BSE and vCJD: Their biology and management
BSE and vCJD: Their biology and management

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BSE and vCJD: Their biology and management

12 The international dimension

12.1 An overview

Although by 2000 the number of new confirmed cases of BSE in the UK was approaching zero (Figure 1), several of these involved cattle born since the tightening of feed controls in August 1996. By then, confirmed cases of BSE were also being officially notified in countries other than the UK (e.g. in cattle born in France, Spain, Germany and Japan; see Figure 2). Indeed, in September 2001 it was predicted that during 2002 there would be more cases of BSE in France than in the UK.

Question 58

What is the likely cause of these BSE cases in animals born in continental Europe or elsewhere?

Answer

As calves, these animals may well have been fed contaminated cattle concentrates exported from the UK before effective controls were introduced in the early 1990s.

The UK exported 25 000 tonnes of MBM in 1991. By then, MBM destined for Member States of the EU did not include SBO. However, MBM sent outside Europe (e.g. to Thailand, Taiwan, Singapore and Indonesia) did contain SBO. Some of this MBM was subsequently sold on to other countries, such as China. When the UK stopped exporting MBM entirely in 1996, other European countries took over the trade - until 2000, when BSE cases were reported from all over Europe. Then the USA took it over, having declared for years that it was entirely free of BSE. This emphasises the truly globalised trade in animal feed - as well as animals destined for human consumption - the origins of which can be extremely difficult to trace. [R E]

In fact, EC scientists predicted in 2000 that BSE might already exist in the USA, because prior to 1996 it had imported British cattle (some of which could have had BSE) and 44 tonnes of British MBM. After 1996, the USA imported 800 cattle from other European countries in which BSE was subsequently discovered.

In May 2003, a Black Angus beef cow born in Canada in 1995 was found to have BSE. Canada's only previous case (in 1993) had been born in Britain in 1987. The USA immediately closed its borders to Canadian cattle. However, in 2002 Canada had sent 500 000 live cattle and a great deal of MBM to the USA. [R D]

After behaving strangely at a slaughterhouse in Washington State in December 2003, a six-year-old Holstein cow was confirmed to have BSE. The cow had almost certainly been infected from cattle feed manufactured in either Canada (where it had been born) or the USA that had included tissue from native-born animals. Confidence in the USA's beef industry was severely damaged. For instance, countries such as Japan (a major purchaser of US beef) - and, indeed, Canada - refused to import beef from the USA. On the basis of BSE in one animal, this might be regarded as an extreme example of the application of the precautionary principle. The USA commenced an enhanced surveillance programme in June 2004 using a rapid test for BSE. This produced two inconclusive results in June/July 2004 and another in November 2004. All three animals tested negative in a confirmatory test based on immunohistochemistry (which detects small quantities of substances based on their binding to specific antibodies). Although the first two animals also tested negative in a further Western blot test (another very sensitive immunohistochemical technique), the November 2004 case came back positive. Meanwhile, Canada confirmed two more BSE cases in January 2005, bringing to four the total number of BSE-infected cows identified or linked to Canada. [R D]

By 2003, diagnostic tests were available to test for BSE in live cattle. This represents an advance on the way BSE cases were recognised in the UK in the 1980s and 1990s, when confirmation of suspected cases required post-mortem examination of brain tissue. However, there were challenges to the science from within the beef industry, not least because there was no consensus on which of several tests was the most effective in identifying BSE or how many - and which - cattle needed to be tested in order to establish the incidence rate of BSE in the North American herd.

Question 59

In 1997, the USA banned the feeding of cattle carcases to other cattle - although it was widely believed that this ban was not enforced properly (a situation reminiscent of that which applied in the UK in the early years of BSE). However, it remained legal to include cattle MBM in feed for pigs and poultry. It was also legal for pigs and poultry - and poultry litter containing poultry feed - to be included in cattle feed. Explain why, in these circumstances, the ruminant feed ban was extremely unlikely to have been adequate to stop the spread of BSE. [R E D]

Answer

For a start, accidental cross-contamination could occur during the manufacture of feed for cattle, pigs and poultry. It is also possible for cattle accidentally or deliberately to be given feed intended for pigs or poultry. Moreover, cattle MBM could legally be incorporated into feed given to pigs or poultry and then tissues from these animals - and also some of their feed - to be incorporated into cattle feed. If even tiny amounts of PrPSc were contained in the cattle MBM, then BSE could spread.

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