Prion disease zoonosis: assessing risk associated with new and emerging prion agents.

Abstract Prion diseases are infectious neurodegenerative diseases associated with the misfolding, aggregation and deposition of host prion protein (PrP) in the brain. These diseases include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease (CWD) in deer and Creutzfeldt-Jakob disease (CJD) in humans. The infectious agent is thought to be composed solely of a misfolded form of PrP (PrP Sc ) that propagates by binding and converting host PrP into the abnormal conformation. Natural routes of transmission appear to be oral, but the disease can also be spread by intravenous, intracerebral or peripheral exposure. Human prion diseases, such as CJD, were generally considered to have a spontaneous or familial (linked to mutations in the PrP gene) aetiology, although subsequent iatrogenic transmission has been demonstrated in some cases. Risks posed to humans from prion diseases in animals were therefore thought to be low. The emergence of BSE in the UK was followed by the appearance of a new variant of human prion disease (vCJD), and transmission studies in rodents confirmed that the two diseases were caused by the same strain of the prion agent. These data proved that some ruminant prion diseases could be zoonotic, and a potential risk to humans may exist from new and emerging prion agents in ruminants. Indeed analysis and characterization of recently identified prion isolates in ruminants (atypical scrapie, H-type BSE, L-type BSE and CWD) has revealed that some may pose a risk of transmission to humans, and are still a concern to public health.