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.
Carpentier-Edwards aortic stented porcine valves. 1  Made of pig aortic valves that have been treated in a preservation liquid and mounted on flexible frames. 2  The frame is designed to be flexible at the opening as well as where the leaflets come together replicating the native aortic valve.