Diarrhea is the passing of loose or liquid stool, more often than normal. The cause could be related to problems with your dog’s small intestine, large intestine, or other organs.
To help your vet determine the cause, be prepared to answer questions about your dog’s diet, habits, and environment, as well as specific details about the diarrhea. Once your veterinarian has narrowed the list of possible culprits, she can plan for specific tests to determine the exact cause.
Small intestinal versus large intestinal diarrhea
Small intestinal and large intestinal diarrhea have different causes, require different tests to diagnose, and are treated differently. With small intestinal diarrhea, a larger amount of stool is passed with a mild increase in frequency–about three to five 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 present and you may hear the rumbling of gas in the belly. If there’s 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 five times daily. The pet strains to pass stool, and if there’s blood in the stool, it will be red and possibly slimy with mucus. The pet doesn’t usually vomit or lose weight with large bowel diarrhea.
Small intestinal diarrhea
- Viruses such as canine distemper, canine parvovirus, and canine coronavirus in young, poorly vaccinated dogs.
- Bacteria such as salmonella, clostridia, or campylobacter–although these same bacteria can be found in the stool of healthy dogs and cats.
- Worms and giardia, mostly in young animals.
- Swallowed objects such as bones and sticks. They may get stuck in the intestine, causing both diarrhea and vomiting; or, they may pass through the intestinal tract but damage its lining on the way, causing diarrhea.
- Dietary indiscretion or a sudden change in diet; diarrhea may or may not be accompanied by vomiting.
- Food allergies. Other symptoms can include vomiting or itchy skin.
- Toxins including lead and insecticides; diarrhea is usually accompanied by vomiting.
- Inflammatory bowel disease (IBD). 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, such as the Basenji.
- Tumors of the intestine, usually in older dogs.
- Fungal infections such as histoplasmosis, in certain parts of the country.
The cause of small intestinal diarrhea may be determined from blood tests, examination of the stool, x-rays, ultrasound of the abdomen, or by endoscopy. Endoscopy involves 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 with endoscopy, as the biopsies taken don’t include the full thickness of the intestinal wall, and the cancerous cells may be deep in the wall. In that case, the vet will need to perform surgery, to take a larger biopsy of the entire thickness of the intestinal wall, to make a diagnosis.
Acute (short-term) small intestinal diarrhea can be managed by withholding food, but not water, for 24 to 48 hours. If diarrhea stops, small amounts of a bland, low-fat food are fed three to six times daily for a few days, with a gradual increase in the amount fed and a slow transition back to the pet’s normal diet. Special diets usually contain rice, which is more digestible than other grains.
Don’t give your dog over-the-counter diarrhea medications without first consulting a veterinarian. If your dog’s active, not dehydrated, and has been previously healthy, acute diarrhea can often be managed at home. If the diarrhea is accompanied by other symptoms such as depression, or it continues for more than a few days, take your dog to a veterinarian.
Large intestinal diarrhea
- Inflammatory bowel disease (IBD)
- Colonic ulcers
- Colonic cancer
- Stress, especially 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 whether rectal polyps or rectal cancer are involved. 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 may be based on a specific diagnosis. Non-specific treatments often include a high fiber diet and sullfasalazine, an anti-inflammatory drug.
If the problem’s outside the intestines
Diseases outside the intestinal tract that may cause diarrhea include kidney failure, liver failure, pancreatic disease, and hyperthyroidism in cats. 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 amount of greasy stool. Pancreatic insufficiency can occur in young animals due to a congenital deficiency of pancreatic enzymes.
Source: Adapted from the Washington State University College of Veterinary Medicine