Eating meat 'may still pose CJD risk'
Robin McKie, science editor
Sunday December 1, 2002
Muscle and flesh of cattle and sheep may harbour deadly levels of prions that cause variant CJD. This stark prospect, raised by the Nobel Prize winner who first discovered that these infective particles can cause brain illnesses, suggests eating meat may still pose a serious health risk.
The prospect that a timebomb may still be ticking in our kitchens was raised by Stanley Prusiner, who revealed yesterday that experiments at the University of California in San Francisco had shown that scrapie-infected mice have unexpectedly high concentrations of prions in their muscles.
'These are just mouse models, but they raise the obvious worry that cows and sheep could be similarly affected,' said Prusiner, who received the 1997 Nobel Prize for medicine for discovering that degenerative brain diseases such as Creutzfeldt-Jakob Disease are caused by prions.
Until now, scientists had assumed that only brains and spinal columns of cows and sheep contain dangerous levels of prions. But it appears that cuts of beef and lamb, which largely consist of muscle, could also be affected.
'The levels we found were a hundredfold less than those found in brains, but were still significant,' he said. 'In particular, we found that the hind legs of mice had high numbers of prions. It remains to be seen if that is mirrored in the hind legs of cattle or sheep.'
Prusiner said that part of the problem could be traced to existing tests for the existence of prions in animals or humans. 'These tests chew up prion molecules in such a way that means we fail to spot them. We were destroying critical evidence of infections, perhaps up to 99 per cent of it. It is very serious.'
As a result, he and his colleagues have perfected a new technique for detecting prions in samples, one that is 10,000 times more sensitive than existing tests used in Europe and America.
The new test - conformation dependent immunoassay (CDI) - works on a completely different principle. Essentially, it recognises the shape of prion molecules. 'We believe that by applying the test to cattle we should significantly reduce human exposure to bovine prions,' said Dr Jiri Safar, one of Prusiner's colleagues.
'Previous attempts to quantify BSE and scrapie prions in milk or non-neural tissue, such as muscle, may have underestimated infectious titers [levels] by as much as a factor of 10,000, raising the possibility that prions could be present in sufficient quantities to pose risk to humans. The high sensitivity of this new test may profoundly alter our view of the epidemiology of prion diseases.'
However, Prusiner had one reassuring message. He dismissed a study, published last week by British researcher John Collinge, which suggested that BSE was responsible for more than one type of CJD in this country. In the journal of the European Molecular Biology Organisation, Collinge suggests that a second form of the disease was also caused by prions from meat. However, Prusiner dismissed the idea. 'I simply do not read the data the way that he does. I can see no evidence of such an effect. I just don't think it is likely.'