I refer to the letter titled Mad Cow And British Blood Donors (December 1). It is pertinent to clarify the issues.

Mad Cow disease is transmitted by an agent (prion) known as vCJD. It is possible to transmit this agent via blood. The British isles were the hardest hit by this agent, so there is a risk (even if very small) that during the period in question (1980-1996) one could have been exposed to the agent within the British isles. As a precaution, most countries decided to adopt a strategy to defer donors who have been to the UK for a cumulative period of about six months within the time period mentioned earlier.

Since our knowledge of the agent and its effects is still limited and since no reliable test is yet available to screen for the condition in humans this deferral is still in force. Obviously, this deferral does not hold in the UK.