Assessing Risks of BSE Transmission to Humans

Abstract

The prion diseases or transmissible spongiform encephalopathies are a group of neurodegenerative conditions affecting humans and animals which are transmissible between mammalian species by inoculation with, or in some cases by dietary exposure to, infected tissues. The transmissible agent or prion consists principally of a modified form of a host encoded glycoprotein, the prion protein (PrP). This disease related isoform, PrP^Sc, differs from the cellular isoform, PrP^C, by a post-translational modification which appears to be conformational rather than covalent. The epidemic of a novel animal prion disease, BSE, in the UK has led to fears of transmission to humans through dietary exposure to beef or beef products. Recently, the occurrence of cases of sporadic Creutzfeldt-Jakob disease (CJD) in young people has caused renewed alarm and these unusual cases appear to represent a novel form of human prion disease. A causal link with BSE is unproven but remains a likely explanation. Transmission of prion diseases between species is usually inefficient and associated with extremely prolonged incubation periods at primary passage, the so-called “species barrier”. The effectiveness of the species barrier between cattle and humans is unknown but the ability of bovine PrP^Sc to induce conversion of human PrP^C can be studied in transgenic mice expressing human PrP, which lack a species barrier to human prions,. These mice can also be utilised in strain typing studies to try and identify whether the “new variant” CJD cases arose from exposure to bovine prions.