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.
For someone having an aortic valve replacement, the new aortic valve will be one of two types. These two types of aortic valve replacements are: Man-made mechanical valves, made of metal and plastic. Tissue valves, which are aortic valves that are taken from infection-free animal hearts.