In May 1996, an outbreak of gastroenteritis occurred among customers who bought dinner from a restaurant that specialized in fried chicken in Abha city, southwestern Saudi Arabia. We conducted an outbreak investigation to determine the source of the outbreak.
Methodology
Cases were defined as persons who developed diarrhea and abdominal pain, with fever, nausea or vomiting within 72 hours of eating at the restaurant "A" on May 7, 1996 in Abha city, and or Salmonella spp. was isolated from stool culture or a rectal swab. Case-patients were interviewed about foods eaten. The relative risk (RR) and the corresponding 95% confidence intervals (95% CI) were calculated for each food item.
Results
Of 228 persons with food poisoning met the case definition (mean age + SD 19 + 9 years), 158 case-patients were interviewed. The median incubation period was 10 hours (range: 3 and 27 hours). Out of ten food items served, only mayonnaise (RR 2.52; 95% CI 1.71-3.73) and minced garlic (RR 1.20; 95% CI 1.02-1.41) were associated with cases. Salmonella enterica was isolated from 124 (84.2%) out of 159 persons with symptoms of food poisoning, and 91 (73.4%) were serogroup enteritidis phage type B14. Mayonnaise was prepared in the restaurant using a regular blender. Lemon-flavored salt was substituted for vinegar. Minced garlic was prepared with same blender immediately after making the mayonnaise. The mayonnaise and garlic were stored as unit servings in little plastic containers at room temperature for a median of 6 hours before being served for dinner.
Conclusion
This outbreak was due to mayonnaise prepared in the restaurant. Unsafe storage of the mayonnaise at room temperature could have resulted in overgrowth of bacteria. The relatively short incubation period is probably due a high infective dose of bacteria per serving. It is advisable that restaurants use commercially prepared mayonnaise or use only pasteurized eggs for preparation of food items that require raw eggs as an ingredient.