The British Inquiry into the BSE epidemic reached the conclusion that the main cause of the disease was the feeding of MBM (meat and bone meal) to previously uninfected cattle. Cattle are normally herbivores, and the MBM included the remains of other cattle, which, when fed to uninfected cattle, caused the disease to spread more rapidly. The infectious agent of BSE is unusual because it remains active even at high temperatures. The rendering process (by which animal remains or byproducts are made into useful byproducts) does involve a heating process, however in Britain the temperatures reached during rendering had been reduced, meaning that the infectious agent remained active and therefore spread more rapidly.
Another contributing factor to the spread of the disease may have been the feeding of protein supplements to young calves, the proteins being infected with the disease.