Such animals would carry human genes and these would order their cells to manufacture human proteins for harvest in their blood or milk. It usually takes a high degree of specialized skill to breed such animals. Of these only a small proportion will transmit the gene to their offspring. At the moment there are no regulations requiring segregation of transgenic livestock from other animals or special disposal of carcasses, although the matter is under review by the government. These animals could be bought and sold as conventional breeding stock and so reach the food chain - an unlikely event just now as such breeding research is just starting in Canada.
Regulations exist that set levels for drugs or pesticides in foods.
Some gene products could be treated in the same way by establishing a safe level or regulatory threshold. This matter is also being reviewed. This will continue to be a concern and needs to be watched. But biotechnology can also help retain biodiversity. Current technology permits saving rare species facing extinction by storing fertilized embryos at low temperatures then, re-implanting them in a surrogate mother long after the original parent has died.