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 prosthetic valves used for replacement: tissue and mechanical. When heart valve replacement is necessary, the decision to implant either a tissue valve or a mechanical valve will be made based on several factors: 1  Health conditions (in addition to your heart issues). 2  Medications.