The reason for your dog pooping blood will depend upon its cause. For example:
Infectious Agents
Parvovirus, corona virus, bacterial infections (Clostridium and Salmonella), protozoal agents like coccidiosis and intestinal parasites like hookworms and whipworms are all infectious agents that can lead to blood in your dog’s stool.
Dietary Intolerance and Allergy
This can occur as the result of eating food that is no longer good, overeating, consuming a material that is not edible, a sudden change in diet or eating their human’s food can cause inflammation in your dog’s lower bowel and blood in their stool. An allergy can occur to a protein in a particular food causing colitis and blood in your dog’s stool.
Masses
These can be present in the colon, rectum or anus and include benign and malignant tumors.
Colitis
When blood in your dog’s stool is ongoing, it can point to his colon being inflamed, which is also called colitis. Colitis can be caused by a parasite, mass, infectious agent, food intolerance or allergy, as well as due to an immune-related or unknown cause.
Trauma
Trauma from a bite wound to your dog’s anal area, a pelvic fracture that interrupts the colon or rectum, a sharp item ingested and passed or an instrument or material being placed in your dog’s rectum can lead to blood in his stool.
Bleeding Disorders
Should your dog have a bleeding disorder, he may bleed from the lining of his lower bowel. Types of bleeding disorders include thrombocytopenia, which is when there is a decrease in the number of your dog’s platelets, inherited clotting disorders like hemophilia and abnormal platelet function problems, and disseminated intravascular coagulation (the result of a severe infection, organ failure or a major liver disorder).
Narrowing of Anus or Colon
This is secondary to other trauma, inflammation, cancer or foreign body. When stools are passed when the anus or colon are narrowed, bleeding can occur.
Anal Sacculitis
An inflammation or an abscess of the anal sacs can lead to the consistency of the fluid in the anal sacs to change into a bloody liquid. This liquid can provide a coating to the stools as your dog defecates.
Perianal Fistulae
Ulcerations that can be seen in the skin around your dog’s anus. If the ulcerations connect to the anus, blood can be present in the stool.