WANHAM, Alberta - This is how quickly a cattle rancher's luck turns bad.

One day, Marwyn Peaster was in business, as successful as the next man in a place where 50-below-zero winters and parching summer droughts sometimes make success more a state of mind than a line on a balance sheet.

Then, last week, a government official called and said one of Peaster's cows had tested positive for ``mad cow'' disease. Almost immediately, the freight trucks showed up and began herding up Peaster's cows, all 150 of them, until his holding pens were empty and the only sounds on the farm were the mooing and stomping in the trucks, and then the rumble of big truck engines, and then nothing.

Within 24 hours, Peaster's cows were slaughtered and tested, and a federal veterinarian drove out to Mel McCrae's small farm in Baldwinton, Saskatchewan. It looked as if Peaster's infected cow might have originated from McCrae's herd, the official said. The 65-year-old rancher of purebred Angus cattle was asked if he might have imported the animal from Britain.

No, McCrae replied. If it was one of his cows, it was born right there in Canada.

The implication was clear: Mad-cow disease, one of the most frightening of livestock illnesses because it can infect the human food supply, had likely reached North America.

A total of 16 ranches are under quarantine across three provinces in central Canada, as investigators attempt to track down the source of an infection -- so far, only a single case -- that could cripple Canada's $21.9 billion livestock industry and threaten the top foreign supplier of live cattle to the United States.

Beef infected with bovine spongiform encephalopathy, or BSE, have been linked to 130 human cases of a variant of Creutzfeldt-Jakob disease, which causes paralysis and death.

With the United States and several other nations imposing a ban on Canadian beef imports, feedlots are backing up, cattle auctions have been canceled, and government investigators are struggling to determine whether contaminated feed was responsible for infecting Peaster's cow -- and if it was, how many other cattle might have been exposed.