Food poisoning, Abha City, Dhul-Hijja, 1416

On Dhul-Hijja 24, 1416 (May 7, 1996), the Department of Preventive Medicine, Asir requested assistance in investigating an outbreak of gastroenteritis that occurred among customers who bought dinner from a restaurant that specialized in fried chicken in Abha city, Asir region southwest Saudi Arabia. The Field Epidemiology Training Program conducted a retrospective cohort study to determine the extent and the source of the outbreak.

A case of food poisoning was defined as diarrhea and abdominal pain, with fever, nausea or vomiting in a person who had onset of illness within 72 hours of eating a meal prepared at take-away restaurant A, on Dhul-Hijja 24, 1416 (May 7, 1996), and/or Salmonella spp. was isolated from stool culture or a rectal swab.

We obtained a list of all patients with food poisoning who were admitted to hospitals in Abha City and neighboring towns. Only cases that met the case definition were included. The socio-demographic characteristics and the results of bacteriologic tests were abstracted from medical records. Patients were interviewed directly or over the phone and were questioned about date and time of eating dinner, symptoms, and food items eaten at dinner from the restaurant. We visited the restaurant to identify procedures including; cooking and storage temperatures, handling and storage of cooked foods, and the number of daily sales of meals on the day of the outbreak.

During the outbreak period 228 persons with food poisoning were admitted into nine hospitals; 200 (88%) males, and 28 (12%) females. The mean (± SD) age of patients was 18.8 + 9. Salmonella spp. was isolated from 124 (84.2%) out of 159 fecal specimens cultured; 91 isolated (73.4%) were found to be Salmonella enterica serotype Enteritidis; and three isolates proved to be phage type B14. The median incubation period was 10 hours (range: 3 and 27 hours). The epidemic curve suggested a common source outbreak (Figure).

Among 10 food items eaten, mayonnaise had the highest risk ratio (RR 2.52; 95% CI 1.7-3.7) followed by minced garlic (RR 1.2; 95% CI 1.02-1.4). Mayonnaise was locally prepared from raw eggs 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 minced garlic, were stored as unit servings in little plastic containers at room temperature unrefrigerated for a median of 6 hours before being served for dinner.

Salmonella spp., a common cause of food poisoning, is usually found in poultry, uncooked egg products, raw milk, and meat; in other foods, including fruits, vegetables, and pasteurized milk, cross contamination also occurs. Food items containing raw eggs, e.g., homemade ice-cream, home-made mayonnaise, cookie batter, and hollandaise sauce have been implicated in many food poisoning outbreaks [1]. Salmonella enterica serotype Enteritidis (S. enteritidis) is transmitted via the chicken's ovarian canal to the yolk [2]; S. enteritidis PT4 has been isolated from the contents of clean unbroken eggs. Ingesting 100,000 to 1,000,000,000 Salmonella bacilli will produce illness [3].

The findings of this study demonstrated that this outbreak was due to a restaurant-made mayonnaise, prepared most probably from infected eggs. Minced garlic, which was prepared immediately after and in the same blender used in preparing mayonnaise, was most likely cross-contaminated. The use of commercially prepared mayonnaise has been shown to decrease the risk of food poisoning outbreaks associated with S. enteritidis from restaurants [4].

In Riyadh City, most outbreaks of food poisoning have been attributed to Salmonella spp. Chicken "Shawrma" sandwiches, a popular fast food, in Saudi Arabia have been repeatedly implicated. Mayonnaise is an ingredient in "Shawrma" sandwiches. It is recommended that restaurants use pasteurized eggs and follow the standards recommended by the Saudi Arabian Standards Organization when making mayonnaise, or use commercial mayonnaise. Food handlers need to be educated on, and inspected for proper storage and handling of food. There is a growing need for establishing a reference public health laboratory to help characterize causative bacteria by phage typing.