On May 10, 1998, students from the hostel of the Institute of Health Sciences in Muscat began to present at major hospitals with gastroenteritis. All ill students normally ate at the hostel restaurant. We began an investigation to determine the cause of the outbreak and to develop measures to prevent similar situations in the future.
Methodology
A case was defined as any student from the hostel who developed diarrhea or abdominal pain between 10 and 17 May 1998. A confirmed case was defined as a case with Salmonella enterica, serovar enteritidis (SE) recovered from a stool specimen. We distributed a questionnaire covering food history and symptoms to all students. In a retrospective cohort study we calculated risk ratios for each food in comparison with not eating the food and in comparison with a reference food. We interviewed the food handlers regarding food preparation.
Results
We identified 123 (attack rate [AR]=49%) cases among 249 students. These included 72 confirmed cases. All 123 (AR=78%) cases appeared among the 158 students who ate the meal of May 9, compared with 0 of 91 students who did not eat this meal (risk ratio [RR]=infinity, p<0.001). Illness developed between 2 and 37 hours (median 13 hours) after eating. AR for chicken was 73% (RR=4.36, 95% confidence interval [CI]=1.66-11.45), for egg mousse 100% (RR=6.0, 95% CI=2.45-14.68) and for chapatti 100% (RR=6.0, 95% CI=1.66-11.45). By using only freshly or commercially prepared food as a reference category and comparing each suspect, the RR increased dramatically -- for chicken RR=13.82 (95% CI=1.98-96.33), egg mousse RR=19.0 (95% CI=2.82-128.02), and chapatti RR=19.0 (95% CI=2.82-128.02). The chicken had been thawed for 12 hours at room temperature and left after cutting an additional nine hours before cooking. The egg mousse contained more than 100 uncooked eggs. One person prepared both the chapatti dough and the egg mousse at the same time. On this same day the water supply to the kitchen was interrupted.
Conclusion
This major outbreak of SE resulted from three foods prepared in the hostel kitchen and subjected to cross-contamination and time-temperature abuse during preparation. Based on these results, we recommend that all kitchens under contract with the Ministry of Health should be subject to standards of safety in food preparation with emphasis on avoidance of cross-contamination and time-temperature abuse.