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.
Tissue heart valves can come from a variety of sources: porcine (pig), bovine (cow) and homografts or allografts (human). The primary advantage of a tissue valve over a mechanical valve is that you do not need to be on lifelong blood-thinner medication.