Epidemiological explanation of an outbreak of gastro-enteritis in Sweden in the absence of detailed microbiological information

Waterborne gastroenteritis outbreaks have often gone undetected or been inc ompletely defined in terms of source and extent. Methods which allow detect ion or clarification of such events are therefore useful. We describe the m ethods used to detect and investigate such an outbreak. In autumn 1996 high school absence rates and the rate of parents absent from work to care for sick children suggested a health problem in a Swedish town which had a hist ory of unexplained outbreaks of gastrointestinal disease. A systematic samp le of 300 households was surveyed by post. Respondents represented 10% of t he total population of the town. Questions concerning symptoms and exposure s were included. The same questionnaire was used in a nearby town as a cont rol. Sixty four percent of respondents reported an acute gastrointestinal i llness during a two month period. Diarrhoea (90%) and abdominal pain (88%) were the most frequent symptoms among the sick. Two percent of those sick s ought medical care. Exposures associated with disease were being a member o f a large household, young age, and consumption of water from the community water supply. Attack rate showed a dose response relationship with increas ing frequency of water consumption. The peak incidence of gastrointestinal illness occurred shortly after raw water quality control data had shown a r ise in indicator bacteria. Further analysis, dividing those infected into g roups according to when they became ill and whether they were the first mem ber of their household to fall ill, supported the hypothesis of primary cas es being infected from the water supply with some secondary person to perso n spread.