Sheep flocks across the North may have foot-and-mouth but are not showing clinical signs of the disease, one of the State's leading veterinarians has said.
The vice-president of Veterinary Ireland, Mr Sean O Laoide, said that because there was no tangible link between the three confirmed outbreaks in the North, the assumption was that sheep were infected.
Because there was such a distance between the outbreaks, it was very hard, epidemiologically, to say how it had moved.
There must be a situation, he said, where the disease had travelled in sheep.
However, this strain of the virus means sheep do not readily exhibit the clinical symptoms. Furthermore, sheep may have shown symptoms but may have since recovered, said Mr O Laoide.
If the virus infects a flock of sheep, only 2 per cent exhibit symptoms, but other animals can continue to be infected. And these sheep remain potential sources of further infection for nine months.
"The danger is that this is what has happened," he said.
There are worries that a similar situation could occur in this State, said Mr O Laoide.
Cattle, on the other hand, more readily show the symptoms of the disease.
The cattle are currently being housed, but there are fears that when they go out to graze on land next to sheep, they will become infected, he said.
A similar situation had occurred with the North's third confirmed outbreak, at Cushen dall, Co Antrim, where the herd-owner first noticed that lots of lambs were dying, he explained.
One of the indicators of the strain was the death of lambs, but foot-and-mouth was diagnosed at the farm only when the virus spread to cattle, said Mr O Laoide.
It was easy to trace the source of the outbreak at Meigh, Co Armagh, from sheep smuggled from Britain, but it was not so easy to determine the source of the latest outbreaks.
Farmers are being urged to check their animals at least twice a day for signs of infection.