THE fatal brain condition known as mad cow disease could potentially be transmitted to humans by sheep carrying scrapie, new research suggests.

Scientists have concluded that scrapie, the sheep equivalent of mad cow disease, or BSE, has the potential to infect humans in a similar way to variant Creuzfeldt Jakob Disease (CJD), which caused a major health scare over beef in the 90s.

Tests on mice found a link between the infectious agent behind scrapie with sporadic CJD (sCJD), a fatal human disease, the cause of which has not been discovered.

The scientists stress they have no proof that eating mutton or lamb infected with scrapie can lead to sCJD in humans, but tests on humanised laboratory mice show that scrapie is capable of infecting humans. And the way the infection spreads in the brain is identical to that seen in cases of sCJD.

The scientists, led by Dr Olivier Andreoletti from the National Veterinary School of Toulouse in France, wrote in the journal Nature Communi-cations: “Our data on their own do not unequivocally establish a causative link between natural exposure to sheep scrapie and the subsequent appearance of sCJD in humans.

“However, our studies clearly point out the need to consider this possibility.”

Both scrapie and different forms of CJD are caused by rogue misshapen prion proteins. Normal prions that come into contact with the defective versions are changed too and turn “bad”.

In this way the infection spreads, inflicting terrible damage to the brain.

Variant CJD first emerged in 1996 and was shown to be the human version of the cattle disease bovine spongiform encephalopathy (BSE).

The prions that caused the disease spread to humans in contaminated beef – especially burgers, cheap cuts and pies – and in some cases were also traced to blood transfusions from infected donors.

However, fears of a catastrophic epidemic did not materialise.

To date, 177 UK deaths from vCJD have been recorded, most occurring in the late 90s and early 2000s.

The mice in the new study are the same strain previously used to confirm the ability of BSE to break the species barrier and infect humans. In the tests, the animals had scrapie prions injected straight into their brains.

The authors point out that prion infection via non-direct routes, such as eating contaminated tissue, can have an incubation period running into decades.

They said: “Furthermore, it is crucial to bear in mind that sporadic sCJD in humans is a rare disease and that scrapie has been circulating in small ruminants populations used for food purposes for centuries. "Consequently, it is our opinion that, even if a causative link was established between sheep scrapie exposure and the occurrence of certain sCJD cases, it would be wrong to consider small ruminant (prion) agents as a new major threat for public health.”