Mayonnaise strikes again, and again, and again, salmonellosis from three consecutive meals in one restaurant

Between September 18 and 22, 1998, 162 gastroenteritis patients suddenly appeared at three general hospitals in southern Saudi Arabia. All reported eating food from one restaurant on September 17 or 18. We began a case-control study to identify the food or drink served that was responsible for the outbreak.

A case was defined as any person who ate from the restaurant on September 17 or 18 and developed diarrhea within the outbreak period. Diarrhea illness was defined as three or more loose stools per 24 hours. The controls were all the well companions of case-patients who had shared the restaurant food. Using a structured questionnaire, we interviewed 107 case-patients and 48 control-companions about consuming food or drinking items from the restaurant. We identified 107 cases-patients, 52 (48.6%) were male and 55 (51.4%) were female. The median age was 18 years (range 1-75 years).

All case-patients reported suffering from diarrhea (100%), abdominal pain (99.1%), vomiting (58.9%), fever (98.1%) and nausea (73.8%). The onset of illness occurred over a 46-hour period after food consumption; the median incubation period was 18 hours (range 8-46 hours) (Figure 1).

A total of 104 case-patients (97%) were more likely than control-companions, 5 (10.6%) to have eaten the mixed salad plate that contained 6 salads (Odds ratio [OR] = 291.2; 95% confidence interval [CI] = 57.4-1802) at any of 3 consecutive meals on September 17 and 18. Case-patients, 94 (88.4%) were more likely than control-companions,5 (10.4%) to have eaten either of the 2 salads with mayonnaise (OR = 68; 95% CI = 21-251). After stratification by salads with mayonnaise, other salads did not show an elevated OR (weighted OR = 1.04, 95% CI = 0.04 €“ 10.2). Salmonella group D non-typhy was isolated from 81 cases of a total of 126 examined cases. Five isolations of Salmonella recovered from patients were serotype enteritidis. Mayonnaise was prepared in the restaurant by blending 5 egg yolks with 1.5 L of oil. Mayonnaise was added to the salads as the salad plates were assembled over a 3-hour period; the mayonnaise was at room temperature the whole time. Eggs had been stored at room temperature for > 3 days.

Salmonella spp., a common cause of food poisoning, is usually found in poultry, uncooked egg products, and meat. Food items containing raw eggs, homemade ice cream, and homemade mayonnaise have all been implicated in many food poisoning outbreaks [1,2]. This common-source outbreak of salmonellosis resulted from restaurant-prepared mayonnaise served at three consecutive meals. The high attack rate, as well as the relatively short incubation period, indicate high contamination of the mayonnaise by Salmonella. Temperature abuse of both eggs and mayonnaise probably resulted in heavy contamination and high infectivity.

The findings of this study demonstrated that this restaurant-made mayonnaise was most probably prepared from infected eggs. The serotype (enteritidis) suggests that the organism originated in eggs.

In 1993, Salmonella serotype enteritidis phage Type 4 was identified from intact eggs produced in Saudi Arabia [3]. Salmonella enterca serotype enteritidis is transmitted via the chicken's ovarian canal to the yolk; ingestion of 1,000 to 100,000 S. enteritidis PT4 will produce the illness [4] Although health and municipal authorities recirculated an existing order to all restaurants requiring them to use only commercial, pasteurized mayonnaise, restaurants continue to make their own. Health inspectors should conduct periodic check-ups to assure that the regulation is followed.