A second Briton has been infected with the human form of BSE after a transfusion of blood from a donor who later went on to develop variant Creutzfeldt- Jakob disease (vCJD).

A second Briton has been infected with the human form of BSE after a transfusion of blood from a donor who later went on to develop variant Creutzfeldt- Jakob disease (vCJD).

John Reid, the Health Secretary, announced yesterday that he was tightening the rules governing blood donors to minimise the risk of the infection being passed on during hospital procedures.

In a separate development, it emerged that the second patient to become infected with vCJD via a blood transfusion belongs to a genetic category comprising about half of the population that was, until now, thought to be resistant to the disease. Scientists said it was too early to predict what that could mean for a future epidemic but one expert said that it might result in a doubling of the number of people who eventually succumb to the lethal brain disease.

Last December, the Department of Health announced that anyone who had received a blood transfusion since January 1980 would be banned from giving blood.

The move was in response to the first person in the world to become infected with vCJD via a blood transfusion - which was also the first case of person-to-person transmission of the disease rather than infection from eating contaminated food.

Two new measures to limit the risk of cross-contamination via donated blood are to be enforced from next month. The department said that anyone who was in any doubt about whether they had received a blood transfusion in the past would be banned from giving blood.

It also said that a second group of specialist blood donors who give certain components of their blood on a regular basis using a process called apheresis would also be excluded.

Mr Reid said the measures were designed to follow the Government's highly precautionary approach to containing the spread of vCJD, especially during surgical procedures.

"Although people may have concerns about the implications of this announcement, I would emphasise again that the exclusion criteria are being tightened because of a small but unquantifiable risk," Mr Reid said.

"People should continue to have a blood transfusion when it is really necessary. Any slight risk associated with receiving blood must be balanced against the significant risk of not receiving that blood when it is most needed," he said.

More worrying for scientists is the fact that the second person to become infected with vCJD has a genetic makeup known as "methionine-valine heterozygous" which is common in more than half of the population and was once thought to offer resistance to the infection.

Until now, all the patients who have developed the disease have an alternative genetic make-up known as methionine homozygous, which comprises some 40 per cent of the population.

Professor Peter Smith, the chairman of the Spongiform Encephalopathy Advisory Committee, said that it was still too early to assess the implications.

The patient who received the blood donation died of other causes, did not show signs of vCJD and was only diagnosed with the infection following an analysis of the spleen, Professor Smith said.

He said the patient may not have gone on to develop the disease and might only have become susceptible as a result of being transfused with a relatively large dose of the infective agent, rather than the small dose taken orally - the usual route of infection for the 147 people who have developed the disease since 1995.

Professor Neil Ferguson of Imperial College London said that estimates of the future spread of the epidemic may have to be revised. "Clearly this is bad news overall but it is still too early to say what the implications are. A pessimistic assumption is that the final numbers affected by the epidemic may be twice as high as we thought possible," Professor Ferguson said.

Some scientists, notably Professor John Collinge of St Mary's Hospital, London, have suggested that the vCJD epidemic could experience a second, larger increase in numbers if the 60 per cent of people who are not in the high-risk category were found to be susceptible, a situation that now appears to be the case.

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