The Minster for Agriculture Mr Walsh today described thesecond outbreak of foot-and-mouth in the North as "harrowing".
He said his experts were baffled by the outbreak as the North had gone six weeks without a reported case.
Expert opinion said the incubation period of the virus was 28 days.
Mr Walsh ordered the total import ban on all animal-based products from the North. He also urged a redoubling of efforts to prevent the spread of the disease to the Republic.
Northern Ireland Agriculture Minister Ms BrídRodgers confirmed the North's second foot-and-mouth case. A second set of tests on the dairy herd in Co Tyrone was positive.
Initial test results on the blood samples from the herd in Ardboe near Cookstown were negative but a statement from Ms Rodgers said her officials were now viewing it as "a positive case."
The statement follows a British government briefing that the average daily number of confirmed foot-and-mouth cases fell by a quarter last week.
Meanwhile the president of the Irish Creamery Milk Suppliers' Association Mr Pat O'Rourke is describing the second outbreak of foot and mouth disease in the North as frightening.
Mr O'Rourke urged all farmers in the State to check their livestock for disease.