All passengers travelling by ferry from Wales to Dublin and Dun Laoghaire are now being disinfected against foot-and-mouth disease.
Foot passengers' shoes and the tyres of all vehicles are being doused in disinfectant in Irish ports.
Holyhead, the main port of exit from Wales, is only 20 miles from the Gaerwen industrial estate, the site of the abattoir where a sheep was confirmed as having foot-and-mouth disease yesterday.
Ferry booking staff at the Holyhead terminal were yesterday questioning customers phoning to book a passage on the ferry about their movements, asking whether they had been on a farm or in recent contact with animals or livestock.
A spokeswoman for the port authority in Holyhead said: "We are responding fully to the guidelines laid down by the Minister for Agriculture in Ireland and the Irish Veterinary division, as well as the United Kingdom Ministry of Agriculture."
Ms Sue Williams, secretary to the port manager, said that from midnight last night a total ban was imposed on all imports of horses, greyhounds and working dogs.
This was in response to a decision by the Irish Minister for Agriculture, she said.
Stena Ferries have a policy of not carrying livestock. However, horses for breeding and racing are carried.
Yesterday horses intended for travel to Ireland were being taken to Carnals Stables in Rhosneigr on Anglesey.
The precautionary measures, however, will do nothing to prevent contamination from passengers who travelled before last weekend.