Volume 2: Science
3. The nature and cause of BSE
Introduction

3.1 In order to respond adequately to the emergence of BSE, the Government required urgent advice about the nature and cause of the disease and the risk that it might pose to humans. Scrapie in sheep was the only common disease of a similar nature known to affect farm animals. Because much had been learned about scrapie over the previous 50 years, it was natural that this provided much of the knowledge of transmissible spongiform encephalopathies (TSEs) available at the time to inform scientists faced with a similar disease in cattle. Creutzfeldt-Jakob Disease (CJD), the human equivalent of animal TSEs, was also well known to medical scientists who, in 1985, had become aware of the occurrence of CJD in recipients of human growth hormone prepared from pooled cadaveric pituitary glands.

3.2 Scrapie had been thoroughly investigated to see if it might have passed to humans, possibly causing CJD. Extensive retrospective studies together with a review of world literature led to the conclusion, published in 1987, that scrapie had never passed to humans despite opportunities to do so over the 250 years during which the disease had contaminated sheepmeat entering the human food chain. ¹

3.3 As recounted below, the early investigations into the epidemiology of BSE strongly suggested that BSE was caused by scrapie in sheep passing the species barrier into cattle through contaminated meat and bone meal (MBM) included in cattle feed. If BSE was caused by the scrapie agent infecting cattle, it might be concluded that BSE, like scrapie, posed no risk to humans. Despite the fact that such a conclusion could not be supported by scientific evidence - the long incubation periods associated with CJD meant that it would be many years before such evidence could be expected - many of those involved took comfort from the evidence that there was no proven link between scrapie and CJD.

3.4 This chapter first considers the investigation into the BSE epidemic and the grounds for reassurance about the risk to humans which was based on the 'scrapie origin theory'. It also describes the 'novel mutation theory' (for which evidence has been accumulating since 1989) and some other alternative hypotheses that have been considered in recent years. However, these alternative theories could provide no reassurance about the risk of transmission to humans.

3.5 The later sections of the chapter move on from the epidemiological studies and cover other areas of research. These include experiments carried out to investigate the infectivity of BSE, and its transmission to other species.