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.
One type of TAVR valve, made of bovine (cow) tissue and supported on a metal stent, is then implanted inside the narrowed valve resulting in a normal functioning aortic valve. With a TAVR procedure, the patient does not need the chest opened for surgery or put on a heart-lung (bypass) machine.