An E.coli alert among nursery school children in Belfast was extended tonight after the detection of two further probable cases.
One child has been under treatment in a Belfast hospital since last week after being diagnosed with the illness.
The child is a pupil at the Ravenscroft Nursery School in East Belfast where screening of all other pupils and staff today led to the detection of the two further probable cases of E.coli 0157.
The results of tests on the last 25 pupils are expected in the next few days.
The Eastern Health and Social Services Board said the two pupils had been involved in an exchange visit last week with a second nursery school in the west of the city.
As a result, around 60 staff and pupils at that school - the Cathedral Nursery School in the Lower Falls area - were now being offered screening for the infection, which causes severe diarrhoea and vomiting.
The health board said it had extended the screening based on the expert national advice of the Health Protection Agency because of the special risk factors associated with E.coli 0157 and children of pre-school age.
Dr Lourda Geoghegan, a public health expert with the board, said the extension of the investigation was precautionary and urged parents, staff and children at Cathedral Nursery School not to be unduly concerned.
The cause of the outbreak has yet to be identified and the board is working with the Belfast City Council's environmental health service, the public health laboratory and the Belfast Education and Library Board.