food Sanitation Law (note: the number of cases was much smaller 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 2005, outbreaks in nursery schools remained many, with 7 outbreaks reported. Since EHEC, as is the case with Shigella , causes infection with a minute quantity of the organisms, infection is liable to expand by person-to-person transmission and food contaminated with a minute quantity of the organisms may cause infection. 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. 144-148 of this issue). Furthermore, a characteristic feature of EHEC infections is the frequent occurrence of secondary infections among family members (Table 2). If a case is found, thorough instruction to family members is required to prevent secondary infections and it is important to keep basic precautions to food poisoning such as thorough cooking of food items.
Update 2006: Reported cases of EHEC infection during the 1st-22nd weeks of this year counted at 488 (Table 1). In the 16th week, a peak, small though, can be seen (Fig. 1). This coming summer, further increases in cases of EHEC infection are anticipated, thereby necessitating further attention to infection control and prevention.