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BSE testing

WHEN IT COMES TO bovine spongiform encephalopathy (BSE), fear of the disease can be worse than the disease itself. Sporadic reports of sick animals can make futures markets shudder and erode consumer confidence, even if initial test reports turn out to be false positives. Although gold-standard tests that reduce false-positive results are available, they are not yet the standard in the U.S. BSE surveillance effort.

Inconclusive tests

Back in July 2003, a Beef magazine article examining the market psychology of inconclusive BSE tests showed an industry that was all but resigned to having to deal with false positives. Beef's report included the following telling quote from John Clifford, DVM and deputy administrator for the Animal and Plant Health Inspection Service Veterinary Services: “Inconclusive tests are a normal component of most screening tests, which are designed to be extremely sensitive. They [rapid tests] are designed to cast a very wide net in order to catch any possible animal that may have a condition, which will end up negative during further testing.”

After the rapid test, the follow-up test, usually the ELISA test, is supposed to remove doubt as to whether or not the disease is indeed present. But the ELISA test can also give false positives, and has. Since the ELISA method tests only one feature of the protein in a sample, positive results must be further confirmed in exhaustive laboratory tests that require scientists to analyze each sample with a battery of complex laboratory assays. This can take days.

False positives avoided

If adopted as the standard by the USDA, a more reliable testing technique, called the Western Blot test, could help avoid these secondary false-positive results by giving information on several independent features of the protein contained in a sample. Swiss company Prionics produces the Western Blot test kit, which is the standard in the Canadian BSE surveillance program.

Markus Moser, CEO of Prionics, is hopeful that the USDA will approve the Western Blot test for the U.S. BSE surveillance program. “Prionics-Check Western test has been used on 18 million BSE tests between 2001 and 2003 without a false-positive result,” Moser says. “False-positive results can cause a significant loss of consumer confidence and have a major economic impact, especially in BSE-free countries and countries with a low incidence of BSE, like the U.S.”

Vigilance required

An optimist might say that the widely adopted ban on animal-based feed components in the U.S. and other countries could soon make BSE just an unpleasant memory. In that rosy scenario, new BSE tests would end up as a solution looking for a problem. Unfortunately, it seems likely that the scourge of prions and the disease they cause could be with us permanently.

Scientists now believe that normal proteins occasionally and spontaneously misfold themselves into prions. Once that happens, adjacent proteins tend to assume the same misfolded shape. Like adding one crystal to a supersaturated jar of sugar water, one prion can create a cascading effect that can rapidly transform other normal proteins into prions. So even though eliminating animal components from all feed will substantially reduce the risk of further spread of BSE, agriculture will have to remain vigilant against new cases popping up that could spread between animals. A better confirmation test would certainly help.

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