The Topic of This Month Vol.20 No.11(No.237)
Small round structured virus (SRSV) and some other viruses were nominated as etiological agents of food poisoning by the enforcement regulations of the Food Sanitation Law amended partially on May 30, 1997 (see IASR, Vol. 19, No. 1). When a food poisoning incident is reported, the nearby health center undertakes epidemiological investigations and collects specimens of patient stools and incriminated foodstuffs for microbiological examinations. If viral origin is suspected, the specimens will be sent to prefectural and municipal public health institutes (PHIs). To detect SRSV, PHIs have disposed essential laboratory systems.
According to the Statistics of Food Poisoning compiled by the Ministry of Health and Welfare (MHW), incidents of microbial food poisoning occurring in 1998 totaled at 2,743 (41,550 cases), of which those of viral origin stood at 123 (5,213 cases), accounting for 4.5% and 13% of all incidents and cases, respectively.
In January 1997, a computer network system, for collecting and feeding back laboratory findings of infectious agents, was shared out between PHIs and the Infectious Disease Surveillance Center, the National Institute of Infectious Diseases. The information on individual outbreaks of gastroenteritis suspected of viral origin has been collected throughout this system to elucidate the actual status of viral gastroenteritis outbreaks. This information involves not only foodborne outbreaks but also those due to person-to-person transmission of the virus. The following is a summary of the 258 outbreak reports suspected of viral origin with special reference to SRSV-positive incidents occurring during October 1997 to September 1999 (as of October 18, 1999) (see IASR, Vol. 19, No. 1 for the reports during January to October 1997).
Figure one shows the monthly incidence; the incidence is high in winter. Since May 1997, when the Food Sanitation Law was partially amended, incidents due to unknown etiological virus have markedly decreased. Of 258 outbreaks occurring during October 1997 to September 1999, the etiological virus was detected in 234 outbreaks; SRSV (or Norwalk-like virus: NLV) in 226, group A rotavirus in three, group C rotavirus in two, and coronavirus in three outbreaks. The routes of SRSV transmission suspected were foodborne in 162 outbreaks, presumably person-to-person transmission in three, and unknown routes in 61.
To find the scales of outbreaks, patients in 213 incidents involving clarified numbers of patients among the 226 incidents suspected of being SRSV origin were grouped with the exponent of 2. The patient numbers per outbreak were distributed with a peak at 9-16 patients (47 incidents) (Fig. 2). Subsequently, outbreaks were grouped into three; small scale involving 2-8 patients, medium-scale involving 9-32 patients, and large scale involving more than 33 patients. Incidents involving more than 65 patients are summarized in Table 1.
Places of acquiring infection with SRSV or those of ingesting incriminated foodstuffs are tabulated as seen in Fig. 3 by outbreak scale. Of 71 small-scale outbreaks, the majority or 37 outbreaks (52%) occurred at restaurants, followed by nine (13%) in homes. Of 90 medium-scale outbreaks, 39 (43%) occurred at restaurants, 12 (13%) at banquet halls, and nine (10%) at hotels. In addition, seven outbreaks (7.8%) occurred at schools, six (6.7%) at homes for the aged or mentally handicapped, five (5.6%) at nursing schools and kindergartens, and three (3.3%) at dormitories. The ratio of incidents occurring at group-populated facilities is higher in medium-scale outbreaks than in small-scale outbreaks. Of 52 large-scale outbreaks, as few as 13 (25%) occurred at restaurants, followed by, similar to medium-scale outbreaks, 10 (19%) at banquet halls, six (12%) at hotels, six (12%) at schools, and five (9.6%) at nursing schools and kindergartens. There have been two outbreaks at hospitals.
With respect to the route of SRSV transmission and the incriminated foodstuff by scale of outbreak (Fig. 4), oysters were incriminated in 41% of small-scale outbreaks and 34% of medium-scale outbreaks. School lunches and catered meals were most often (17%) incriminated in large-scale outbreaks, in which the rate of oyster-associated incidents was low to the contrary. Besides, neither the route of transmission nor the foodstuff involved has been clarified in many large-scale outbreaks. Since SRSV is uncultivable at the moment, it is hardly possible to detect a small amount of the virus in the incriminated foodstuffs in many incidents.
SRSV has principally been detected by electron microscopy at PHIs; nowadays RT-PCR (PCR) is becoming more popular. By combining PCR with hybridization, genogrouping (genogroup I and II) is possible. In Japan, it has been found that genogroup II is more prevalent. Moreover, analyzing the nucleotide sequence made differentiation of strains possible nowadays. It has been suggested that the prevalent strains differ from one district to another (see p. 267 of this issue).
In addition, analysis of nucleotide sequence has made it possible to trace the route of transmission of SRSV. An outbreak caused by SRSV, which was prevailing among children in an area, transmitted to their family members, and then to foodstuffs they handled, has been reported (see p. 267 of this issue). Weekly SRSV gastroenteritis outbreaks were compared with the number of cases of SRSV gastroenteritis in children (under 15 years of age) (see IASR, Vol. 19, No. 11) (Fig. 5); oyster-unassociated outbreaks and SRSV gastroenteritis incidents in children show similar trends. If an oyster-unassociated outbreak occurs, it is also important to investigate the epidemiological relationship with SRSV gastroenteritis in children in the neighboring areas. Oyster-unassociated outbreaks are occurring perennially (see IASR, Vol. 20, No. 8, and p. 268 and 270 of this issue).
If no virus is detected from stool specimens of patients or food handlers, ELISA for antibody titration with the empty particles of SRSV as the antigen (see IASR, Vol. 19, No. 11) will be useful to investigate the outbreaks and the route of transmission (see p. 267-268 of this issue). SRSV infection can be suspected by detecting an elevation of the antibody titer in paired sera taken in acute and convalescent phases. Cooperation of examinees is indispensable to collect stool and serum specimens in the acute phase soon after onset of symptoms and again serum specimens in the convalescent phase.
Once a patient occurs at such facilities as homes for the aged, infection may spread through person-to-person transmission. In an outbreak occurring in a facility in Akita Prefecture in May 1999, it was reported that the countermeasure for preventing the secondary infection worked out successfully (see p. 269 of this issue).
As described above, small-scale outbreaks of SRSV food poisoning presumably due to raw oysters often occur in winter of every year. Since October 1, 1999, labeling the packages of eating-raw oysters to express the sea area of harvest has legally been required to help trace from the patients back to the harvest place for preventing expansion of food poisoning injuries (Notification No. 1825 by the Environmental Health Bureau, MHW issued on December 28, 1998; see p. 271 of this issue).
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