Tests to reveal where Canada’s single BSE-infected cow picked up the disease have proved inconclusive, meaning officials will have to slaughter and test around 650 more cattle to be sure that no others are infected with the disease.

The genetic tests compared the DNA of the infected cow with that of bulls on farms at which the cow had been kept during its lifetime. Despite its exact origin remaining unclear, investigators still believe the cow came from one of a series of farms in Saskatchewan and Alberta, according to chief veterinarian Brian Evans.

“It was recognized from the outset that because we did not have genetic material or entire breeding records for all of the potential herd sires on all of the premises… dating back to 1996, that we may come to this point,” he was quoted by Reuters as saying.

Investigators will also quarantine another herd of 140 cattle that may have shared feed from the same source as a farm where officials believe the cow spent four years.