Search for BSE in muscle meat draws blank17:05 27 March 02
NewScientist.com news service
Tests by government scientists in France have allayed renewed fears that eating beef can cause variant Creutzfeldt Jakob Disease, the human form of BSE. The fears were heightened on 18 March when scientists unexpectedly reported finding traces of infective material in the muscles of mice.
The findings carried extra weight because they came from the lab of Stanley Prusiner, the scientist at the University of California in San Francisco who won a Nobel Prize for discovering mutated "prions". These are the defective proteins believed to cause brain-wasting diseases like BSE, vCJD and scrapie.
Prions are known to collect in brains, spinal cords, spleens and other lymphoid tissue and these parts have long been banned for human consumption. Muscle tissue in meat was assumed to be free of prion contamination and safe to eat.
But Prusiner's team found prions in the hind leg muscles of mice whose brains had been injected with BSE-like prions. He reported his results in Proceedings of the National Academy of Sciences.
The new twist in the story came on Wednesday, when French government scientists announced that they had drawn a complete blank when they looked for prions in muscles from several BSE or scrapie-infected animals including mice, sheep, goats and cows.
"The tests proved negative in the search for pathological prions in the set of samples, including those taken from the hind limb muscles," says the AFSSA, France's food safety watchdog.
Reassuringly, tests on peripheral nerves and lymphoid tissue in the cow also came up negative, suggesting that meat will be safe even if it contains this type of tissue. "These observations are consistent with the findings made to date concerning the distribution of the infectivity linked to BSE in cows," they say.
The researchers detected prions with two tests. The first, an "ELISA" test, detects antibodies made by animals against prions. The second "Western blotting" test isolates and identifies fragments of the prion protein itself. The scientists will discuss their results in more detail at a meeting of France's top BSE specialists on 11 April.
Low level infectivity
The significance of Prusiner's results has also been questioned in Britain. Peter Smith, chairman of the government's Spongiform Encephalopathy Advisory Committee, told New Scientist that a much more significant experiment had been under way for five years in Britain, and had yet to give any cause for concern.
Instead of using a surrogate animal like the mouse, this experiment is focusing on which cattle tissues can transmit BSE to other cattle. Scientists at the Veterinary Laboratories Agency in Weybridge, Surrey, injected the brains of live calves with liquidised tissue from various parts of the bodies of BSE-infected cows.
"None injected with muscle have gone down yet," says Smith. "But if you inject BSE-infected brain tissue, the calves come down with BSE in about two years, as expected," he says.
"If there is infectivity in muscle, it must be at a much lower level than in other tissue, particularly that from the brain or central nervous system," says Smith. You can never prove a negative, he adds, but the results so far have been reassuring.
Journal reference: Proceedings of the National Academy of Sciences (vol 99, p 3812)