An agent that is smaller than a virus causes scrapie. It is either a protein called a prion or it is another substance closely associated with the prion protein. Scientists still have a lot to learn about the scrapie agent and exactly how it works.
Tissue valves are made with tissues from porcine (pig) heart valves or bovine (cow) cardiac tissue because they function like human heart valves. Once the tissue is removed from the animal, it is chemically treated to preserve the tissue and prevent immulogic reactions once it is placed in a patient.
Biological Heart Valves. Biological Heart Valves are made of tissues and valves derived from animals or human donors. Their biggest advantage is that they do not cause blood clotting on their surface and truly replicate the form and function of the normal human valve.