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.
1 Biological valves are made from animal tissue (called a xenograft) or taken from the human tissue of a donated heart (called an allograft or homograft). 2  Sometimes, a patient's own tissue can be used for valve replacement (called an autograft).early all of these operations are done to repair or replace the mitral or aortic valves. These valves are on the left side of the heart, which works harder than the right. They control the flow of oxygen-rich blood from the lungs to the rest of the body.