My two kitties had a few bouts with blood in their stools and the Vet gave me different dry food for them. It is Science Diet WD, high fiber. Once I started feeding them the WD, the bloody stools lessened, although they still have it from time to time. The Vet tested them for every thing under the sun .....and they are happy kitties, so we are just going to keep an eye on it/them. There seems to be a connection between what they eat and their stools. If I give them "off their diet foods", and I do, they seem to have the problem return, just not as badly as when they were on different dry food. I would have the cat checked for parasites etc. and if all is fine, then switch the food. Some cats have sensitive digestive tracts. Mine do for sure. Hope this helps! I think that is why they remain so thin. Vets rule, eh?