Bright red blood in the stools in animals is called hematochezia and it signifies lower intestinal tract bleeding. Black, tarry blood in the stools in animals is called melena and it signifies upper intestinal tract bleeding. Blood in the stools is often caused by internal parasites, allergies, bleeding ulcers, ingested blood or gastrointestinal diseases.

Internal parasites cause blood in the stools in animals due to the irritation the parasites create in the digestive tract. Hookworms survive on the blood in the small intestine and often cause anemia which can often be noticed by pale gums as well as black blood in the stools in animals. Puppies are especially susceptible to serious harm or death from hookworms.

Whipworm infestations affect the colon and can cause bright red blood in the stools in animals. Whipworm is quite rare in cats, but fairly common in dogs. Whipworms look like long whips and the eggs can be seen in a fecal exam. By the time bright red blood in the stools in animals appears, there is likely to be a large number of the parasites in the animal's colon.

Bright red blood in the stools in animals may also be caused by food allergies. An allergic reaction may be likely if blood in the stool is accompanied by other allergy symptoms in animals such as constipation or diarrhea, ear infections, rashes or skin infections. Itchiness is another symptom of allergies in animals.

Bleeding ulcers in the stomach or the esophagus causes black blood in the stools in animals. Bleeding ulcers are also called peptic ulcers. An ulcer is an open sore caused by an irritant such as an acid, drug or bacterium. Ingested blood, such as from killing and eating prey, is another cause of black blood in the stools in animals.

Gastrointestinal diseases also cause blood to appear in the stools of animals. Black blood in the stools is often associated with lymphoma. Lymphoma is a cancerous condition of the white blood cells and is fairly common in dogs. Inflammatory Bowel Disease (IBD)is fairly common in cats. Blood in the stools in animals with IBD is sometimes the only symptom of the disease. The blood occurs as the cells in the digestive tract become inflamed.