A new blood filter device could in future prevent people being infected with the human form of mad cow disease through transfusions, it was revealed today.

The technique can effectively remove the rogue prion proteins responsible for transmitting brain diseases such as variant CJD.

Although so far only tested on hamster blood containing the prions that cause scrapie, a related disease affecting sheep, scientists believe it can be developed for humans.

Earlier this month it was disclosed that 24 people given blood transfusions contaminated with vCJD face the awful knowledge that they are likely to succumb to the disease.

Professor John Collinge, a leading expert from the Medical Research Council's Prion Unit based at Imperial College London, said the risk was "substantial".

He conducted a study of three cases of patients acquiring vCJD from a blood transfusion which showed the disease was easily transmitted by this route.

A total of 66 transfusion patients are known to have received infected blood from donors who developed vCJD.

Many of the recipients were already seriously ill from unrelated causes and subsequently died from them.

Today, a total of 24 remain alive. They face an uncertain future, since the disease has a long incubation period which varies according to an individual's genetic make-up and may span decades.

Currently, white blood cells are removed from donated blood to minimise the risk of transmitting vCJD. Donors are also carefully screened, and no-one who has received blood is subsequently allowed to give it.

But these measures alone are not thought to make donated blood 100% safe.

The new filter system was developed by Pathogen Removal and Diagnostic Technologies Inc (PRDT), a joint venture between Canadian-based biotech company ProMetic Life Sciences Inc, and the American National Red Cross.

A report in The Lancet medical journal described how scientists screened millions of molecules to find one that selectively binds to the prion protein.

The molecule, called L13, was incorporated into a device that filters blood and extracts prions.

It was tested on scrapie-infected hamster blood that was first stripped of white blood cells, in the same way as blood used for human transfusions.

Despite this precaution, 15 hamsters became infected with scrapie after the blood was injected straight into 99 animals.

But when the blood was passed through the filter device before being injected into another 96 hamsters, none fell ill.

One of the scientists who conducted the research, Dr Robert Rohwer from the University of Maryland in Baltimore, US, said: "Removal of vCJD infectivity by adsorption gets round the extremely difficult problem of detecting the very low concentrations of these agents in blood, especially during the long asymptomatic period of the disease when people donate.

"For these disease agents, removal may be our best and perhaps our only option."

The French company MacoPharma, which makes equipment for removing white blood cells from blood, is helping to develop the new system, called P-Capt, as a commercial partner.

Together with PRDT, it is collaborating with National Blood Transfusion Services in the UK and Ireland to conduct an evaluation of P-Capt.

MacoPharma project manager Iwona Walicka said: "Implementation of the product for the filtration of donated blood should significantly reduce the risk of vCJD transmission by transfusion."