An update on the epidemiology and the physiopathology of spongiform encephalopathies in ruminants

Abstract

The bovine spongiform encephalopathy epidemic resulted from cattle feeding with contaminated meat and bone meal (MBM). The recycling of infected carcases into MBM amplified the dissemination of the infectious agent. The origin of the disease and the nature of the agent - infectious protein, virus, … - as well as the possibility of its transmission to humans remain unknown. Most of the current knowledge about spongiform encephalopathies, including the physiopathology of BSE, has been extrapolated from results of many year studies of scrapie in sheep. But some differences exist between diseases according to the host species. One of these differences is the absence of infection in lymphoid tissue of cattle whereas it has been showed to be one of the main features of the disease in sheep. It can then be postulated that dissemination of the agent could occur mainly by the nervous route in the bovine, whereas lymphoid system is likely to be a important and early replication site in sheep.

Origin of neuronal death, which could explain symptoms encountered in these conditions, is still poorly understood. PrP, a normal protein found at the membrane of numerous cell types, accumulates in its pathological isoform, PrPsc, in the central nervous system and is strongly involved in the pathological process, since its presence and its genetic polymorphism considerably influences the progress of infection. But PrP is probably not the only protein involved in the transmission and the spread of the agent in the infected host.