Diarrhea is the passing of loose or liquid stool, more often than normal. Diarrhea can be caused by diseases of the small intestine, large intestine or by diseases of organs other than the intestinal tract. Your ability to answer questions about your pet’s diet, habits, environment and specific details about the diarrhea can help the veterinarian narrow the list of possible causes, and to plan for specific tests to determine the cause of diarrhea.
Small intestinal and large intestinal diarrhea have different causes, require different tests to diagnose and are treated differently. Small intestinal diseases result in a larger amount of stool passed with a mild increase in frequency; about 3 to 5 bowel movements per day. The pet doesn’t strain or have difficulty passing stool. Animals with small intestinal disease may also vomit and lose weight. Excess gas production is sometimes seen and you may hear the rumbling of gas in the belly. If there is blood in the stool it is digested and black in color.

Disease of the large intestine including the colon and rectum cause the pet to pass small amounts of loose stool very often, usually more than 5 times daily. The pet strains to pass stool. If there is blood in the stool, it is red in color. The stool may be slimy with mucus. The pet does not usually vomit or lose weight with large bowel diarrhea. A sudden onset of small intestinal diarrhea may be caused by viruses including canine distemper, canine parvovirus, canine coronavirus, feline panleukopenia virus or feline coronavirus, in young, poorly vaccinated pets. Small intestinal diarrhea can be caused by bacteria such as salmonella, clostridia or campylobacter although these same bacteria can be found in the stool of normal dogs and cats.

Worms and giardia can cause small intestinal diarrhea, mostly in young animals. Foreign bodies including bones, sticks and other objects can pass through the stomach and get stuck in the intestine causing both diarrhea and vomiting. These same foreign materials may pass through the intestinal tract without getting stuck but may damage the lining of the intestinal tract causing diarrhea. Dietary indiscretion or a sudden change in diet can cause diarrhea with or without vomiting. Food allergies in dogs and cats can cause diarrhea, vomiting or itchy skin. Toxins including lead and insecticides can cause diarrhea usually with vomiting. Inflammatory bowel disease (IBD) occurs commonly in both dogs and cats. In IBD the walls of the intestine contain abnormal numbers of inflammatory cells which can be eosinophils, lymphocytes or plasma cells. The cause of IBD is not known but is suspected to be an allergic reaction to components of food, bacteria or parasites. IBD can be congenital in some breeds of dogs, for example Basenji dogs may develop a severe inflammatory bowel disease. Tumors of the intestine are another cause of diarrhea usually occurring in older pets. The tumor may be a single mass when the tumor is from the glands of the intestine (adenocarcinoma) and may be removed by surgery or the tumor may occur diffusely along the intestine. Lymphosarcoma occurs in both dogs and cats and can either be a single or multiple masses in the intestine or the abnormal lymphocytes may be spread through out the intestine. Lymphosarcoma is often responsive to anti-cancer drugs in cats but rarely responds to anti-cancer drugs in dogs.

In certain parts of the country small intestinal disease can be caused by fungal infections including histoplasmosis. Your veterinarian can discuss with you whether histoplasmosis is seen in your part of the country. Diseases outside the intestinal tract that may cause diarrhea include kidney failure, liver failure, pancreatic disease and hyperthyroidism in the cat. Severe inflammation of the pancreas (pancreatitis) can lead to damage of the pancreas and an inability to make enough enzymes to digest fat. This is called pancreatic insufficiency and causes diarrhea with a large volume of greasy stool. Pancreatic insufficiency can occur in young animals due to a congenital deficiency of pancreatic enzymes.

The cause of small intestinal diarrhea may be determined from blood tests, examination of the stool, x-rays or ultrasound of the abdomen or by endoscopy. Endoscopy is the technique of passing a flexible scope through the stomach into the upper intestine. Small biopsies of the lining of the intestine can be taken for microscopic evaluation. Endoscopy requires general anesthesia. A diagnosis of intestinal lymphosarcoma may be missed on endoscopy as the biopsies taken using endoscopy do not include the full thickness of the wall of the intestine and the cancerous cells may be deep in the wall of the intestine. A diagnosis in that case requires surgery in order to take a larger biopsy of the entire thickness of the intestine.

Dogs and cats with chronic small intestinal diarrhea will lose weight as they are unable to properly absorb nutrients and may develop edema of the legs or fluid accumulation in the belly or chest. A small protein, albumin may be lost in diarrhea. Albumin acts like a sponge to keep water in the blood vessels. When albumin is lost in the stool, blood albumin gets low and water leaks out of blood vessels to accumulate in other locations. Chronic diarrhea may cause the fur to look dull and brittle due to nutrient deficiencies. Acute small intestinal diarrhea can be managed by withholding food, but not water for 24 – 48 hours. If diarrhea stops, small amounts of a bland low-fat food are fed 3 to 6 times daily for a few days, with a gradual increase in the amount fed and a gradual transition to the pet’s normal diet. Foods designed as intestinal diets usually contain rice as rice is more digestible than other grains. You are discouraged from administering over-the-counter diarrhea medications without first consulting a veterinarian. If the pet is active, not dehydrated and has been previously healthy, acute diarrhea can often be managed at home. Diarrhea that continues for more than a few days or is accompanied by depression or other signs is an indication to take your pet to a veterinarian.

Diarrhea of large intestinal origin can be caused by whipworms, polyps, inflammatory bowel disease, colonic ulcers or colonic cancer. Stress can cause large bowel diarrhea in excitable dogs. The diagnosis of large intestinal diarrhea is also made by blood tests and examination of the stool. A rectal examination using a gloved finger may provide some information about the cause of large bowel problems including rectal polyps and rectal cancer. Endoscopy to examine the large intestine is performed using a rigid or flexible scope passed up the rectum. Because the rectum is often very irritated, colon exams are usually performed under general anesthesia.

The treatment of large bowel diarrhea may be based on a specific diagnosis. Non specific treatment of large bowel diarrhea often includes a high fiber diet and sullfasalazine, an anti-inflammatory drug. .