There are several causes of blood in a puppy's stool. Blood in the stool may be indicated by an obvious bright red color. Tarry-black soft droppings also indicate the presence of blood in the stool.
Bright red blood in a puppy's stool is called hematochezia. The tar-like stools, melena, are black because the blood was partially digested by the puppy.
According to veterinarian Dr. Bari Spielman, the most common causes of blood in a puppy's stool are parasites, such as whipworms, hookworms and roundworms. Bright red blood may also come from constipation if the puppy has been straining to produce hard stool.
Illnesses or infections that can cause bloody stool in puppies include salmonella, parvovirus, colitis or Guardia.
Another serious cause of blood in a puppy's stool is by swallowing a foreign object or human medication. The object or drug can damage the puppy's internal organs.
"The Veterinarian's Guide to Your Dog's Symptoms" (Michael S. Garvey, DVM, et al.; 1999) warns that any puppy or dog that passes only bright red blood and nothing else may have hemorrhagic gastroenteritis, a potentially lethal condition. The puppy needs to see a vet immediately.