Potential blood donors from Northern Ireland are being turned away by the Republic’s transfusion service because of the continuing “unknown but serious threat” from vCJD, the human form of mad cow disease.

Donors who have lived for a more than a year in Northern Ireland and the rest of the UK are still banned from donating blood, said Dr William Murphy, Medical Director of the Irish Blood Transfusion Service (IBTS).

Launching its annual report, he said they also have to ban people who had transfusions in the past and “these constitute a large percentage of potential donors”.

There is no test yet for vCJD, which caused the death of four people in the Republic, none of which were related to blood or blood products.

The IBTS applied to the Department of Health for a new blood filter, designed to remove the vCJD infectious agent, but officials say more evidence of its effectiveness is needed. He also warned other donors are having to be deferred for weeks or months because of other emerging threats, including the West Nile virus and chikungunya, two viral infections spread by infected mosquitoes.

Dr Murphy said that no cases of these viruses have been transmitted through blood in the Republic but ongoing surveillance is needed — there were cases of chikungunya in northern Italy last year and the West Nile virus has emerged around the Mediterranean and North America.