Yes, tapeworms can cause diarrhea in dogs. Tapeworms can cause diarrhea because they produce large amounts of a chemical called proglottid segments. When these segments break off, they contain enzymes that digest food in the host's intestine-digested food leaks out into the dog's poop, which is why it looks like pieces of red rice. You might notice that your dog has tapeworms when you see little white dots on his poop. Those are tapeworm eggs.