The Topic of This Month Vol.18 No.3(No.205)
According to the Statistics of Food Poisoning in Japan compiled by the Statistics and Information Department, Minister's Secretariat, the Ministry of Health and Welfare, cases of bacterial food poisoning numbered 29,513 in 1994 and 22,329 in 1995, surpassing 19,089 in 1993. Cases due to Salmonella as an etiological agent accounted for 36% in 1993 (6,954 cases), 49% in 1994 (14,410 cases) and 36% in 1995 ( 7,996 cases), being still ranked top. In number of outbreaks, those due to Vibrio parahaemolyticus were ranked top and those due to Salmonella second in 1994 and 1995. Regarding cases per outbreak, however, those due to Vibrio parahaemolyticus numbered 26 and 23 in each of these years, while those due to Salmonella outnumbered these, being 70 and 45, respectively, showing the same tendency as usual.
Reports on isolation of Salmonella sent in from prefectural and municipal public health institutes in the whole country have been the largest in number among those of all pathogens dealt with by IASR. The reports on isolation of Salmonella from human sources during 1986-1995 show a tendency that after an increase during 1986-1989, they have remained on the same level, although minor fluctuations can be seen. In regard to the predominant serovars, however, such a tendency can be seen that S. Enteritidis has been increasing, while S. Typhimurium and other serovars have been decreasing (Fig. 1).
Such a tendency is clearly shown also by the predominant serovars that provoked outbreaks of Salmonella food poisoning. There were 10 to 12 different serovars among the isolates from outbreaks involving 10 or more cases reported to IASR in every year during 1993-1996. Of these outbreaks, those due to S. Enteritidis numbered 41 in 1993 (55%), 75 in 1994 (70%), 69 in 1995 (71%), and 84 in 1996 (76%), indicating a continuously increasing tendency (Table 1).
Increase in S. Enteritidis isolation is also shown clearly by the 15 most common serovars of human isolates in this country (Table 2). The proportion of S. Enteritidis isolates to the total Salmonella isolates increased suddenly from 5% in 1988 to 24% in 1989, and thereafter increased gradually year after year (see IASR, Vol. 16, No. 1). It was isolated in as high as 47% in 1993 and the proportion changed to 56% in 1994 and again to 47% in 1995.
S. Enteritidis isolates involved in outbreaks were sent to the National Institute of Health for phage-typing (Table 3.). The predominant phage types (PT) during the period from 1990 to 1996 were PT1, which increased to 32% in 1992 (35/110) and gave an average rate of 41% during the following five years, PT4, which suddenly increased to 57% in 1990 (26/46) and gave an average rate of 40% during the following seven years, and PT34, which gave a rate of 26% in 1990 (12/46) and decreased gradually until none was isolated in 1996. Thus, the sum of the rates of PT1 and PT4 isolates attained 84% in 1996, indicating the prevalence of these two PTs of S. Enteritidis. The isolates from sporadic cases and the environment are of various PTs and no one can deny the possible future emergence of S. Enteritidis food poisoning with these PTs.
Since the rate of S. Enteritidis isolation is continuously high, collection of detailed information including that of epidemiology seems to be essential to comprehend its trend.