The Topic of This Month Vol.17 No.7 (No.197)
Vibrio parahaemolyticus food poisoning, incidents of which used to account for nearly half of all those of bacterial food poisoning, markedly decreased in 1992 and 1993 and was outnumbered by Salmonella food poisoning in incidents and cases. This was due to the markedly increased cases of Salmonella food poisoning caused by ingestion of hen's eggs laid by layers originating from imported chicks contaminated with Salmonella Enteritidis.
In 1994, incidents of V. parahaemolyticus food poisoning again slightly outnumbered those of Salmonella food poisoning; in 1995, those tended to increase further. The number of cases of Salmonella food poisoning, however, is still being outstanding (Figs. 1 and 2; from the statistics of food poisoning in Japan by the Ministry of Health and Welfare). This fact indicates that the scale of Salmonella food poisoning is likely becoming larger.
According to the statistics of food poisoning, all the outbreaks in 1994 totaled 830 (151% of those in the preceding year), involving 35,735 cases (139%) and two deaths (both were due to biotoxins). Outbreaks and cases with agents identified numbered 709 and 29,894, respectively. Incidents of V. parahaemolyticus food poisoning numbered 224 (32%), followed by those of Salmonella food poisoning, numbering 205 (29%). Cases of Salmonella food poisoning, being 14,410 (48%), however, largely outnumbered those of V. parahaemolyticus food poisoning, being 5,849 (20%).
In 1995, outbreaks of food poisoning totaled 699 (84% of those in the preceding year) involving 26,325 cases (74%) and five deaths (three were due to biotoxins, another to Salmonella, and the other to Staphylococcus aureus), thus outbreaks as well as cases decreased in number. The agents were identified in 627 outbreaks involving 22,660 cases. Of these outbreaks, those of V. parahaemolyticus food poisoning numbered 245 (39%) involving 5,515 cases (24%) and those of Salmonella food poisoning 179 (29%) involving 7,996 cases (35%). As was the case in the preceding year, V. parahaemolyticus food poisoning exceeded Salmonella food poisoning in number of outbreaks but were outnumbered in number of cases.
In 1994 and 1995, reports from prefectural and municipal public health institutes on isolation of V. parahaemolyticus from human sources numbered 1,280 (including those from 108 imported cases) and 1,304 (85 imported ones), respectively. The monthly reports on isolation of V. parahaemolyticus in 1994 were 271 (21%) in July, 577 (45%) in August, and 222 (17%) in September; those in 1995 were 230 (18%) in July, 624 (48%) in August, and 335 (26%) in September. As usual (see IASR, Vol. 15, No. 8, 1994), both years showed the summer-prevalent tendency with the most cases in August (Fig. 3).
Of the outbreaks of V. parahaemolyticus food poisoning reported in IASR during 1994 and 1995, those involving more than 10 cases are shown by scale in Table 1. Such outbreaks counted 157 in the two years; 79 in 1994 and 78 in 1995. Of these outbreaks, 121 (77%) were small-scale ones with 10-49 cases. According to the previous Topic of This Month on V. parahaemolyticus (see IASR, Vol. 15, No. 8, 1994), of 533 outbreaks with more than 10 cases having occurred during 1987 through 1993, small-scale outbreaks with 10-49 cases accounted for 82%. The corresponding ratio during the last two years did not differ significantly from that in the preceding years. During the last two years, 11 of such large-scale outbreaks involving 101-400 cases were reported, but no such extraordinarily large-scale outbreak involving more than 500 cases was reported.
Food poisoning outbreaks in this country, however, tend to be in such an extraordinarily large scale involving more than 500 or even more than 1,000 cases in recent years. It seems that food poisoning outbreaks tend to become larger in scale. School lunches have been incriminated for such large-scale food poisoning outbreaks, but no V. parahaemolyticus food poisoning due to school lunch has recently been reported. V. parahaemolyticus food poisoning in this country seems to be restricted in relatively a small scale.
Of the serotypes of V. parahaemolyticus isolated in outbreaks involving more than 10 cases, O4:K8 was highly prevalent in both years accounting for 48% (38 outbreaks) in 1994 and 24% (19 outbreaks) in 1995. The tendency was also the case with the matching incidents during 1987-1993, yielding isolates of O4:K8 in 26%. This indicates that O4:K8 is the most important serotype responsible for food poisoning in this country.