They found the rogue prions are able to cross the intestinal barrier which blocks the path of potential toxins, by hitching a ride on another protein.

It is hoped the finding could lead to new ways to block the disease.

The research, by Case Western Reserve University, is published in the Journal of Neuroscience.

It is now widely accepted that vCJD - a progressive brain disease - results from eating infectious prion proteins contained in beef products from cattle infected with BSE.

These prions accumulate in the tissue of the nervous system, where they cause damage that is ultimately fatal.

To date 155 cases of confirmed and probable vCJD have been reported worldwide. It is unclear how many other people may be harbouring the disease.

The prions are tough enough to survive attack by the digestive enzymes found in the stomach.

And the new study shows that they are able to pass across the intestinal barrier by riding piggyback on ferritin.

This is another protein found in meat that plays a key role in mopping up excess iron in the digestive tract, and which is normally absorbed by the intestine.

Simulated digestion

Lead researcher Dr Neena Singh said: "The mad cow epidemic is far from over. It is therefore essential to understand how this disease is transmitted from one species to another, especially in the case of humans where the infectious prions survive through stages of cooking and digestion."

The researchers used brain tissues infected with the spontaneously occurring version of CJD, which is also caused by prions.

They simulated the human digestive process by subjecting the tissue to treatment with the same digestive fluids they would encounter in the intestinal tract.

Dr Singh said ferritin was found in a similar form in different species.

Thus it may be the key which explains how prions are able to jump so effectively across the species barrier.

She said: "If we can develop a method of blocking the binding of prions to ferritin, we may be able to prevent animals from getting this disease through feed, and stop the transmission to humans."

Professor James Ironside, of the CJD Surveillance Unit at Edinburgh University, told the BBC News website the study seemed like "an interesting piece of work".

But he added: "It might well be possible that there are other routes for the abnormal prion protein to cross the gut wall, perhaps via immunologically active cells known as M cells, or perhaps through defects in the wall of the gut (including the mouth and gums).

"I would agree that blocking the entry of the abnormal prion to the bloodstream would be desirable as a prophylaxis to oral exposure to prions, but there will probably be a long way to go before this can be achieved."