Prions are normal proteins, present in all mammals and some non-mammalian species such as salmon and ostriches. According to the prion theory of disease, some prions can fold abnormally and then they can kill nerve cells. Prions are remarkably hardy. They are not destroyed by the digestive system of humans or other animals. And they are very heat resistant.
Prions are hard to stop.
Under FDA's rule, ruminants can be fed to pigs and pigs can be fed to ruminants. Under the rule, even ruminants that are known to be infected with a TSE can be fed to pigs. This does not answer the question whether pigs can be infected through their normal diet, but it indisputably establishes that pigs, like many other species, are susceptible to TSEs. During a 16-month period, Dr. Doi observed CNS symptoms in 106 pigs, taking careful notes and retaining tissue samples, including brains. Dr. Doi, and Dr. Langeheinreich, the pathologist who examined the brain tissues, both say they believe they were dealing with a single disease in all the pigs.