On Jan. 7, 1998, the Field Epidemiology Training Program received a telephone report from a physician about patients with gastroenteritis who had eaten at a restaurant in Riyadh. One hospitalized woman stated that similar cases had been reported after a dinner hosted by a bank on December 28, 1997 catered by the same restaurant. We began an outbreak investigation to identify the source of the infection and to make preventive recommendation in order to prevent any future foodborne outbreaks.
Methodology
A retrospective cohort study was carried out. The 37 bank party attendees and a total of 12 members in 7 families, which included 7 community cases, who shared food from restaurant A between Dec. 29-31, 1997, were asked about the restaurant foods they ate and any illness that followed to identify the food items responsible for the outbreak. The attack rate (AR) was then calculated and the relative risk (RR) for each food item, p-values ≤ 0.05 were considered statistically significant. We visited the restaurant and inspected egg storage and tiramisu preparation method. A thermometer was used to measure the internal temperature at various points in the process.
Results
In the party outbreak, 37 Saudi women who attended the bank party on Dec. 28, 1997 were interviewed; 18 had gastroenteritis symptoms. The attack rate for diarrhea among 14 women who ate tiramisu was 100% (RR=4.25, P-value=0.00002, 95% CI 1.80 -10). In the community outbreak, there were 9 cases among 12 persons in 7 families who ate food from the restaurant. Case-patients had eaten tiramisu in the restaurant from Dec. 29-31, but none had eaten tiramisu on the day of the bank party. The AR for diarrhea among family member who ate tiramisu was 100% (RR= infinity, P-value 0.007). All epidemiological evidences in both the bank party and the community outbreaks showed that there was a high significant association between eating tiramisu and developing illness. No other food item was associated with illness. Of all the patients, nine cases of Salmonella group D non-typhi were grown from stool samples. Raw egg yolks were used in the tiramisu preparation, all from local farms. Eggs were stored after purchase at ambient temperature in the restaurant. The tiramisu was left at ambient temperature (25C) throughout loading and transportation to the bank party until serving time - more than 4.5 hours of time-temperature abuse.
Conclusion
Two salmonella outbreaks were caused by tiramisu dessert served by restaurant A, Riyadh. Raw eggs were the likely source of infection. Time-temperature abuse of the tiramisu used for the bank party and leftover tiramisu subsequently served in the restaurant were the major factors in producing these explosive outbreaks.