Tuesday, June 14, 2005

Mad cow back on the USDA PR menu

Don't worry. Be happy. That same old song from US Department of Agriculture as they soft-shoe their way through the latest BSE debacle.
"Consumer confidence, I am very confident, will remain," Agriculture Secretary Mike Johanns said. "This is a situation where the firewalls worked. We do not have a human health risk. This animal did not enter the food chain. This animal never got near the food or feed chain."

Johanns, former governor of beef-producing Nebraska, told reporters during a conference call that he intended "to enjoy a good steak."
On June 10 it was announced that tissue from a second cow (this one apparently native born as opposed to the first one which was born in Canada) was being sent for confirmatory BSE tests. But it turns out this isn't a "new" cow but a check of a supposedly negative cow from November, 2004.

Since the US started testing high risk "downer" cattle for BSE, 375,000 tests have been done and all have been negative. Canada has found 3 positives in 30,000 cattle tested. (New Scientist). The question is why the rate is so much lower in the US. It could be because there are fewer cases, but there is no reason why this should be. Or it could be the test.

The November cow tested positive with the screening test (an ELISA). Industry sources say the false positive rate for this test is about 1 in 1000. However if many cattle are tested and BSE is rare it can still turn out that most positives are false positives (this is the inverse conditional probability of the false positive probability, the positive predictive value, and depends on the prevalence of true positives in the population). But it still wouldn't explain why the rates in the US and Canada are so different.

It turns out there is also some question about the accuracy of the follow-up confirmatory test, an immunohistochemistry (IHC) method. In IHC a thin slice of brain is stained with fluorescent antibodies against prions and examined optically, but according to Marcus Doherr of the University of Bern in Switzerland, one of the developers of Swiss BSE surveillance, ". . . if the prion is diffuse enough in the brain tissue, you can get a weak signal with the ELISA, and a negative with IHC.” (New Scientist) Apparently these questions were sufficiently serious that the Inspector General of USDA aksed for additional confirmatory tests with western blot (also a technique that uses antibodies against the prion, but doesn't depend upon spatial distribution as in IHC).

So at this point, pending the outcome of the western blot in the international reference lab for BSE in the UK and a USDA lab in Iowa, this is not a BSE positive animal. Reuters is reporting the tests might take as long as two weeks. Whatever the outcome, though, the questions that gave rise to the retest remain. Are the methods used in the US sensitive enough, is the confirmatory test sufficiently sensitive, and why do US results differ so markedly from Canada's?

Addendum: See excellent coverage over at U.S. Food Policy, with link to Secretary's Press Conference transcript.