The Topic of This Month Vol.20 No.7(No.233)
Vibrio parahaemolyticus food poisoning, having shown a decreasing tendency in recent years, started to increase again in around 1994. The information on the incidents of V. parahaemolyticus food poisoning in Japan is independently being collected by the following routes; (1) Mandatory notification complying with the Food Sanitation Law (the Statistics of Food Poisoning compiled by the Food Sanitation Division, the Ministry of Health and Welfare). (2) The reports of the examinations on the cases implicated in food poisoning outbreaks performed by the prefectural and municipal public health institutes and health centers (PHI/HC) (Isolation Reports from PHI/HC, compiled by the Infectious Disease Surveillance Center).
The following is a summary of the incidence in the whole country during the past three years based on the information gathered from the above routes (for the incidence before 1995, see IASR, Vol. 17, No. 7).
The Statistics of Food Poisoning: The outbreaks of all kinds of food poisoning in 1996 totaled at 1,217, involving 46,327 cases, of which etiological agents were identified in 1,047 outbreaks (86%), involving 41,300 cases (89%). Incidents of V. parahaemolyticus food poisoning were the second most prevalent after those of Salmonella food poisoning, involving the third largest number of cases after Salmonella and enteropathogenic Escherichia coli food poisoning (Table 1). (The abruptly increased cases of enteropathogenic E. coli food poisoning in this year were due to the large-scale outbreaks of enterohemorrhagic E. coli O157:H7 infection: Fig. 2). The incidents of food poisoning in 1997 totaled at 1,960, involving 39,989 cases, of which etiological agents were identified in 1,723 (88%), involving 29,625 cases (74%). Although there were slightly more incidents of V. parahaemolyticus food poisoning than that of Salmonella (Fig. 1), the number of cases of Salmonella food poisoning was the largest as before (Fig. 2). In 1998, the incidents totaled 3,059, implicating 44,645 cases, of which etiological agents were identified in 2,953 incidents (97%), implicating 43,536 cases (93%). The number of V. parahaemolyticus food poisoning cases doubled that of the preceding year, exceeding that of Salmonella food poisoning cases (Fig. 2).
Isolation reports from PHI/HC: The annual reports from PHI/HC on isolation of V. parahaemolyticus from human sources have tended to increase since 1997 (Fig. 3). The monthly isolation during 1996-1998 shows a peak in August of every consecutive year and is concentrated upon the period of July-September as before. However, frequent isolation was also reported in October of 1998 (Fig. 4).
According to the outbreak reports (including those implicating two or more cases) during the three years from 1996 through 1998, the monthly outbreaks of V. parahaemolyticus food poisoning show a summer-prevalent tendency with a peak in August and few outbreaks in winter, a repetition in tendency (Fig. 4).
Outbreaks of Vibrio parahaemolyticus food poisoning reported during 1996-1998 are grouped by outbreak scale (Fig. 5). Outbreaks during the last three years totaled at 496 (102 in 1996, 160 in 1997, and 234 in 1998), of which those involving 2-49 cases accounted for 94% (220 outbreaks each involving 2-9 cases and 244 each involving 10-49 cases). The outbreaks involving 50-499 cases were relatively few, accounting for 6% (30 incidents), and such extraordinarily large-scale outbreaks, involving more than 500 cases, occurred only twice. Recent V. parahaemolyticus food poisoning in Japan tends to be small in scale but occurs rather frequently.
One of the extraordinarily large-scale outbreaks was the one, affecting 691 cases, due to boiled red queen crabs sold at a crab shop in Niigata Prefecture in August 1996 (the serotype of the agent was O3:K6). The other one was that, involving 1,167 cases, due to catered meals in Shiga Prefecture in July 1998 (the serotypes of the agent were O1:K56, O3:K6 and other five types; see p. 166 of this issue).
The serotype O4:K8 of V. parahaemolyticus, which used to be predominant in previous outbreaks, has been replaced with O3:K6 since 1996 (Fig. 6). The increased incidents of V. parahaemolyticus food poisoning during 1997-1998 have been ascribed to the increased incidents of O3:K6 food poisoning. It is not known, however, what has caused the change in the V. parahaemolyticus serotype.
V. parahaemolyticus O3:K6 food poisoning has rapidly been increasing also in India and Bangladesh since 1996. It is noteworthy that incidents due to the organisms of the same serotype have also been increasing in Southeast Asian countries such as Thailand and Taiwan. V. parahaemolyticus O3:K6 outbreaks from consumption of raw oysters were reported in July-August 1997 (involving 209 cases) and July-September 1998 (23 cases) in the US (see CDC, MMWR, 47, 457-462, 1998; 48, 48-51, 1999). Attention should be paid to the trend of future world-wide outbreaks of V. parahaemolyticus O3:K6 food poisoning.
The pulsed-field gel electrophoresis (PFGE) patterns of the O3:K6 strains recently isolated in Japan and other countries were different from those of the previously isolated strains and show close similarity from one another suggesting the possibility of a single clone origin (see p. 161 of this issue).
Six outbreaks due to serotype O4:K68, which had never been implicated in food poisoning, were reported in 1998 (Fig. 6: see p. 167 of this issue). Since this new serotype has also been isolated in Thailand and India, attention must also be paid to the future trend of V. parahaemolyticus O4:K68 food poisoning.