The Topic of This Month Vol.19 No.1(No.215)
Outbreaks of viral gastroenteritis have recently been surveyed in Japan as well as in UK and USA (see IASR,Vol. 17, No. 11 and p. 14 of this issue). The Study Group on Food-borne Viral Gastroenteritis Outbreaks in Japan reported in 1995 that there were 908 outbreaks of nonbacterial gastroenteritis during the five years from September 1990 through August 1994. In 360 of these outbreaks, the etiological agents were attributed to viruses, of which 330 were small round structured virus (SRSV) (see IASR, Vol. 17, No. 2).
SRSV is electron microscopically a spherical virus with a diameter of 25-35 nm. No cultured cells supporting the growth of SRSV have been found. Its prototype is Norwalk virus first found in 1972 from patients in an outbreak of gastroenteritis. SRSV genome cloning has recently been accomplished and it has been classified into Caliciviridae of RNA virus.
Since January 1997, a computer network system for laboratory findings of infectious agents has started between prefectural and municipal public health institutes (PHIs) and the Infectious Disease Surveillance Center (IDSC), and information on individual outbreaks of gastroenteritis suspected of being viral origin has been collected through this system. This Topic summarizes 198 outbreaks reported in 1997 before November 21.
Table 1 shows the monthly incidence by the estimated route of infection and by the incriminated foodstuff implicated in the incidents suspected of being food borne. Reports of outbreaks of gastroenteritis suspected of being food borne (single exposure) are concentrated upon January-March; especially a large number of those presumably caused by raw oysters upon January-February. Such outbreaks that were suspected of person-to-person transmission (successive exposure) numbered three. In 39% (59/152) of the incidents suspected of being food borne and 52% (37/71) of those giving positive SRSV detection, raw oysters were incriminated as the vehicle. Besides, school lunch and sushi were reported to be the vehicles.
In 101 of the 198 incidents, viruses were detected from stools. The viruses were identified as SRSV in 97 and group A rotavirus (Table 2, No. 19), group C rotavirus (Table 2, No. 15), coxsackievirus A9, and coronavirus in each of the other incidents.
In 64 of the 198 outbreaks, the patient numbers were reported. The greater parts were small-scale outbreaks involving two to four patients in 15 and five to nine patients in 14 outbreaks. Three large-scale outbreaks involving more than 100 patients have also been reported (Fig. 1).
The places of infection/eating food in 97 of the 198 episodes in which SRSV detection was positive involved restaurants in 45%, hotels and inns in 14%, schools in 12%, and homes in 8% (Fig. 2). Outbreaks implicating 20 or more patients took place at schools, a company, homes for the aged, hotels and inns, a nursery school, a home for the handicapped, a dormitory, and a hospital (Table 2).
The duration of such outbreaks suspected of being food borne is usually 2-3 days, whereas that suspected of person-to-person transmission is as long as 2-3 weeks. Once patients occur in an institution, they are liable to develop into a large-scale outbreak by person-to-person transmission among inhabitants and attendants. It is, therefore, important to cope with SRSV infection not only with food hygienic precaution but also by prevention of possible transmission within the institutions (see IASR, Vol. 18, Nos. 5 and 6 and p. 3 of this issue). SRSV may occasionally be detected in vomits of patients; therefore, care must be taken on not only stools but also vomits as the possible sources of infection (see IASR, Vol. 17, No. 2).
SRSV gastroenteritis is characterized by severe vomiting, though lasting only for such a short period as a day or two, and diarrhea may develop later (see IASR, Vol. 17, No. 2). Of the 97 SRSV-positive incidents, average incubation periods were recorded in 12 ones: 34 to 39 hours in eight, 42 to 47 hours in other three, and 25 hours in the other one of the episodes.
Recently, RT-PCR (PCR) has become applicable to SRSV detection and this has been used in combination with electron microscopy (EM)(see IASR, Vol. 17, No. 2). Of the 97 SRSV-positive incidents, both EM and PCR detected the virus in 57, PCR alone in other 22 and EM alone in the other 18 incidents. Although the sensitivity of PCR is considered to surpass that of EM, the standard method of SRSV detection is still EM (see p. 6 of this issue). It has been reported that the use of primers originating from multiple strains can increase the rate of detection by PCR (see p. 5 of this issue).
According to the investigations that the Food Sanitation Division, the Ministry of Health and Welfare entrusted to the prefectures, there were 149 incidents of nonbacterial food poisoning during January through April 1997 in 33 prefectures and major cities in the whole country. These incidents included those reported to IDSC and their overall features are similar to those reported. Three of 31 oyster specimens in one of 38 incidents in which PCR was performed on the specimens of the incriminated foodstuffs resulted in SRSV positivity.
Virus was formerly not indicated as an etiological agent of food poisoning in the reporting form (in the enforcement regulation) in compliance with the Food Sanitation Law. On June 1, 1997, the enforcement regulation was partially amended on the basis of the deliberation by the Food Sanitation Council, and SRSV and some other viruses have clearly been indicated as etiological agents of food poisoning. In addition, they have replaced the term 'bacteria' in the notification by 'microorganisms' comprising the concept of viruses (see p. 6 of this issue). In compliance with this amendment, a technical workshop on the procedures for examination for SRSV was held for the staff members of PHIs in November 1997 at the National Institute of Public Health (see p. 6 of this issue).