1. Water and Health Unit, Department of Environmental Health, National Institute for Health and Welfare (THL), P.O. Box 95, FI-70701 Kuopio, Finland
2. Department of Environmental Science, Faculty of Science and Forestry, University of Eastern Finland (UEF), P.O. Box 1627, FI- 70211 Kuopio, Finland
3. Bacteriology Unit, Department of Infectious Disease Surveillance and Control, National Institute for Health and Welfare (THL), P.O. Box 30, FI-00271 Helsinki, Finland