The Topic of This Month Vol.21 No.8(No.246)
According to the Statistics of Food Poisoning in Japan, Ministry of Health and Welfare, cases of bacterial food poisoning totaled at 38,408 in 1996, 29,104 in 1997, 36,337 in 1998, and 27,741 in 1999. Salmonella cases accounted for 43% (16,334 cases) in 1996, 38% (10,926 cases) in 1997, 32% (11,471 cases) in 1998, and 43% (11,888 cases) in 1999. Except in 1998, when Vibrio parahaemolyticus was the most predominant (see IASR, Vol. 20, No. 7), Salmonella has been the most predominant etiological agent. The number of Salmonella food poisoning cases per incident has been on a decrease; 47 in 1996, 21 in 1997, 15 in 1998 and 14 in 1999. This decrease is due to the overall reports of sporadic cases since the second half of 1997. Thereafter, in 1998, the number of cases per incident, involving two or more cases, was 35. Six of those incidents involved as many as 500 or more cases through the period of 1996 to 1998 (Table 1). Many incidents occurred on a yearly basis during the summer, showing a peak in September, as shown in Fig. 1.
The yearly reports of isolation of Salmonella from the prefectural and municipal public health institutes (PHIs) to the Infectious Disease Surveillance Center (IDSC), the National Institute of Infectious Diseases (NIID) numbered at 5,000 to 6,000 in recent years as shown in Fig. 2. The top 15 most common serovars are seen in Table 2. Since 1989, Salmonella enterica subsp. enterica serovar Enteritidis (S. Enteritidis) has been the most predominant serovar. It accounted for 58% in 1996, 55% in 1997, 62% in 1998, and 46% in 1999. These figures were more than 10 times larger than those of the second ranked serovar (except 1999).
S. Typhimurium, which used to be the most predominant serovar by 1988 (see IASR, Vol. 14, No.1, Vol. 16, No. 1 and Vol. 18, No. 3), ranked 3rd in 1996, 4th in 1997, 2nd in 1998, and 5th in 1999. In Western countries, multidrug-resistant S. Typhimurium (resistant mainly to ampicillin, chloramphenicol, streptomycin, sulfanilamide, and tetracycline), with DT (definitive phage type) 104 is now prevalent (see IASR, Vol. 18, No. 6 and Vol. 21, No. 6). S. Typhimurium DT104 is also being isolated in Japan, but not showing sudden increase as S. Enteritidis, has been observed (Fig. 3).
A sudden increase of another serovar, S. Oranienburg, was observed in 1999. S. Oranienburg has never been ranked in the 15 most common serovars in the past 15 years except in 1986 and in 1991 when it was ranked 8th (104/3,384 isolations) and 9th (130/5,550 isolations), respectively. In 1999, however, its isolation counted at 1,375 of 6,315, accounting for 22% (Table 2). The sudden increase was due to a series of food poisoning outbreaks (diffuse outbreak) occurring between the end of 1998 and May 1999 due to consumption of snacks made of S. Oranienburg-contaminated semidried squid. Since the contaminated raw material was processed into snacks, which were consumed mainly by children and distributed to the whole country, the cases totaled at as many as 1,505 (see IASR, Vol. 20, Nos.4-7). In this incident, there was no death, but cases of retroperitoneal abscess (see IASR, Vol. 20, No. 6) and purulent spondylitis (see IASR, Vol. 20, No. 10) were reported.
Such a tendency in the isolation by serovar is also seen in the organisms causing outbreaks (Table 3). Of the outbreaks reported to IDSC during 1996 through 1999, the serovars of Salmonella isolated from incidents involving 10 or more cases showed about 10 different serovars yearly. Such incidents numbered at 116 in 1996, 103 in 1997, 80 in 1998, and 102 in 1999. Those caused by S. Enteritidis accounted for 76% in 1996, 88% in 1997, 80% in 1998, and 66% in 1999, of all Salmonella food poisonig incidents. The outbreaks caused by S. Enteritidis often involved egg dishes; secondary recontamination is considered to be another important cause of infection (see IASR, Vol. 18, No. 9).
Of the S. Enteritidis strains sent to the Department of Bacteriology, NIID, were phage typed. The results with the strains derived from outbreaks including familial infection are shown in Table 4. Phage type (PT) 4 kept ranking top, accounting for 44% in 1996, 47% in 1997, 42% in 1998, and 33% in 1999, followed by PT1 ranking second. However, the sum of PT1 and PT4 accounted for 85% in 1996, decreasing to 60% in 1999; nevertheless PTs 6, 6a, 21, 47 and RDNC have shown frequent isolation.
Salmonella causes not only enteric infection accompanied with diarrhea but also such fatal systemic infection as sepsis (see IASR, Vol. 19, No. 2 and Vol. 20, No. 11). It is, therefore, important to see a doctor as soon as possible and, for the doctor, to pay a careful attention to any possible change in patients' conditions. In Japan, S. Enteritidis is nowadays continuously prevalent, and as of July 25 of this year, 173 isolates have been reported (Table 2). There are, however, more than 2,300 serovars in Salmonella, so a large-scale outbreak due to a hitherto rare serovar may occur as exemplified by the 1999 semidried squid incident. Under the circumstances that multidrug-resistant S. Typhimurium DT104, being prevalent in Western countries, are found in Japan, it is necessary to pay attention to the trend of Salmonella food poisoning outbreaks and the serovars of the organisms involved. It is important to enlighten the public on prevention of food poisoning. The right storage and handling food materials for the forthcoming summer is also needed.