In the nick of time

 作者:符舛洄     |      日期:2019-03-08 06:11:23
By Debora MacKenzie A SWISS cow infected with BSE has been removed from the human food chain after testing positive for the disease at an abattoir. This is the first confirmed case of infection in an apparently healthy animal destined for the dinner table. During the past three months Switzerland’s Federal Veterinary Office has organised tests of brains from 3000 randomly selected cattle more than 30 months old, at abattoirs across the country. All the brains have now been tested with a fast immunological assay for the prion protein thought to cause BSE, developed by Prionics, a company based in Zürich. An older, slower assay is also being used on the brain samples, and so far all the results from the first 2200 match the results of the fast assay. In September, both tests found the rogue prion protein in the brain of a four-year-old cow. The speed of the Prionics test meant the carcass was kept off the market. The sample of 3000 is too small to give an accurate incidence of BSE infection, but the results suggest that around 50 infected cows per year are eaten in Switzerland. Earlier this year, tests conducted by the Federal Veterinary Office and Prionics suggested a rate of infection of 4.5 per thousand in apparently healthy herd mates of cattle which had developed BSE (This Week, 13 June, p 4). In Switzerland, herds in which there is a case of BSE are destroyed. But the results raised the question of how many infected animals were lurking in apparently healthy herds. “BSE has a long incubation time, so we always knew that in theory, some infected animals might be entering the food chain,” says Markus Moser of Prionics. “Now we have actually observed it.” Prionics is now pressing the Swiss government to test all carcasses in abattoirs. In Britain, cows older than 30 months do not enter the food chain. The Swiss did not test younger cows because, even if they are infected as calves, it takes time for the prion to multiply to detectable levels. “We would have needed a very large sample size to have detected infection in younger cows,” says Moser. “But cows 20 months old have developed BSE in Britain.” Roy Anderson of the University of Oxford, a member of Britain’s Spongiform Encephalopathy Advisory Committee, says epidemiological models of the disease predict that per year, between 200 and 300 cows incubating BSE enter the food chain in Britain. It is not clear if any of these contain sufficient prion in their tissues to infect people, however. And attempting to exclude these animals using a test like the one developed by Prionics could be expensive. “If it is not too expensive and reliable, testing at abattoirs would make sense,” says Anderson. But so far,