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.
Surgical options for valve replacement include: 1  Mechanical valve â a long-lasting valve made of durable materials. 2  Tissue valve (which may include human or animal donor tissue) 3  Ross Procedure â âBorrowingâ your healthy valve and moving it into the position of the damaged valve aortic valve.