A warning that all water in the public supply in Knock, Co Mayo, must be boiled before consumption has been issued following the discovery of E.coli bacteria.
Its presence indicates contamination by faecal matter from humans or farm animals.
People using group water schemes connected to the Knock supply have been advised to boil water before consuming it. Up to 500 homes are connected to the supply. It also serves thousands of pilgrims visiting the Knock Shrine daily. The alert was issued by Mayo County Council and has been in operation since last weekend. The source of the contamination has not been confirmed, according to Mayo's chief health officer, Mr Cathal Carney of the Western Health Board.
He added: "During sampling on Friday last, we discovered E.coli in two water samples taken in separate locations along the Knock supply. Automatically when E.coli is identified we inform the county council, the body responsible for the supply, and we inform food premises in the area.
"We are continuing to sample the water in the Knock supply and as soon as we are satisfied that the samples are clear we will be informing the local authority to lift the boil alert. The alert is often lifted without ever locating the cause of the contamination," Mr Carney said.
