The approach worked in the test tube

Potentially, they have found a way to kill only the prion proteins that cause disease, and leave normal prions alone.

This removes another obstacle to an eventual vaccine that could form the basis of a treatment for vCJD.

Experts are increasingly convinced that the progressive and invariably fatal brain illness is caused by clumps of prion proteins building up and killing brain cells.

However, not all prions are bad - only prions which have become misshapen through contact with other "misfolded" prions cause the problem.

Several teams around the world are hoping that a vaccine can be developed which would prime the immune system to destroy the misfolded clumps of protein.

However, any vaccine would have to differentiate between "rogue" prions and normal prions, which perform a useful role in the brain.

A team of researchers from the University of Toronto believe they have achieved this.

Weak spot

They have found an area on the the vCJD prion which they think is unique to the disease-causing form.

With this knowledge, they can prime antibodies to "spread the word" to other immune cells to launch attacks on any proteins which have the same unique appearance.

The Toronto team managed to prove that this worked in principle in the laboratory, developing an immune response that would mark the rogue prions for destruction - but spare normal, healthy prions.

The treatment worked not only on human vCJD prions, but also on many other types of related "spongiform" diseases, such as BSE in cattle and scrapie in sheep.

Dr Neil Cashman, who led the study, which was published in Nature Medicine, told The Guardian: "It worked for every species."

Other teams

This is not the first research to try to develop antibodies to treat and prevent vCJD.

A team at Imperial College London vaccinated mice who had been infected with scrapie prions.

Normally, the mice would be expected to fall ill and die swiftly, but the treatment appeared to delay the onset of disease indefinitely.

The application of this breakthrough as a vaccine is doubtful, as mass immunisation against vCJD is highly unlikely.

It is likely to be years at the earliest before a vaccine could become widely available.

The latest estimates suggest that the number of people who develop vCJD may be far fewer than first feared.