Pupils and staff are to be tested for the E.coli infection
An open farm has begun an intensive cleaning programme as a precaution after a child who visited it was found to have the E.coli infection.
Five people who tested positive for the bug visited or had an association with the Streamvale farm outside Belfast.
It closed voluntarily this weekend as 90 children and staff at three primary schools are set to be tested for the infection.
A P1 pupil at Rosetta Primary School was found to be carrying the bug.
Pupils at it and two other south Belfast schools recently took part in an excursion to the open farm.
The other pupils are from Cregagh and Holy Rosary primaries.
The Eastern Health Board said other people who had contracted the infection in recent weeks had some contact with the farm.
A spokeswoman for the farm said it was closing as a precautionary measure.
"The source of the infection remains unclear and the EHSSB stresses that the risk to the wider community remains very low," Judith Morrow said.
"We are confident that our intensive cleaning programme over the weekend will further reassure the Health and Safety Executive (HSE) on Monday and that we will immediately reopen for business."
Dr Philip Donaghy, from the Eastern board, said: "What we're doing is screening all those children that visited that particular farm on that particular day.
"They did it almost as part of a joint visit so we want to be sure that all these children are looked at and tested."
The Eastern board has screened more than 500 people for E.coli since the beginning of March after previous cases were identified.
To date, a total of 14 cases have been now confirmed as having had the infection, of whom three live outside of the board's area.
Five of these people are now known to have either visited or had an association with the farm shortly before testing positive for the bug.
Children and pensioners are especially vulnerable to E.coli 0157 which is normally found in the intestines of people and cattle and can be passed on by eating infected food and liquid.