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.
There are two types of artificial heart valves: mechanical devices made from synthetic materials; and biological or tissue valves made from animal or human tissue. In general, biological valves are used for patients who are over 65 or cannot take anticoagulants.