Methods
On 30th July 2002, the Suffolk Communicable Disease Control team (SCDC) was informed by the consultant microbiologist that S. Enteritidis had been isolated from stool samples of five patients. All had recently eaten a meal in a local Chinese restaurant. Further enquires revealed that there were more patients with similar food history and gastrointestinal symptoms. An Outbreak Control Team was convened on 31st July and it was decided that a full investigation should be carried out to identify the extent of the outbreak, the probable vehicle of infection and to advise on the appropriate control measures.

Epidemiological
The environmental health department (EHD) staff initially gathered information from people who had become ill on a standard data collection form. In the initial stages of the investigation, it became apparent that all those who became ill had eaten or had bought a take away at the restaurant on 27th July 2002. The information collected included name, address, sex, their symptoms and date of onset. The restaurant provided the list of food items that were served/sold on the day in question. This menu extended to 40 food items. This list was shown to the restaurant patrons and they were asked to state the food items they had eaten. A variety of ways was used to identify further cases including the technique of snowball sampling. This involved asking the patrons whether they were aware of any others who had similar symptoms and had eaten in the restaurant. General Practitioners providing primary care in the area were contacted and were requested to check for patients with gastrointestinal symptoms. The presenting symptoms of the patrons were diarrhoea, headache, abdominal pain and fever.
The next step involved interviewing all those who had eaten/bought food on 27th July whether they became ill or not. The restaurant provided table-booking details. The following case definition was adopted for the outbreak. "Symptoms of acute gastroenteritis including one of the following: diarrhoea, vomiting or abdominal pain up to 96 hrs after having had a meal from the said restaurant including takeaway between 22 and 30 July 2002 and/or individuals who have positive stool sample for S. Enteritidis up to 96 hrs after having a meal from the restaurant including a takeaway between 22 and 30 July 2002".
An analytical investigation was carried out using a retrospective cohort design. Efforts were made to identify anyone who ate or bought food at the restaurant on 27th July. Eligibility for membership of the cohort was defined as a person having the opportunity to eat any of the food items available on the day.

Statistical methods
Data was entered in to Statistical Package of Social Sciences version 10 [5]. Food specific attack rates and the corresponding two tailed p vales were derived by Fisher's exact test [6]. Univariate relative risk (RR) and 95% Confidence Intervals (CI) were calculated using standard cohort analysis [7].

Microbiological
Stool samples were requested from all who had eaten food from the restaurant on 27th of July. Environmental sampling was not carried out as this was considered to be of limited value. There was no food left over from 27th July, but three food samples were taken on 30th July and sent for analysis to the food laboratory at Chelmsford Public Health Laboratory. Stool specimens were sent to Ipswich Hospital microbiology laboratory and were cultured for the presence of Salmonella sp. Isolates of Salmonella were forwarded to the Laboratory of Enteric Pathogens at Central Public Health Laboratory, Colindale for phage typing. Standard procedures were adopted for phage typing at the laboratory [8].

Environmental
The EHD staff inspected the premises including verifying the procedures for hazard analysis and critical control point (HACCP). Egg storage and preparation of egg items were also investigated during the visit. Efforts were made to trace the egg trail back to the supplier.