The most likely source of infection was, indeed, commercially manufactured pet foods. In 1989 the pet food industry in the UK removed the dangerous tissues from cows voluntarily from pet food and in 1990 a statutory ban was put in place. Only one cat,an adopted stray, apparently born after the ban has developed the condition.
Most pet foods are labelled as chicken flavour or beef flavour. This has little to do with what sort of meat is in the food.
I would suggest that anyone worried should find out the regulations concerning pet food manufacturing in their own country. Or find a food that states the contents fully or a company that garuntees the quality.