A new study adds to concern that the deadly human form of mad-cow disease may have infected far more people than previously thought.

Assumption that most people are genetically shielded from the brain-destroying disease could be wrong, it says, cautioning though that the evidence for this remains sketchy.

Variant Creutzfelt-Jakob disease (vCJD) is linked to eating meat infected with bovine spongiform encephalopathy (BSE), the term for mad-cow disease.

A rogue version of a prion protein proliferates in the brain, turning it spongy and eventually causing death.

After vCJD was first identified in March 1996, some experts calculated it could inflict a death toll in the tens of thousands, especially in Britain, where the BSE epidemic began.

One estimate ran as high as 136,000 deaths.

These calculations were swiftly revised downwards to a few hundred or even less when it was realized that the toll was rising far slower than expected.

At present, Britain has recorded 161 definite and probable cases of vCJD, six of whom are still alive.

One reason for optimism about vCJD has been genetic.

All of the deaths have occurred among people with a so-called MM variation in part of a key gene, PRNP, located on Chromosome 20 of the genome.

In the white British population, 42 per cent of people have the MM variant. The others have different types - MV (47 per cent) and VV (11 per cent) and the fact that no MV or VV cases had arisen led many to believe that this was a protection against the rogue protein.

But the new study, which appears in the British Medical Journal, places a cloud of doubt over this assumption.

Researchers led by James Ironside, a professor at the University of Edinburgh, Scotland, carried out a DNA analysis of three appendices found to carry the prion protein.

The tissues were part of a vast earlier study in which British labs screened thousands of appendices and tonsils for the protein in order to get an idea of the spread of vCJD.

Ironside's team say they were extremely surprised to find that two out of three screened samples came from people with the VV variant. Neither individual, both aged in their 20s at time of surgery, has the symptoms of vCJD.

The paper warns against dramatism. Only two VV samples have so far been identified, and just because a VV individual has the protein does not mean that he or she will develop vCJD.

On the other hand, no one knows how long it takes for vCJD to incubate, which raises the possibility that VV individuals may fall sick years from now. In classic Creutzfeldt-Jakob disease, this can take up to 30 years.

And another, and as-yet unquantifiable, risk is that VV individuals with the prion may unwittingly pass it on to others through blood donations.

In 2004, a person with the MV gene variant was found to be infected with vCJD, but again this individual had no clinical disease.

And in March, a study found that mice, which had been genetically modified to produce all three PRNP variants and were directly injected with prion into their brains all fell sick with vCJD, although the disease took a long time to incubate.

"There are compelling reasons why health officials should take notice," said two Canadian specialists, Kumanan Wilson and Maura Ricketts, in a BMJ commentary on the latest paper.

"It is conceivable that, having jumped the species barrier (from cows to humans), transmission of the prion within the species becomes easier."

Source: China Daily