A second cow in the US has produced an initial positive result for BSE. US officials are saying it is 鈥渧ery likely鈥� the cow will turn out to be negative. However, in most countries, the test used is wrong about a positive result only about once in a thousand times.
The US Department of Agriculture announced the result late on Friday. It was the first positive test since the US began testing thousands of cattle for BSE on 1 June. The testing is in response to the discovery of its first mad cow in December 2003.
However, the USDA is calling the result 鈥渋nconclusive鈥� until it is confirmed by immunohistochemistry (IHC) at the US鈥檚 National Veterinary Services Laboratories in Ames, Iowa.
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IHC is a highly reliable but slower BSE test. In a briefing on Friday, John Clifford, deputy head of the US Department of Agriculture鈥檚 Veterinary Services Programme, said the IHC results will take four to seven days.
The USDA will only give the location of the animal was and why it was tested if that result comes back positive, said Clifford. The USDA is targeting animals that show BSE-like symptoms, or which die for unknown reasons, or are unable to stand. Such 鈥渉igh risk鈥� cattle are 10 to 15 times more likely to have BSE.
Extremely sensitive
But Clifford stated repeatedly that 鈥渋t鈥檚 very likely this animal could be negative鈥�. He stressed that the BioRad test used was 鈥渄esigned to be extremely sensitive鈥� to catch any possibly infected animal, some of which 鈥渨ill end up negative during further testing鈥�.
Asked the odds of the result being a false positive, Clifford said 鈥渨e wouldn鈥檛 want to provide that type of information鈥�. But the rate of false positives for the BioRad procedure was measured by the European Commission when it evaluated BSE tests for use in the EU in 2003.
Those data have never been published, but industry sources who have seen them say the BioRad test had a false positive rate of about one in a thousand initial tests, a rate borne out subsequently in practical experience. BioRad is used in Germany and Belgium.
In Europe and Japan, if a cow鈥檚 brain initially tests positive with BioRad, it is tested twice more. Only if one of those repeated tests is also positive is the sample sent for IHC confirmation. The false positive rate after such repeated testing is even lower, around one in 100,000 for BioRad. It is not known whether the USDA is doing this.
True incidence
However, Markus Moser of the Swiss firm Prionics, which makes a rival ELISA test for BSE, points out that if the true incidence of BSE in cattle is much lower that the test鈥檚 false positive rate, most of the positive tests obtained with the test will indeed be false.
In Japan, where the BSE incidence is just one tenth of the BioRad false positive rate, only eight of the first 113 positive BioRad tests were confirmed.
It may be that this is what USDA is counting on when it says its positive test is 鈥渧ery likely鈥� to turn out false. But no one knows if the true incidence of BSE in the US is so low 鈥� that is what the testing programme is supposed to measure.
