BSE infectivity in the absence of detectable PrPSc accumulation in the tongue and nasal mucosa of terminally diseased cattle
The pathogenesis of bovine spongiform encephalopathy (BSE) infections in cattle has been studied in recent years by using highly sensitive transgenic-mouse bioassays. It has been shown that in this species, the BSE agent amplifies almost exclusively in the central and peripheral nervous system. Even in animals that were killed in the clinical end stage of the disease, the lymphoreticular system was shown to be free of the infectious agent. No other animal species investigated to date exhibits such a restricted BSE-infectivity distribution pattern. However, there is growing evidence for a radial spread of infection from the central nervous system (CNS) into the periphery during the late stages of the disease. In this study, we challenged transgenic mice overexpressing the bovine prion protein with homogenates prepared from a wide variety of tissue samples collected from BSE-infected cattle. As prion infections involve the conversion of the cellular prion protein into its abnormally folded isoform (PrPSc), we applied various detection methods, such as the purification of scrapie-associated fibrils, immunohistochemistry, and the protein misfolding cyclic amplification technique. Despite negative results using these highly sensitive biochemical methods, we were, for the first time, able to detect BSE infectivity in the tongue and in the nasal mucosa of terminally diseased BSE field cases as well as experimentally challenged cattle by transgenic-mouse bioassay. This shows that BSE infectivity can be present in the peripheral tissues of terminally diseased cattle, including tissues used for human consumption.