Blood in stool

My 2 year old shitzu occasionally has diarreah (diarrhea) with blood droppings.  His stool was checked for parasites with negative results.  He acts normal, plays, eats, sleeps normally - What could be the problem? the vet recommended changing his diet from all hard food to chicken and rice.  The problem went away but occasionally resurfaces.  I currently feed him a mix of hard and soft food.

[Aleda M Cheng, D.V.M., C.V.A]

I would eliminate all hard food, and try putting your dog on a grain free or home-made diet. Good choices are Wellness canned grain-free, Orijin canned, grain free, Evo/Inova grain free.  California Naturals grain free, and others.  

Make sure that your vet tested your dog's stool for Giardia.  This is not a routine parasite check.  Giardia is difficult to find in the average stool and may require several tests to find the protozoa, but it does cause blood in the stool.