Gardai began to seal off all but 30 of the roads crossing the Border yesterday in an attempt to stem any further movement of livestock into the Republic.
The exercise is identical to that of 1967, when there was a major outbreak of foot-and-mouth in Britain. On that occasion the disease did not reach the island of Ireland.
The Garda Commissioner, Mr Pat Byrne, began moving additional personnel to the Border last Thursday at the behest of the Government.
His spokesman, Supt John Farrelly, said yesterday that 442 gardai had been posted to the Border by Wednesday evening and a further 55 had arrived yesterday.
There were already 1,300 gardai stationed in the Border area and many of them had been working lengthy shifts.
About half the road crossings between Northern Ireland and the Republic had checkpoints by yesterday, according to the Garda.
However, roads were still open yesterday afternoon in the so-called exclusion zone around Meigh in south Armagh. There had been no request by yesterday evening for the Defence Forces to move troops up from its Southern Command.
About 200 soldiers from the Western and Eastern Brigades were on Border duty, but only about a quarter of these were on patrol at any given time.
A spokesman for the Defence Forces said they had not, by last evening, received any request for additional resources at the Border. But the Chief of Staff, Lieut Gen Colm Mangan, had directed that the Southern Brigade prepare to move northwards.
Last night the Department of Defence was considering the legality of a request from the Department of Agriculture to provide soldiers to monitor traffic at sea ports. There was no precedent for the military to respond to a request from any branch of Government other than the Garda.
The "approved" roads include such routes as from Dublin to Belfast, Derry and Letterkenny, and crossing points such as Aughnacloy, Strabane and Swanlinbar.
The small "unapproved" roads were beginning to be sealed off last night and people were being refused permission to pass in either direction. Gardai at the crossing points were directing traffic to "approved" routes where, in some instances, very large queues were forming.
But yesterday afternoon traffic, including lorries, was still crossing via side roads near the infected farm in south Armagh.
On RTE Radio last evening Supt Farrelly said a further 13 checkpoints had been set up during the day in the Louth/Armagh area.
Some of the checkpoints were half a mile or so inside the Republic, he said, but this was because the checks were at points were roads converged.
There were 141 checkpoints in position along the Border by Wednesday evening. There are approximately 300 crossings and in some cases the same road crosses the Border more than once.