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A severe waterborne outbreak occurred in Nokia city, Finland in November 2007. A cross-connection between waste water system and drinking water pipeline caused a severe contamination of drinking water distribution network. The first signals of contamination include odd colour, taste and smell in the tap water. The appearance of the first illness cases finally resulted in the recognition of the outbreak. Water analyses showed extremely high counts of indicator bacteria in drinking water. Later on Campylobacter sp., Giardia sp., noroviruses , Salmonella enteritidis, rota-, entero-, astro- , rota- and adenoviruses were also detected from water. Flushing of the network was started immediately after the faulty connection was removed. The first mitigation action besides the boiling water advisory was chlorination (1.5 - 3.0 mg/l). Although, the indicators disappeared within a few days, pathogenic microbes were found for over two months. Even enhanced chlorination was found insufficient to clean the badly contaminated pipelines. This resulted in the use of pipeline internal gauging (pigging), air/water pulsed flushing and shock chlorination (10 mg/l, 24 h) to clean the pipelines. The network was finally declared to be clean in February 2008 when no adenoviruses nor noroviruses could be found from water or deposit samples. The Nokia case was the largest waterborne outbreak in Finland for at least 20 years: a population survey conducted in Nokia and a neighbouring town showed a total of 8,450 cases of gastroenteritis during the outbreak; 1000 persons sought care at the municipal health centre and nearly 200 were treated in hospital.

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