The rate of onset of new cases of variant Creutzfeldt-Jakob Disease in Britain has increased by 20 per cent over the last 12 months, according to the latest figures.
It has also emerged that the increase is not uniform, with more cases arising in the north.
Uncertainties about factors such as the incubation period for the disease made it difficult to know how many more cases were likely, said Prof James Ironside of the Western General Hospital in Edinburgh. "No one is in a position to predict where the upward curve will go." He was speaking yesterday at the British Association Science Festival.
He added, however, that the rise in cases over the past year could alter estimates about how the disease would progress. "This obviously influences the predictions about future numbers."
Prof Ironside is a leading figure in the investigation of variant CJD and has given evidence to the British government inquiry on BSE. He was the leader of the team of scientists which first linked the new form of CJD in humans to mad cow disease.
The new figures reveal that nearly twice as many cases are occurring in the north of Britain as in the south. There were 2.71 cases per million of population in the north and 1.47 cases per million in the south. The total number of cases reported in the UK stands at 102 as of July.
The best estimates of the final toll vary from just over 100 to many thousands.
Some predictions were that millions could develop the illness but despite the reported rise in numbers, the worst of the predictions were unlikely to come true, he said. "The worse case scenarios have been excluded by the findings so far."
The reasons for the increase in numbers of cases and the geographical distribution were still unknown but the primary source for infection was still believed to be from the food chain, he said.
The continuing investigations would look at other possible means of exposure to infection, such as medicinal products and occupational routes, like working in abattoirs and butchers' shops.
Prof Ironside also said there was a genetic component to the disease in the cases seen so far. All victims had shared a common prion protein type.
Prions are the proteins that change from the normal to an aberrant form to cause the disease.
He said it was not known whether other prion protein types would emerge as being susceptible to the disease.