The researchers have discovered that mad cow disease (BSE) and its human equivalent, Creutzfeldt- Jakob Disease (CJD), are spread by a deformed protein in the brains of infected animals.
The infectious agents are not bacteria or viruses. Instead, the deformed protein seeds itself through a sort of biological 'domino effect', tipping normal proteins into the abnormal shape which causes disease.
The process is previously unknown to science. It resolves a puzzle which has baffled scientists for years, because the diseases did not appear to be spread by conventional micro-organisms. Instead, the results confirm earlier suggestions that the deformed proteins, known as prions, were themselves the infectious agents.
The experiments, reported in today's issue of the scientific journal Nature, were conducted by a team led by Dr Byron Caughey, of the US National Institute of Allergy and Infectious Diseases in Montana. They isolated the deformed protein from infected hamsters and then, under strictly controlled conditions, mixed it in a test tube with the normally shaped protein. They discovered that the normal protein changed shape, converted into malignant form by the influence of the abnormal prion.
The experiments were greeted enthusiastically by other scientists. One of Britain's leading researchers into CJD, Dr John Collinge of St Mary's Hospital Medical School in London, said the results were 'very exciting' and represented 'a key piece in the jigsaw puzzle'.
Dr Collinge pointed out that a logical next step would be to mix prions - deformed protein - extracted from the brains of cattle which have died of bovine spongiform encephalopathy with normal human protein.
He said: 'The real question is how efficiently BSE prion converts human protein. If it works easily, then that is worrying. If it is difficult and it takes laboratories many years to achieve, then that is reassuring. One would hope that the bovine prion barely converts the human protein, if at all.'
Normally, cells in the body have chemical defences to break down and remove defective proteins. Called proteases, these chemicals are enzymes broadly similar to those in 'biological' washing powders, but prions are protease resistant, so the abnormal proteins accumulate in the brains of infected animals and cannot be got rid of. Dr Collinge warned, however, that the experiments needed more refinement.
The discovery by the US team may also resolve the puzzle that CJD can be both an infectious and an inherited disease. According to Dr Collinge, studies of how CJD is inherited have indicated since about 1989 that abnormal, prion protein rather than a conventional virus or bacterium might be the infectious agent.