The amino acid sequence in the prion protein of cattle differs from that of sheep at 7 positions, and that of humans at 30 positions. It is thought that greater differences provide a more formidable "species barrier" to transmission. Scrapie strains have not been shown to transmit to cattle; however, BSE has been experimentally transmitted to sheep and several other species. Prions from cattle and humans, but not sheep, share specific amino acid sequences in two areas of the protein, possibly accounting for the transmission in spite of the amino acid sequence differences (2,3,21,28).

All cases of v-CJD have been recorded in people homozygous for methionine at prion protein codon 129. This genotype is found in 38% of the UK population. Genotypes that are heterozygous for methionine/valine (UK frequency 51%), or homozygous valine (UK frequency 11%), may have different incubation periods or increased resistance to infection (15,29).

BSE has a characteristic pattern of lesions in all species examined when changes are mapped by region of the brain. Also, protein digestion patterns of BSE prion are identical between species and differ from other TSEs. Other TSEs, however, demonstrate "strain" differences using these tests and in clinical progression of the disease. Strain differences are believed to represent differing molecular conformations of identical proteins (13,14).

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