The life and times of a cow from the Canadian province of Alberta are currently the subject of intense scrutiny, as officials struggle to understand how it became infected with bovine spongiform encephalopathy (BSE).

Canadian agriculture minister Lyle Vanclief announced on 20 May that the animal, which was slaughtered in January, had tested positive for BSE. Within hours, the US Department of Agriculture (USDA) banned Canadian beef imports. As of 26 May, 17 herds in Alberta, Saskatchewan and British Columbia had been quarantined.

How the cow contracted the disease continues to perplex officials. Experts say that it is likely that the animal was infected through feed made from the remains of cattle with the disease. Canada banned the use of animal remains in cattle feed in 1997, around the time that the cow was born.

Officials now have to determine what proportion of the country's cattle herd, if any, has the disease. Canada continues to allow cattle remains to be used in feed for animals such as chickens, raising the possibility that infected feed could be finding its way to cattle.

But experts are betting against a large-scale outbreak or any danger to consumers. About one million cows are imported into the United States from Canada every year. Lisa Ferguson, who works on BSE-like diseases at the USDA, says that the United States runs BSE tests on these cattle, and would have detected any sign of an epidemic.