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TORONTO (AP) - Investigators placed four more farms under quarantine Friday, including three involving feed production, in a widening search for the cause of North America's first case of mad cow disease in a decade.

The Canadian Food Inspection Agency said 13 farms were now under quarantine - eight in Alberta, where the recent case of bovine spongiform encephalopathy, or BSE, was found; two in Saskatchewan to the east; and three in British Columbia to the west.

The farms quarantined in British Colombia were feed suppliers, said Brian Evans, the agency's chief veterinary officer. Cattle feed from animal sources contaminated with BSE is considered the most likely cause of the infected cow.

Agriculture and veterinary investigators were tracing that cow's history to learn where it lived, how many calves it produced and what it ate. The growing list of quarantined farms reflects the thoroughness of the investigation, rather than any indication of further spread of BSE, said George Luterbach of the CFIA.

Early indications showed the infected cow might have been born on a Canadian farm, which would make it the first case of a North American-born animal contracting the illness known as mad cow disease which decimated the British beef industry in the 1990s.

The outbreak has caused the United States, Japan, Australia, South Korea, Singapore and New Zealand to ban all beef imports from Canada, despite reassurances from Canadian government and industry officials that the beef was safe.

Canadian investigators removed all the cattle from one Alberta farm and were destroying the herd to examine the brains for further possible cases of BSE. Test results were expected early next week.

"I don't believe that cow came in contact with anything that gave it that disease on my farm," owner Marwyn Peaster said Thursday.

While Canadian authorities and farmers say the lone case of BSE presents minimal public risk, the closing of major foreign markets to Canada's beef products brought immediate cuts in production and uncertainty to a $22 billion industry.

The only previous case of BSE in North America was in 1993, involving a bull imported from Britain. The animal and its herd were slaughtered, but no trade bans resulted.

The recent infected cow was slaughtered Jan. 31 but kept out of the food chain because it was believed to have pneumonia, officials said. Testing was delayed several months because there was no suspicion of BSE, as well as a backlog of higher priority cases, officials said.

Mad cow disease first erupted in Britain in 1986 and is thought to have spread through cow feed made with protein and bone meal from mammals.

Canada and the United States outlawed the feeding of meat and bone meal to cattle, sheep and goats in 1997, a rule believed to be the main defense against the disease.

The human form of the illness is Creutzfeldt-Jakob disease, which causes paralysis and death. Scientists believe humans develop new variants of Creutzfeldt-Jakob when they eat meat from infected animals. More than 130 people have died of the disease, mostly in Britain.

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