Canada had previously been considered free of BSE, although there had been one earlier reported case of a cow in Canada testing positive for BSE, in December of 1993. That cow, however, had been imported from Britain, and was immediately destroyed along with the rest of its five-member herd.
Officials are currently unsure if the recently-infected cow was born and raised in Canada or had been imported. It has been destroyed, as will be all 150 members of its herd. For additional details on this story, read the coverage by CNN or The New York Times (free registration required).
BSE is a degenerative neurological disease affecting adult cattle. It is believed by many to cause variant Creutzfeldt-Jakob disease or vCJD, in humans, although this is widely disputed. For more details on BSE and vCJD, and the possible link between them, see this earlier SFT/SciScoop story from February.