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Testing differences

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Feb 10, 2005
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Canada says no change needed in its mad cow tests


By Roberta Rampton

WINNIPEG, Manitoba (Reuters) - Canada is not looking at changing the way it confirms tests for mad cow disease because it is confident it is finding positive cases, a senior food safety official said in an interview.

The U.S. Agriculture Department said on Friday it will revise its protocol for dealing with suspect cases of bovine spongiform encephalopathy, or mad cow disease, after it failed to immediately confirm a positive case in November.

The USDA said it will change the way it runs the "gold standard" immunohistochemistry (IHC) confirmatory test, and add a second type of test called Western Blot to its regimen.

Those are tools that Canada is using already, said Darcy Undseth, a senior veterinarian at the Canadian Food Inspection Agency.

"We already have those elements in place, so at this point, there is no indication that we would have to change our protocol," Undseth told Reuters.

A sample from a U.S. beef cow tested positive for the disease last November using a rapid screening test.

But when the USDA ran the IHC test on the sample, it came back negative.

The sample was found to be positive this month when it was retested by the USDA and by a lab in Weybridge, England, that is considered to be the world expert on mad cow disease.

The IHC test identifies the prions that cause mad cow disease with a stain. The stain is attached to an antibody that sticks to the prion, Undseth said.

The USDA had used only one antibody, but Canada has been constantly adjusting the antibodies it uses in consultation with the Weybridge lab, Undseth said.

"We have a panel of antibodies, and we use usually nine or 10 antibodies," he said.

The USDA said on Friday it will work with the Weybridge lab to determine what antibodies it should use in its IHC test.

Canada has confirmed three cases of mad cow disease in its herd in the past two years, all using the IHC method, Undseth said.

Out of more than 61,000 animals screened for the disease using rapid tests since 2003, another three returned inconclusive results, Undseth said.

The three samples were thought to be the result of laboratory error, but were retested at Canada's federal animal disease lab in Winnipeg, first with a rapid test, which came back negative, and then with the IHC test, which was also negative, he said.

Samples from another three animals found in the field showing symptoms of mad cow disease were sent directly to the Winnipeg lab last year, Undseth said.

On those animals, both the rapid screening tests and the IHC tests were negative, he said.

"So far we've had very clearly negative or clearly positive cases," Undseth said.

But should Canada find cases that were strongly positive using the rapid test, but returned negative results using the IHC method, Canada's policy is to run the Western Blot test, Undseth said.

The Western Blot test is also useful for samples that have partially decomposed, he said.

The Canadian Food Inspection Agency is not concerned that it took seven months for the USDA to confirm the positive case, Undseth said.

Out of 388,000 cattle screened by the USDA with rapid tests in the past year, only three initially came back inconclusive, with only one of those found positive, Undseth said.

"We know that they test a very large number ... and they've had very few screening positives," he said.

"We have full confidence that it's showing there's a very low prevalence of BSE in North America."

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