a1 National Institute for Health and Welfare, Helsinki and Kuopio, Finland
a2 Tampere University Hospital, Tampere, Finland
a3 University of Tampere, Tampere, Finland
a4 Nokia Health Centre, Nokia, Finland
a5 Centre for Laboratory Medicine, Tampere, Finland
a6 Environmental Health Control Unit, Pirkkala, Finland
a7 University of Helsinki, Helsinki, Finland
An inappropriate cross-connection between sewage- and drinking-water pipelines contaminated tap water in a Finnish town, resulting in an extensive waterborne gastroenteritis outbreak in this developed country. According to a database and a line-list, altogether 1222 subjects sought medical care as a result of this exposure. Seven pathogens were found in patient samples of those who sought treatment. To establish the true disease burden from this exposure, we undertook a population-based questionnaire investigation with a control population, infrequently used to study waterborne outbreaks. The study covered three areas, contaminated and uncontaminated parts of the town and a control town. An estimated 8453 residents fell ill during the outbreak, the excess number of illnesses being 6501. Attack rates were 53% [95% confidence interval (CI) 49·5–56·4] in the contaminated area, 15·6% (95% CI 13·1–18·5) in the uncontaminated area and 6·5% (95% CI 4·8–8·8) in the control population. Using a control population allowed us to differentiate baseline morbidity from the observed morbidity caused by the water contamination, thus enabling a more accurate estimate of the disease burden of this outbreak.
(Accepted August 10 2010)
(Online publication September 15 2010)
† The Pirkanmaa Waterborne Outbreak Study Group are listed in the Appendix.