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The causes of canine diarrhea with blood are varied and can range from nothing serious to a life-threatening virus. Blood in a dog’s stool should raise immediate concern if the dog is younger than two years and especially if the dog hasn’t been vaccinated against parvovirus. A generally less serious but common cause of canine diarrhea with blood in dogs of all ages is the dog eating something it shouldn’t have. This can include a food its body is having difficulty digesting or a foreign object that is not passing easily through the intestinal tract. Other causes might include intestinal parasites, injury, or medications.
One of the primary symptoms of parvovirus is bloody diarrhea. The virus is most common in puppies and is nearly unheard of in dogs older than two years. Puppies are routinely vaccinated against the virus in a series of four to five inoculations given before the puppy is six months of age, and dogs routinely receive booster vaccinations every one to three years throughout their lives. Left untreated, death from parvovirus occurs in more than 90 percent of cases, and even with treatment death occurs in more than half of cases. Parvovirus is highly contagious among unvaccinated canines.
Perhaps the most common cause of canine diarrhea with blood is the dog eating something that can’t be digested or can't be digested easily. The dog may have eaten spoiled foods, sharp bones, or other objects that cut the intestines as they pass through the dog’s body. In many of these cases, the dog’s body will correct the problem within several hours. If large amounts of blood are present with bowel movements, if the dog appears to be in pain, or if the dog is exhibiting other symptoms, the dog should be taken to a veterinarian immediately.
Also related to eating habits, canine diarrhea with blood can also be caused by feeding the dog a poor-quality pet food. If the dog is prone to overeating or eating too quickly, this can result in bloody stool. Dogs may also develop diarrhea with blood present if they become overly stressed.
Another common cause of canine diarrhea with blood, especially in puppies but also in adult dogs, is the presence of intestinal parasites. These can include hookworms, whipworms, and roundworms. Less common intestinal parasites that can cause bloody stool are giardia and coccidia.
There are other conditions, though less common, that may include bloody diarrhea as a symptom. These include Addison’s disease, hyperthyroidism, and pancreatitis. Some medications can cause bloody stool, especially medications intended for human use, which can be toxic to canines.