The Topic of This Month Vol.22 No.8(No.258)

In Japan, Staphylococcus food poisoning used to occur frequently in the 1970s and the early 1980s, but thereafter it was on the decrease. In the late 1990s, both incidents and cases accounted for only 2 to 5% of all bacterial food poisoning. People's attention to Staphylococcus as an etiological agent of food poisoning was on the gradual decrease. An outburst of Staphylococcus food poisoning broke out due to tainted milk products in Osaka and neighboring prefectures in June-July 2000 (the Snow Brand low-fat milk incident). This outbreak, being the largest scale ever occurred in Japan after the end of World War II, implicated 13,420 notified cases, greatly outnumbering the 10,476 cases of the Salmonella food poisoning incident that occurred in Hokkaido in 1988. This incident has exerted a vast social influence and people have recognized anew the importance of Staphylococcus food poisoning as a foodborne bacterial intoxication (see p. 188 of this issue).

Staphylococcus is a gram-positive, facultative anaerobe, and asporogenous coccus of a member of Micrococcus . At present, it is classified into 34 species. Since it forms normal flora of the skin, upper respiratory and intestinal mucosa of man and animals of different species, Staphylococcus is detected ubiquitously in the human environment and in food. Staphylococcus food poisoning is caused exclusively by Staphylococcus aureus . S. aureus multiplies and produces heat-stable enterotoxin in food, and when such food containing the preformed toxin is ingested, intoxication will follow (see p. 187 of this issue for Staphylococcus enterotoxin). Although some strains of S. intermedius and S. hyicus are known also to produce enterotoxin, they have never been reported to cause foodborne intoxication. Since S. aureus is able to grow rapidly in the presence of 10% sodium chloride and retarded growth could be seen in its presence in even 15%, growth inhibition with sodium chloride alone is not feasible. Since it does not grow at temperatures below 10��, growth inhibition is possible by temperature control.

Bacterial foodborne intoxication is characterized by a short incubation period in contrast to foodborne infections with Salmonella or Vibrio parahaemolyticus . Symptoms of Staphylococcus food poisoning, such as nausea and vomiting, often accompanied with diarrhea, usually develop in 1-6 hours after ingestion of enterotoxin. Sometimes, fever has also been reported. When healthy persons are affected, they will recover in 6-24 hours without any special treatment, and the prognosis is usually good. However, severe cases must see a physician for receiving adequate symptomatic treatments.

The annual incidents and cases of Staphylococcus food poisoning during the past 30 years in Japan are shown in Fig. 1. Before 1984, there used to be about 200 incidents every year, accounting for 25-35% of all bacterial food poisoning incidents. After 1985, incidents were on the rapid decrease, and in 1996, there were fewer than 50 incidents, accounting for lower than 5% of all bacterial food poisoning incidents. Yearly cases reached 8,000 some years in the 1970s, but in the 1980s, cases decreased gradually from 5,000 to 3,000, and in the 1990s, there were less than 1,000 cases in some years. However, in 2000, there were over 14,000 cases due to the extraordinary large-scale outbreak.

The foodstuffs incriminated during 1995-1999 were grains and composite ready-to-eat food (Fig. 2) (see p. 191-193 of this issue and IASR, Vol. 22, No. 2). The foodstuffs under category of grains often included "rice balls". With regard to the place of preparing food, both many cases and incidents were ascribed to restaurants (Fig. 3). Outbreaks at homes ranked second in regard to the number of incidents, but receded to fourth in regard to the number of cases. This is due to the small number of cases per incident occurring at home. The tendency of many incidents from homemade rice balls has little changed. As can be seen from the monthly incidence, Staphylococcus food poisoning is concentrated in May to October, the season of high temperature (Fig. 4).

Because of such short incubation period of Staphylococcus food poisoning, samples of incriminated foodstuffs can easily be obtained. Usually, the organisms grow in the incriminated food, so the patient should have ingested both enterotoxin and a large number of viable organisms. Therefore, from both incriminated food and patient specimens (vomit and stool), S. aureus could be isolated. Causal relation between an incident and the incriminated foodstuff largely depends on the coincidence in the coagulase, phage and enterotoxin types of the isolates. Since Staphylococcus enterotoxin is heat-stable, food poisoning may sometimes occur from such food that has sufficiently been cooked to kill S. aureus . In such cases, no organisms could be isolated from incriminated food or patient specimens.

Such was the case in the 2000 Snow Brand incident. Viable S. aureus was not detected from the incriminated food or patients' stool, and therefore it was necessary to directly detect enterotoxin (see p. 188-191 of this issue). For enterotoxin detection, simple and rapid diagnostic reagents and kits, which are now on market, are used usually (see p. 187 of this issue). In the previous incidents, the incriminated foodstuffs used to contain enterotoxin in a concentration higher than the sensitivity of such kits (0.2-2 ng/ml) making it possible to detect the toxin directly from incriminated food. Nevertheless, in the Snow Brand incident, it was necessary to treat the specimens by some appropriate method and to concentrate them to detect enterotoxin (see p. 190 of this issue).

The Snow Brand incident had some characteristic features distinct from those of previous Staphylococcus food poisoning incidents:
1. This was such a large-scale incident, implicating 13,420 notified cases.
2. The incriminated foodstuffs were such processed milk as low-fat milk and milk beverage.
3. No viable organisms but enterotoxin in such a low level (0.05-1.6ng/ml) was detected in the incriminated milk products.
4. From the skimmed milk powder, the source material for the incriminated milk products, enterotoxin (4 ng/g) was detected.
5. The temperature control of the processes of producing skimmed milk powder was defective due to an electric power cut.

From the above features, such problems as reconsideration of the conventional detection method mainly of organism detection and the establishment of a more sensitive method for enterotoxin detection, and the necessity for prevention of recurrence of such accidents as the present one have been pointed out.

The incidence of Staphylococcus food poisoning has markedly decreased from the middle 1980s, although that of other bacterial food poisoning has been on a similar level. The decrease used to be ascribable to the efforts of food processors, food dealers and those concerned with food hygiene, particularly to the wearing of gloves by all workers during food handling and thorough practice of temperature control after cooking.

On the occasion of the tragic Snow Brand incident, however, the Ministry of Health, Labour and Welfare (former the Ministry of Health and Welfare) gave notice of "a revision of the enforcement of the approval system for comprehensive sanitary control of manufacturing food (based on the HACCP system)" (Notice No. 1634 by the Environmental Health Bureau, MHW) to prefectural governors on November 6, 2000 to further promote sanitary control of food production.

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