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BSE: Disease control & eradication - Causes of BSE

The BSE Inquiry Report

The BSE Inquiry Report, published in October 2000, concluded that the development of the BSE epidemic resulted from the use of infectious meat and bone meal (MBM) in cattle feed. The MBM had been produced by the rendering (industrial cooking) of carcases of cattle infected with BSE. Cattle feed contaminated with pig or poultry feed containing MBM continued to infect cattle after the 1988 ban on the use of ruminant MBM in cattle feed, although the feed ban and 1990 Specified Bovine Offal (SBO) controls markedly reduced the risk of infection. The feeding of MBM to all farmed animals was banned in 1996 and an EU-wide ban has been in place since 2001.

Exactly what caused the first case of BSE before the disease was amplified by feeding infectious bovine MBM back to cattle remains uncertain. The BSE Inquiry suggested that a new transmissible spongiform encephalopathy (TSE) agent emerged in the 1970s and that the first cases of BSE in the 1970s and 1980s escaped detection. The cause of this new agent was likely to have been a new mutation affecting the prion protein (PrP) in cattle, or possibly in sheep.

The BSE Inquiry commented that it was a common misconception that a reduction in rendering temperature or a failure to prescribe minimum rendering times led to a failure of inactivation of the scrapie agent and its subsequent transmission to cattle. Changes in the rendering industry in the 1970s and early 1980s – a switch from batch to continuous processing and the abandonment of solvent extraction of tallow – might have led to a reduction in the inactivation of the BSE agent. However, the BSE Inquiry noted that processes used previously were also incapable of completely inactivating TSE agents.

The BSE Inquiry concluded that all the evidence pointed to the specific association of an abnormal form of the prion protein (PrP) and TSEs. Normal PrPC is harmless. In TSEs, PrPC is converted to an abnormal form known as PrPSc resulting in the death of nerve cells in the brain and spinal cord.

The BSE Inquiry noted that all the sources of BSE examined had shown the same characteristics in mice. The same strain had been detected in exotic ungulates and carnivores from zoos and in cats developing feline spongiform encephalopathy (FSE).

The BSE Inquiry also discounted alternative theories for the cause of BSE such as the organophosphate and the autoimmune theories.

Development of Rendering Controls

Great Britain first introduced statutory rendering controls in 1981. These were aimed at reducing the contamination of animal feed with Salmonella. European Council Directive 90/667/EC introduced harmonised rendering standards across the European Community and these were implemented in Great Britain by the Animal By-Products Order 1992. These were the first statutory time/temperature controls to be applied to the rendering process. That year the European Commission also commissioned a study to assess how effective the different rendering processes operating in the EC were at inactivating TSE agents. The most effective rendering process involved a temperature of 133oC and 3-bar pressure for a minimum of 20 minutes. As a result the Commission introduced new standards in 1997. In October 2002, the EU adopted Regulation (EC) No.1774/2002. The Regulation ensured that only materials derived from animal declared fit for human consumption could be used for the production of animal feed. It also banned intra-species recycling.

The Horn Review

In the light of the BSE Inquiry’s conclusions, the Government asked a committee led by Professor Gabriel Horn to review the scientific findings on the origins of BSE, including evidence presented to the BSE Inquiry and emerging findings. Their Review of the Origin of BSE Adobe acrobat pdf file (257 KB) was considered by the Spongiform Encephalopathy Advisory Committee (SEAC) and published on 19 July 2001.

The Horn Review agreed with most of the conclusions of the BSE Inquiry. The Horn Review explored the reasons for the BSE epidemic beginning in Britain in the 1970s/early 1980s. MBM had been fed for several years in a number of countries, many of which also had sheep with scrapie.

The Horn Review concluded that an unusual combination of events occurred in the UK in the 1970s/1980s. These related to changes in rendering practices producing a ten-fold increase in the amount of TSE infectivity surviving, the feeding of rations containing MBM to very young calves, a high ratio of sheep to cattle and an infectious agent in the MBM – possibly derived from sheep with scrapie, a sporadic mutation in a cow or other sources such as goats or exotic ungulates.

Standing Scientific Committee

In November 2001, the European Commission’s Standing Scientific Committee published a scientific report Adobe acrobat pdf file (647 KB) on the origin of BSE and the transmission of BSE.

The Hill Review

In 2005, Defra published an independent review Adobe acrobat pdf file (181 KB) of BSE cases born after the 1996 reinforced feed ban (BARB cases), carried out by Professor William Hill FRS of the University of Edinburgh. Further information on BARB cases and the Government’s response to the Hill Review is available Adobe acrobat pdf file (553 KB).

Different (Atypical) Forms of BSE

In 2004, Italy and France reported the detection of a small number of BSE cases with different molecular characteristics to BSE cases seen previously. Several other European countries (including Denmark, Poland, Belgium, The Netherlands, Sweden, Switzerland, and Germany) as well as Japan, the United States of America and Canada have also reported similar cases.

These (atypical) BSE cases have mainly been detected by active surveillance in cattle over 10 years of age, on the basis of the molecular characteristics of the disease-associated prion protein (PrPsc). They fall into two groups - those with Lower molecular weight PrPsc (L-Type) and those with Higher molecular weight PrPsc (H-Type). The Italian L-Type cases have also been described as Bovine Amyloidotic Spongiform Encephalopathy (BASE) due to the due to the presence of amyloid plaques in their brains. Experiments have shown that both H-Type and L-Type BSE retain their molecular characteristics following intra-cerebral transmission to mice.

The Spongiform Encephalopathy Advisory Committee’s August 2007 statement on new forms of BSE is available. Details of atypical BSE cases in Great Britain (PDF 50KB) are attached. The first was detected in 2006 during a retrospective examination of brain samples from previous BSE cases. The aim of the retrospective studies was to determine whether different forms of BSE had occurred in the UK in the past. Further information on the retrospective studies (SE1795 and SE1796) is available.

References:

Biacabe, A.-G., Laplance, J,-L., Ryder, S. And Baron, T. (2004) Distinct Molecular Phenotypes In Bovine Prion Diseases. European Molecular Biology Organization Reports 5, 110-114.

Casalone, C., Zanusso, G., Acutis, P., Ferrari, S., Capucci, L., Tagliavini, F., Monaco, S. And Caramelli, M. (2004) Identification Of A Second Amyloidotic Spongiform Encephalopathy: Molecular Similarities With Sporadic Creutzfeldt-Jakob Disease. Proceedings Of The National Academy Of Sciences (USA) 101, 3065-3070.

Terry, L.A., Jenkins, R., Thorne, L., Everest, S.J., Chaplin, M.C., Davis, L.A. And Stack, M.J. (2007) First Case Of H-Type Bovine Spongiform Encephalopathy Identified In Great Britain. Veterinary Record 160: 873 - 874.

 

Page last reviewed: 4 June 2009

Department for Environment, Food and Rural Affairs