INACTIVATION OF THE BSE AGENT

Bovine spongiform encephalopathy (BSE) belongs to a distinct group of transmissible degenerative encephalopathies (TDE) that includes scrapie in sheep and Creutzfeldt-Jakob disease (CJD) of humans. These fatal neurological diseases are caused by unconventional but uncharacterized transmissible agents that have a number of unusual properties; this includes a high degree of resistance to inactivation. The only disinfectant that appears to be completely effective against high titres of TDE infectivity is sodium hypochlorite. Exposure to 1 to 2 M sodium hydroxide has a substantial but incomplete effect, as does autoclaving at temperatures between 132 and 138C for up to an hour; however, a combination of 2 M sodium hydroxide and autoclaving, even at the more modest temperature of 121C for 30 min, appears to be completely effective. With lower titres of infectivity, less rigorous regimes may be effective. For example, although most of the rendering procedures used to manufacture meat and bone meal (MBM) throughout the European Union (EU) have been found to be incapable of inactivating BSE and scrapie agents, one method which appeared to be effective with moderate titres of BSE and scrapie agent, involved exposure to steam at 133C for 20 min. This procedure is now the only one approved within the EU for the manufacture of MBM for feeding animals, excluding ruminants; however, the UK has introduced a ban on feeding MBM to any farmed species because of the occurrence of a new variant form of CJD in the UK which appears to have been caused by the BSE agent. Although the clinical signs of scrapie in sheep are entirely attributable to neurological dysfunction, tissues such as spleen and lymph nodes are known to become infected before those of the central nervous system. In contrast, the only tissues which have been found to become infected in cattle with natural BSE are brain, spinal cord and retina. These are no longer used in animal or human foodstuff. Because scrapie agent has no known association with human disease there has been no restriction in the past on using any sheep tissues, including brain, as human food; however, it is known from experimental studies that the BSE agent can infect sheep by the oral route to cause a disease that is clinically and neurohistopathologically indistinguishable from scrapie. It is also known that the spleen becomes infected in such sheep. Sheep in the UK could theoretically have become infected with the BSE agent through the feeding of infected MBM before the feed-ban in 1988. There are now measures in place that prevent the head, spinal cord, and spleen of sheep being incorporated into foodstuff.