Do the American health authorities know something we don't about BSE (bovine spongiform encephalopathy), or "mad cow disease"? Just as the British population received the all clear about the safety of its beef, the US health authorities are considering a blanket ban on donated blood from frequent visitors to the UK.

They are prepared to lose about a quarter of the blood which is normally supplied for transfusions, rather than run the risk of a major outbreak of the disease in the US, transmitted by the donated blood.

The ban has been recommended by the influential federal advisory panel to the US Food & Drug Administration (FDA), and so it is likely to be implemented.

The panel members are concerned that scientists do not fully understand how the disease is spread, even though there is no evidence that the disease has been transmitted through blood except during experiments on mice.

Their position seems to be contrary to that of some British scientists, who have been reassuring the public of the low risk of developing BSE.

Some panel members felt that a stay in the UK of just four months was enough to put a person at reasonable risk of contracting BSE, so possibly contaminating his blood (BMJ, 1999; 318: 1574).