13 course summary
BSE is a TSE disease of cattle that was formally recognised in 1986. It developed to epidemic proportions in the UK, reaching a peak in 1992. Although BSE is now fading away in the UK, cases have eventually turned up in many other countries.
Mainly through epidemiological studies, veterinary scientists quickly established (at least to their satisfaction) that BSE was caused by the inclusion in cattle feed of ruminant-derived MBM contaminated with a TSE-causing biological agent of some kind, following a change in the rendering process. The initial source of contamination may have been material from sheep carrying the TSE scrapie. Subsequently, increasing amounts of material from cattle carrying BSE would have become included in cattle feed.
Human TSEs include (classical) CJD (which may be sporadic, inherited or iatrogenic), GSS, kuru and vCJD.
vCJD was recognised in 1996 and linked to exposure to BSE in beef and beef products before 1989.
The number of suspected cases of human TSE diseases in the UK referred to the CJD Surveillance Unit rose from about 50 in 1990 to over 150 per year during 1997-2003. This increase may partly reflect greater awareness of TSEs among members of the medical professions.
Periodically, epidemiologists have predicted the eventual total number of vCJD cases in the UK in the form of a best estimate plus upper and lower 95% confidence limits. The upper 95% confidence limit, which was 10 million in 1997, had fallen to 50 000 by 2002. The 2003 prediction gave a best estimate of about 200 deaths by 2080, with upper and lower 95% confidence limits of 7000 and 10 respectively.
These predictions assume that vCJD is contracted only from having eaten contaminated beef (and not from infected surgical instruments or through blood transfusion). They also assume that only people with a particular genotype (MM) are susceptible to vCJD (which is now known not to be the case).
Analysis of tonsil samples is also being used to predict the likely total number of vCJD cases.
By the end of 2004, there had been 148 deaths in the UK from definite or probable vCJD.
In 1982, Stanley Prusiner first proposed that TSEs are caused by proteinaceous infectious particles or prions. These are protease-resistant particles (PrPs) that exist in (at least) two conformations - 'normal' PrPC molecules (relatively rich in α-helices) and TSE-causing PrPSc molecules (relatively rich in β-sheets).
When PrPSc molecules interact with PrPC molecules, the latter can be converted to PrPSc. Since PrP molecules from different species - with slightly different sequences of amino acids - can also interact in this way, TSEs can cross species barriers. Prusiner's protein-only hypothesis of TSEs is now very widely - but not universally - accepted.
There are some indications that prion-like behaviour might be quite widespread in nature and represents a previously unsuspected aspect of 'normal' biology.
Once BSE had been identified as a new disease in cattle and veterinary scientists had become convinced that it was caused and spread by the inclusion of contaminated ruminant MBM in cattle concentrates, a series of decisions were taken at local, national and international level with the aim of bringing BSE under control and safeguarding human health.
At least initially, these measures proved inadequate to stem the BSE epidemic. This was partly due to the controls often being flouted, either accidentally or deliberately.
Nevertheless, there were repeated assurances from politicians and officials that British beef posed no health risk to the public.
When the likely link between BSE and vCJD was recognised in 1996, further precautionary measures were introduced in the UK and the EC imposed a ban on the export of British beef. Over several years, the UK government expended considerable effort to get this ban lifted. Although the ban was finally abolished in 1999, the French government refused to lift its own ban until 2002.
There have now been cases of BSE in many other countries, including the USA and Canada.
Since BSE is unlikely to flare up again as a major disease in cattle anywhere in the world, the emphasis now is on ensuring that even a low rate of BSE does not lead to cases of vCJD.
Efforts are also being made to ensure that vCJD is not spread through blood transfusions or the use of contaminated surgical instruments, and to find treatments for vCJD.
Those responsible for managing the BSE/vCJD episode often had to take decisions that affected people's lives and livelihoods on the basis of incomplete and sometimes contradictory scientific data and understanding.