Method

To investigate the outbreak, a retrospective cohort study was performed among guests of an organisation's anniversary on 10 October 2007, in Stockholm, Sweden. A case was defined as a person who experienced stomach problems following the event. People who had not been eating from the buffet were excluded from the analysis. An electronic web-based questionnaire (Figure 1) was sent to people who had given their email addresses, and paper questionnaires to the remaining guests.

Epidemiological investigation

The contact person in the celebrating organisation provided a list of what had been served on the buffet. A list with contact details for the invited guests was also obtained. Of the 93 people on the list, 58 had given their email address and 35 their postal address.

A questionnaire was developed containing questions about onset of disease, symptoms and food items consumed from the buffet. The questionnaire was designed to be web-based with the help of a commercially available programme [2]. An individual link to the web-based questionnaire was distributed to the 58 guests with an email address, while the rest of the guests received a modified questionnaire through the post with a pre-paid response envelope included. Because an individual link was used, no names or addresses were recorded in the response database.

Results

Sixty-seven people (72%) answered the questionnaire. Among those who had received the questionnaire through a personal link, 78% (45/58) responded, while the response rate among those who had received it via ordinary mail was 63% (22/35). The database created by the web programme was updated with the results from the ordinary mail questionnaires.

Ten people had not consumed anything from the buffet and all of them remained healthy. Fifty-seven people had consumed food from the buffet, and 22 of them experienced stomach problems following the event. The day of onset of symptoms for each patient is shown in Figure 2.

Four patients had stool cultures positive for campylobacter and two of them were hospitalised due to a severe infection. It is not known how many people were sampled in connection to this outbreak, since there is no register on negative stool cultures. The median age was 54 years among the affected guests and 52 years among the healthy guests, with no significant difference regarding gender. Diarrhoea dominated among the symptoms, see Figure 3.

Discussion

The cohort study revealed that the marinated chicken was the food item suspected to have harboured the campylobacter bacteria. Raw or under-done chicken is a well-known source of campylobacter infection. Of the 206 reported Swedish food-borne during 2005, nine outbreaks with a total of 38 affected persons were caused by campylobacter, and among the sources stated were chicken and chicken liver [3].

Since the outbreak was detected rather late, we will not be able to clarify fully what had happened during the preparation of the chicken that was served at this event.

This outbreak investigation showed how convenient it is to use web-based questionnaires in this kind of investigations. Most advantageous was that data did not have to be entered from a questionnaire form into a database, which saved a lot of time. The programme also created a code book in which every question and the alternative answers were stated. Another advantage over questionnaires sent by ordinary mail was that the response from the people who had received web-based questionnaires was much quicker. A problem with web-based questionnaires at the moment is that the servers on which all the data is stored, belongs to the companies from which the data programmes are hired. If the programme is bought, an extra server is necessary that is not protected by a firewall. This must be considered when thinking of the private data and personal integrity of the people answering the web-based questionnaires. A solution to this problem would be welcome.