An outbreak of Salmonellosis in an extended family in Riyadh

On January 21, 1995, the preventive department in Al Malaz selected primary health care center in Riyadh city was notified by Al Salame primary health care center that 14 members of four related families (median age 23 years) with acute febrile gastroenteritis had been treated at the center. All had diarrhea, fever (median 38.8C), abdominal cramps, and 60% had vomiting. No one was hospitalized. Rectal swabs from the 10 patients yielded Salmonella enteritidis.

The four families had gathered at their grandfather's house on Friday, January 20, 1995. At 9 p.m. they brought four plates of chicken shwarma (equivalent to 40 sandwiches), mayonnaise, taheena and French fries for dinner from a restaurant. During the three days preceding the onset of illness they did not share any other meal. Of the 21 persons in the four families, 17 ate the food. Of the 17 who ate, 14 (82%) became ill from 6 to 21 hours (median 13) after eating. The four persons who did not share the meal remained well. The attack rate (AR) among persons exposed (eating shwarma) was 82%, and the AR among those who did eat shwarma was 0% (RR= INF; p<0.005, Fisher's exact test). There was no association of illness with eating any other food items at that Friday dinner. In addition, S. enteriditis was isolated from left over shwarma in the patients' house.

None of the restaurant staff had been ill but a rectal swab from one yielded S. enteritidis and nasal swabs from three Staphylococcus aureus. The restaurant inspection revealed that only one table was used for all food preparation such as cutting the raw chicken, preparing shwarmas, preparing hamburgers from raw meat, and mixing raw eggs for mayonnaise. The same utensils were used for all food preparations. Each day, 80 frozen chickens (each weighing 1300g) were thawed in two containers of water large enough to hold 40 chickens per container. The chickens were kept immersed in the water for 10 hours at room temperature to thaw. After thawing, the staff cut the chicken into small pieces, removed the skin and bones, and added salt and spices. The chicken was then stored in the refrigerator until the next day when the staff started putting the chicken on the spit. It took a total of 14 hours to thaw and prepare the chicken for cooking.

Chicken shwarma was the most likely vehicle of transmission of salmonellosis in this outbreak because all 14 persons who became ill had eaten chicken shwarma and there was no association of illness with any other foods served for the Friday dinner Moreover, the ill people shared no other common meal during the four days (maximum incubation period for salmonellosis) before the outbreak. The isolation of Salmonella enteriditis from the leftover chicken shwarma also supports this conclusion. The relatively short incubation period (13 hours) and the high attack rate indicate that these people had been exposed to a relatively heavy dose (over 106 organisms per person) of Salmonella in the chicken shwarma.[1]

Salmonella may have originated in the frozen chicken or by cross-contamination or from surfaces contaminated by raw eggs, raw meat or other raw foods of animal origin. Since fresh and frozen raw chicken is often contaminated with Salmonella and since S. enteriditis frequently infects chickens and eggs, the chicken is the most likely source.[2]

Proper preparation and cooking of chicken is therefore always necessary to assure safety of any food prepared from chicken. The infected food handler is a less likely source because he did not have diarrhea and was cultured after the outbreak. His asymptomatic infection may have resulted from eating the same chicken as the affected family.

Temperatures during the thawing, preparation and cooking were not measured but were probably within the range conducive for the multiplication of Salmonella (7 to 55 C).[2] Since Salmonella bacteria divide every 20 minutes, a single bacteria may multiply to a population exceeding 106 after six hours. Cooking at temperatures above 65C for 12 minutes will kill Salmonella.[3,4] However, shwarmas are cooked only on the surface and the interior left under cooked. This family bought the equivalent of 40 sandwiches and we suspect that under cooked chicken below the cooked surface of the chicken on the shwarma spit was included in the shwarma bought by this family.

To prevent foodborne disease, emphasis should be placed on educating and training food handlers in proper food handling practices, appropriate thawing and cooking temperatures and times, and proper storage temperatures. Raw chicken should be either thawed at warm temperatures in less than one hour, or in the refrigerator at temperatures under 4C for longer periods. Preparation surfaces for raw and cooked food should be separate to avoid contact. Utensils should be washed after every use and not used for multiple purposes. Reporting outbreaks of salmonellosis and other foodborne infections helps identify and correct the type of incorrect foodhandling practices seen in this outbreak.