FEARS of a new wave of deaths caused by the human form of mad cow disease have been raised by a type of variant CJD not seen before.

Scientists who examined the brain of a 39-year-old woman killed by the disease found unusual patterns of damage.

The woman also had a distinct genetic make-up associated with her illness not shared by other victims.

Investigations are at an early stage, but the findings point to the possibility of a new group of people succumbing to vCJD as a result of eating infected beef in the 1980s.

The disease, linked to misshapen prion proteins in the brain, has varying incubation periods according to genetic make-up.

Experts believe for many of those infected the timer could still be ticking. They might not develop symptoms for decades after acquiring the disease, or die from other causes before becoming ill. In some cases the incubation period might exceed 50 years.

VCJD arose in cattle as bovine spongiform encepha- lopathy (BSE) before appearing in a new form in humans. The incurable disease leaves the brain riddled with holes, like a sponge, causing a loss of mental faculties and death.

Since the disease was recognised in 1995 there have been 162 confirmed or suspected deaths in the UK.

Fears of an epidemic involving hundreds of thousands of deaths abated after the number of annual deaths peaked at 28 in 2000 and then quickly fell. In the year up to November 30, 2007 there were just four definite or probable cases.

All these people had the MM version of the gene that makes prions, which is carried by around 40% of the population.

The woman recently examined by scientists at the Medical Research Council's Prion Unit at University College London had a different gene variant, known as VV, found in about 10% of Britons. She could be the first of a new wave of cases with the same genetic make-up who have only now reached the end of their incubation period, say researchers.

Another possibility is that the woman simply had a rare form of "sporadic" CJD which has nothing to do with mad cows or infected beef.

The case was described in the journal Archives of Neurology and reported today in New Scientist magazine.

Dr Simon Mead, from the MRC Prion Unit, who examined the woman's brain, said: "The final conclusion remains open. It is waving the flag for neurologists to watch for other cases."

The case of the woman appears to be more similar to vCJD than sporadic CJD. She was relatively young, whereas most cases of sporadic CJD occur in people over 50, and the abnormal prions were typical of vCJD, not sporadic CJD.

The patterns of damage to her brain did not correspond either to MM cases of vCJD or the sporadic disease. This is to be expected, said the scientists, because the prion gene variant affects the brain damage seen in both humans and animals with the disease. It could be that people with the VV gene variant are next in the queue to fall victim to the disease after those with the MM variant.

A similar pattern was seen with kuru, a similar prion-related brain disease, which affected cannibals in Papua New Guinea.