updated 3/3/2006 10:00:36 PM ET 2006-03-04T03:00:36

Canada’s latest case of mad cow disease probably came from tainted feed, raising questions about safeguards designed to keep the disease from spreading.

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Canada confirmed its fourth case of the disease in January. The cow was born in 2000, three years after Canada banned cattle protein in cattle feed.

The cow’s age raises questions about the effectiveness of the ban, because the disease spreads only when cattle eat feed containing certain tissue from infected cattle.

Cattle protein was commonly added to cattle feed in North America to speed growth until Canada and the United States banned the practice in 1997.

Feed for some other animals may still contain cattle remains and can be manufactured at the same mills that make cattle feed.

“Cross-contamination is the most likely source of this,” said Gary Little, senior staff veterinarian for the Canadian Food Inspection Agency.

He added, “We’re not ever going to be able to say with certainty that’s where it happened and point to a particular instance.”

Canada’s investigation found two feed suppliers for the farm where the cow was born and reared. One of the suppliers makes both types of feed, containing cattle remains for other animals and without cattle remains for cattle.

The probe “identified incidents of concern” where cattle feed was manufactured or distributed immediately after banned material had been processed. Equipment may not have been cleaned properly, the report said.

The report also raised the possibility the infection might have come from residue on bins at the farm that had not been cleaned.

Little said Canada has continued to improve its feed ban and has increased inspections at feed plants.

Canada’s feed ban is stricter than the U.S. feed ban; the Canadians do not allow feed with chicken litter or restaurant leftovers, both potential pathways for mad cow disease.

Both governments are working on rules that would strengthen the feed bans, although Canada’s would go further than the that of the United States.

Critics of the U.S. approach include McDonald’s Corp. and leading mad cow disease researchers, who say loopholes would remain.

In the latest case of mad cow disease, “clearly, the feed ban wasn’t working,” said Michael Hansen of Consumers Union. “Everything comes down to how well the feed rule is enforced.”

Besides the four cases in Canada, another Canadian cow was diagnosed after crossing into the United States in 2003. It became the first U.S. case of mad cow disease. A second case was diagnosed last June in a Texas-born cow.

Mad cow disease is the common name for bovine spongiform encephalopathy, or BSE, a degenerative nerve disease in cattle. In people, eating contaminated meat products has been linked to the rare but fatal variant Creutzfeldt-Jakob disease.

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