The first case of BSE was identified in 1986 in Britain. The first human case of vCJD was also diagnosed in Britain in 1995. It had always been thought that the brain-wasting mad cow disease passed to cattle through remains of sheep infected with scrapie, the sheep equivalent of BSE, that were added to cattle feed.
The previous theory was that humans who ate infected beef developed a human form of BSE. It became known as variant Creutzfeldt-Jakob disease or vCJD, to distinguish it from the classic human forms of the disease, which occurs sporadically or runs in families. But the British authors have suggested that the remains of humans infected with classic CJD were fed to cattle, which became ill with a bovine version of the human disease. The remains of those cattle were rendered and mixed into new batches of feed, infecting more animals. Eventually a new version of the disease passed back into humans and was called vCJD.
The authors admitted that their hypothesis is based on a compilation of circumstantial evidence. They said that the theory is not proved but is good enough for conducting further investigations.
The problem is that proving this theory may require feeding infected human brain material to cattle and that is something the public may not tolerate.