There is a statistical species barrier that depresses cross-species transmission of BSE. The concept of a "Species barrier" is based on the observation that interspecies transmission is less efficient than within species transmission.
For example, LD50 concentrations for transmission of BSE to mice are 107, but for mouse TSE transmission to mice of different strains is 104. This would suggest a species genetic barrier as high as 1:1,000

Based on this information, minimal barriers between species would be at least as high as the 1:1000 barrier within a species, but probably higher.