[pdonis]

A representative sample of the current understanding is here:

http://www.ncbi.nlm.nih.gov/books/NBK22600/

"Proteins ingested in the diet are digested into amino acids or small peptides that can be absorbed by the intestine and transported in the blood."

If by "animal-source protein strings" you mean "small peptides", then yes. But in general they still have to be resynthesized into the proteins used by the body. There might be a few useful proteins that are small enough to fall into the "small peptide" range and would therefore be absorbed and used as-is.

Prions have a little over 200 amino acids, so they are on the large side to be considered "small peptides" (which AFAIK are typically a few to a couple of dozen amino acids). They appear to be resistant to the enzymes that normally digest proteins, and it doesn't appear to be clearly understood how they get from the digestive tract into the bloodstream. See, for example, here:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2538961/