but I have never seen a mutt come down with parvo
I work in a shelter, I have seen plenty of it. I've even seen an adult mixed breed dog come down with the virus. We got a whole bunch (20-some) puppies from a resuce up north, all mixed breeds- mutts, all positive for parvo. This was upon their arrival. I've seen pits (another mixed breed) come down with it too. Most of the cases come in with parvo, they do not, usually, pick it up during their stay.