Epidemiology and Infection

Water and gastroenteritis

An extensive gastroenteritis outbreak after drinking-water contamination by sewage effluent, Finland

J. LAINEa1a2 c1, E. HUOVINENa1, M. J. VIRTANENa1, M. SNELLMANa1, J. LUMIOa2a3, P. RUUTUa1, E. KUJANSUUa4, R. VUENTOa3a5, T. PITKÄNENa1, I. MIETTINENa1, J. HERRALAa2, O. LEPISTÖa6, J. ANTONENa2a4, J. HELENIUSa3, M.-L. HÄNNINENa7, L. MAUNULAa7, J. MUSTONENa2a4, M. KUUSIa1 and Pirkanmaa Waterborne Outbreak Study Group

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)


c1 Author for correspondence: Dr J. Laine, Lylykuja 3, 33100 Tampere, Finland. (Email: janne.laine@fimnet.fi)


† The Pirkanmaa Waterborne Outbreak Study Group are listed in the Appendix.