Between the 15^th to 17^th of January 2008, 92 patients sought medical care at hospitals and primary health care centers in Najran city, complaining of gastrointestinal symptoms: diarrhea, fever, vomiting, nausea, and abdominal pain, after eating from a newly opened restaurant. An epidemiological investigation was started to identify the food item(s) responsible for the outbreak and determine the source of infection.

A case control study was conducted. A case was defined as any person who ate from the restaurant between 14/1/2000 to 16/1/2008 and developed diarrheal illness within two days of food consumption. A control was defined as any person who ate from the same restaurant within the same time period and had not developed diarrheal illness during the period of the outbreak. A sample of 50 cases and 50 controls were obtained.

All the cases developed diarrhea (100%), fever (92%), abdominal pain (88%), nausea (84%), vomiting 36(72%) and chills (32%). Mayonnaise salad demonstrated the highest attack rate (AR) and Odds Ratio (OR) (AR = 93.8%, OR = 21.0, 95% CI = 2.6 – 166.5, P < 0.001), followed by broasted meal (AR = 84.4, OR= 19.5, 95% C.I = 6.9 – 54.4, P<0.001). Among those who had eaten chicken shawarma, 84.6% became sick (OR=6.8, 95% C.I = 1.4 – 32.4, P<0.007).

Salmonella enteritidis group D was isolated from 80% of the patients who consented to give stool or rectal swab specimens. All cultures taken from the restaurant food handlers and food items showed no growth for any pathogens. On inquiry, it was found that Mayonnaise was prepared at the restaurant from blending egg yolk, oil and garlic. This was done by restaurant staff two to three times a week. During the day, most of the prepared mayonnaise was distributed in small containers to be served with the broasted meal and kept not far from the oven. At the end of each day, unused mayonnaise was kept for use on the following day, when it was sometimes mixed with a new batch of mayonnaise.

This study is a classical example for a Salmonella food poisoning outbreak, where the clinical, epidemiological, and laboratory data point to Salmonella enteritidis group D as the most likely causative organism.

In the USA, review of results of laboratory-confirmed food poisoning surveillance showed that Salmonellosis was the second cause of food poisoning in 1997.⁵ A similar study in Saudi Arabia reviewed all computerized data for foodborne diseases for the years 1411 – 1413 H (1991 – 1993) reported 781 events of food-borne diseases from 18 regions. There were 6,052 cases, of which 3,515 required hospitalization. No deaths due to foodborne disease were reported. The highest rate was reported from Riyadh region, followed by Taif. Food prepared in restaurants accounted for 32% of events. Staphylococcus aureus was the most commonly implicated organism, followed by Salmonella. The most common contributing factors were poor storage, unsafe food sources and inadequate refrigeration.⁶ Another study in the Eastern province reported that Salmonella was the causative organism in 33% of food poisoning outbreaks during the period 1991-1996.⁷

The most common source of Salmonella food poisoning is poultry, meat, milk, cream and eggs. Investigations of Salmonella outbreaks indicate that its emergence is largely related to consumption of poultry or eggs.¹⁴

This common source outbreak of Salmonella resulted from restaurant prepared mayonnaise. The serotype enteritidis suggest that the organism originated in the egg and raw egg product. Heavy contamination probably resulted from temperature abuse of mayonnaise and eggs causing high infectivity.

To prevent foodborne outbreaks in general, it is required that food handlers wash their hands thoroughly and frequently, before, during and after handling the food; use clean gloves or utensils while handling food; maintain a sanitary kitchen; thoroughly cook meats; avoid cross contamination between raw and cooked food; protect prepared foods against rodent and insect contamination; reduce time between food handling and service; and maintain proper temperatures of cooked foods.^2,9

Specific additional measures for prevention of Salmonellosis outbreaks include improved hygienic practices in poultry farms and abattoirs, avoidance of eating raw or cracked eggs, and effective chemotherapy of infected food handlers and discouraging them to handle food while shedding the organism.⁹

It was recommended to prohibit the practice of mayonnaise preparation at restaurants and advocate the use of packed commercial pasteurized mayonnaise instead. Restaurant supplies of raw food should be kept immediately after purchasing in refrigerators. Other concerned Saudi authorities should be involved in order to intensify the supervision of restaurants and food handlers.