BSE has been transmitted, in the laboratory, to cattle, mice, sheep and goats both orally or through the injection of infected tissue. There is still some doubt as to the infectious agent involved in transmissible SEs. Such organisms could be destroyed in the laboratory by using radiation, heat or chemicals, yet the tissue found in transmissible SEs remains infectious when subjected to these treatments. This is a slightly modified protein which can convert a normal protein in the brain into a dangerous one simply by altering its physical structure. The immune system does not recognise these changes and does not combat the disease.
Prion diseases appear to arise in three ways.
Even in CJD, up to 15 per cent of cases have a familial connection. Secondly, as is hypothesised in the remaining cases of sporadic CJD, there may be a random mutation of a prion protein which sets in train a chain reaction amongst normal proteins. Thirdly, the agent may be transmitted via food or tissue. Prions from one species appear capable of infecting certain other species. It is likely that the closer the molecular structure of the prion protein of different species, the easier it is for cross infection to occur.