Results

Epidemiological
Data were gathered from 52 subjects who had eaten food from the restaurant on 27th July of whom 38 developed symptoms and 14 were free of symptoms. Of the 38 who became ill 16 were male and 22 were female. The mean age was 27 years. The mean incubation period was 30 hours with a range of 6 to 90 hours suggesting a point source outbreak. Two patients received hospital care and there were no deaths. No gastrointestinal illness was reported among the kitchen staff of the restaurant in the weeks before or during the outbreak.
On investigation of food preparation practices at the restaurant, it appeared dishes containing egg were the most likely vehicle for this outbreak. However, this information was not discussed when gathering data from the subjects. Data was gathered in a standardised format from all subjects to avoid any interviewer or recall bias.
We had a strong "a priori" hypothesis that people who had eaten egg or food that had come in to contact with egg were at an increased risk even before looking at the data and these were analysed first. During analysis it became apparent that illness was significantly associated with the three food items that contained egg or the egg rice mixture as shown by the food specific attack rates (Table 1) and the increased relative risks (Table 2). When many other food items eaten on the day including pork-fried rice and a variety of fish dishes were analysed none of which showed an increased attack rate or was significant in the cohort analysis.
Table 1  Specific attack rates of suspected foods  * Fisher exact test
Table 2  Relative risk and 95% Confidence Intervals of suspected foods

Microbiological
A total of 31 stool specimens were submitted to the laboratory from which S. Enteritidis was isolated in 29. Twenty-eight of these 29 isolates were confirmed to be PT 34a and one as PT4. No pathogens were isolated from food samples taken from the restaurant on 30th July.

Environmental
During the visit to the restaurant, the EHD staff reported that there was no evidence of hazard analysis and noted many cleaning and maintenance issues. Hand wash facilities were inadequate. The chef explained that after preparing the egg rice mixture, it was left out at room temperature for the rest of the evening, and reheated when ordered. This mixture was used for some of the other fried rice items. It was estimated that on the evening of the 27th of July the egg rice mixture was left at room temperature for seven hours. The restaurant received eggs from a supplier in London every week and they were not refrigerated. Attempts to trace the egg trail were not successful.

Control measures
The restaurant closed voluntarily on 31st July and the EHD staff reassessed the situation on the evening of 1st of August. As they were satisfied with the arrangements, the restaurant was allowed to reopen. The owner and restaurant staff were provided with information on proper cooking methods and the importance of undertaking HACCP.