Fears that the human version of “mad cow” disease could cause further waves of infection have been heightened by the first case yet identified in a person who is genetically distinct from previous patients.

All the deaths in Britain from the-brain condition have so far occurred among people with a particular genetic profile carried by about 40 per cent of the population. However, a 39-year-old woman who died recently of Creutzfeldt-Jakob disease (CJD) has now been found to have had a different genetic type to other cases caused by eating beef infected with rogue prion proteins.

If she is found to have had the new variant of CJD linked to bovine spongiform encephalopathy (BSE), her case could signal that the disease is starting to affect a second genetic group in whom it has a longer incubation period. Most scientists think it more likely that she developed the sporadic version of CJD that is not caused by eating infected beef.

Simon Mead, from University College London, who examined the woman’s brain after her death, said that although there was not yet any cause for alarm the case showed the importance of continued surveillance.

“The final conclusion remains open,” he told New Scientist magazine. “It is waving the flag for neurologists to watch for other cases.”

The results of genetic tests on the woman have been published in the journal Archives of Neurology.

Confirmation of which form of CJD the patient had will be important because predictions for the number of cases that will be caused by exposure to infected beef in the 1980s and early 1990s depend on how the disease affects people with three particular genetic profiles.

Initial forecasts of up to 500,000 infections have been cut radically, and more recent estimates for future deaths range from only 70 to about 4,000. The likely scale depends on the susceptibility of two genetic groups in whom the disease has yet to be found, and whose susceptibility is not known.

The prion protein that malfunctions in CJD has three versions. About 40 per cent of the population have two copies of the amino acid methionine at a key point in the protein. All cases of new variant CJD reported so far have occurred among people with this genotype, known as MM.

Another 50 per cent of the population have one copy of methionine and one copy of a different amino acid, valine, giving them the genotype MV. Another 10 per cent have two copies of valine, and are described as VV.

The new case is the first with similarities to new variant CJD to be identified in a VV patient, which could suggest that it is starting to affect a second genetic group in whom there is a longer incubation period.

If people with the VV and MV variants are also susceptible, which is unknown, they may have longer incubation periods than those with MM, and there may be further peaks of infection in the future. Mouse research suggests that both groups are susceptible, but that their incubation periods are so long that most carriers will die of another cause before falling ill with CJD.

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