A number of theories exist regarding the origin of BSE. An early and prominent theory was that BSE is derived from scrapie. Along with other wastes, sheep offals, which would have included material from scrapie-infected sheep, were rendered into meat and bone meal (MBM) used in animal feed. This theory hypothesizes that the changes in rendering and feeding practices around 1980 allowed the scrapie agent to survive and enter the cattle feed chain where it overcame the species barrier and adapted to cattle.

The evidence is very strong that the spread of BSE to the point at which it became an epidemic in the U.K. arose through the use of MBM in cattle feed. The inclusion of animal-derived protein in rations was largely determined by market prices for commodities, as rations were formulated on a least-cost basis.

A U.K. review of BSE origins noted it seems likely that from 1970 to the 1980s when MBM was widely used in cattle feed, an unusual linkage of events occurred. The diet of calves, in particular a high proportion of dairy calves, was changed so that MBM was included in their starter rations. Furthermore, because of the large sheep population, the MBM is likely to have included significant contamination with scrapie-infected material. Changes in rendering processes may have resulted in a small but clinically significant increase in the degree of infectivity of this material in MBM.

Infected cattle carcasses were rendered into MBM, increasing the levels of the cattle-adapted scrapie agent in the protein supplement and eventually causing a full-scale BSE epidemic.

Several other possibilities have been proposed regarding the origin of BSE. The official U.K. government inquiry into the BSE epidemic, The BSE Inquiry, was conducted from 1998 to 2000. The inquiry concluded that BSE probably originated from a novel source early in the 1970s, possibly a cow or other animal that developed disease as a consequence of a gene mutation, and said the origin of the disease will probably never be known with certainty. The evidence for such an origin remains elusive and is not universally accepted.

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