CJD epidemic alert

Experts were today revising their estimates of people carrying mad cow disease after discovering that a large percentage of the population could be carrying it without knowing.

The findings emerged from the case of a second patient believed to have contracted the human form of BSE through a blood transfusion.

The patient's genetic makeup is crucially different from that of any known previous victim of the disease.

Scientists fear it could mean that many more people may be susceptible to variant CJD - the human form of the disease - than was previously supposed.

In these individuals, the incubation period of the disease may be much longer raising the possibility of a major epidemic of cases occurring at some unknown time in the future.

Meanwhile there could be a large pool of symptomless people who do not realise they are carrying the infectious prion protein responsible for both the cattle brain disease BSE and vCJD.

Professor James Ironside, from the National CJD Surveillance Unit in Edinburgh, who took part in the study, said: "This finding has major implications for future estimations of numbers of vCJD cases in the UK.

Individuals with this genotype constitute the largest genetic subgroup in the population.

"It's absolutely possible that there may be a new epidemic, because the cases we've seen so far may only be those who are unusually susceptible or have the shortest incubation periods.

"I'm not in the business of scaremongering, but quite clearly the idea that this problem is on the way out is not, unfortunately, the case at all." Professor Ironside said it was highly significant that the elderly patient did not die from vCJD, and the infection would never have been detected without a postmortem examination.

"Although there were no symptoms the patient could still have been a source of infection," he said.

Traces of the prion agent were found in the spleen and a lymph node, but not the brain.

It was therefore "very likely indeed" that the infection had been acquired from a blood transfusion, said Professor Ironside.

However, researchers stressed it was still unclear how dangerous such new cases might be.

But Professor Ironside added: "It's possible that the incubation period for the disease in this group will be longer than for the group we've seen clinically so far."

The finding means that forecasts of how many people are likely to come down with the disease need to be radically revised, Professor Ironside said.

Even recent predictions have ranged widely, from 10 more cases to more than 10,000, but all the estimates have assumed that the disease strikes only those with the less common genetic profile and that the incubation period lasts up to 30 years.

"We have to be very cautious about this and I think we have to go back and redo all the calculations based on this case," Professor Ironside said.