US 'rediscovers' its second mad cow

2019-03-02 09:08:08

By The US has found its second case of mad cow disease in a cow suspected, but cleared, of having BSE in November 2004. Although meat from the cow did not enter the food chain, the finding calls into question the accuracy of the country’s BSE surveillance programme. The cow might also be the first case born in the US. The first US case was in a cow imported from Canada in 2003. In 2004 the country started testing “high-risk” cattle – those that show neurological symptoms, are found dead or are “downers” (unable to stand). Since then it has tested 375,000 cattle. None were declared positive. In contrast, Canada has tested 30,000 cattle and found three positives. The rate at which the tests uncover positive cattle depends on the sample size, stresses Marcus Doherr of the University of Bern in Switzerland, who helped develop Swiss BSE surveillance. This means either that BSE is less evenly distributed in North America than thought, or that the US is missing cases. Unlike Canada, which uses the rapid “western blot” test, the US uses a test called ELISA, which is more prone to false positives. In 2004 the ELISA test detected three BSE positive cattle in the US. When these brains were re-tested, the ELISA was negative. Then they were subjected to immunohistochemistry (IHC) testing – a thin slice of brain is stained with antibodies for the prion protein that causes BSE. All were negative, and the cattle were declared BSE-free. “But if the prion is diffuse enough in the brain tissue, you can get a weak signal with the ELISA, and a negative with IHC,” says Doherr. Another test is needed to be certain, he says. It was revealed on 10 June that the US Department of Agriculture’s own Inspector General asked the USDA to carry out western blot tests on the three conflicting samples from 2004. One sample – a downer from November – came back positive. The animal was reportedly nine years old – born just before the US banned the use of cattle remains in cattle feed, which can spread BSE. USDA would not confirm this or the origin of the animal, though John Clifford, the USDA’s chief veterinary officer, notes that “we have no information that it was an imported animal”. But the animal is still not officially BSE positive. Because of the conflicting results, says Clifford, the sample will be re-tested at the USDA lab in Iowa, and the international BSE reference lab at Weybridge, UK. More on these topics: