If the tests are successful, the vaccine will first be used to wipe out the devastating infections, caused by rogue proteins called prions, in national herds of cattle and sheep.

Prions occur naturally in all mammals and usually cause no ill-effects. But sometimes, the proteins fold into abnormal shapes. These can then spread around the body and convert other normal prions into abnormal, disease-causing prions on contact.

A team led by Neil Cashman at the University of Toronto has now found what could be the achilles heel of infectious prions. The team took normal prions and forced them to fold into abnormal shapes in a petri dish. They found that the prions exposed sections of protein that were usually tucked inside normal proteins, remaining hidden.

Dr Cashman realised that the section of protein that was exposed in abnormal prions, a short strand of three amino acids, was their weakness.

To target the abnormal prions, he cut the three amino acids out of the prions and tagged them to a molecule known to cause an immune response in animals. He then injected them into mice. As Dr Cashman hoped, the mice began to churn out antibodies against the amino acids. Antibodies let the immune system identify anything in the blood that should be destroyed.

The team then purified the antibodies from the mice and tested them to see if they could identify infectious prions from other animals.

"The amazing thing is that once we found it worked for hamster scrapie and mouse scrapie, we tested all the samples we could lay our hands on, from vCJD in humans, to BSE in cattle and scrapie in sheep. It worked for every species," he said.

Dr Cashman, whose study was published yesterday in the journal Nature Medicine, is now testing a vaccine which targets abnormal prions in mice. If it works, the team will move on to sheep and cattle.