Canine Colitis
by Rob Hilsenroth, DVM

Diarrhea is a common condition that almost every dog owner will experience at some point -- in the dog, not necessarily the owner. Most often, the condition is inconvenient but brief, lasting only a day or two. Diarrhea that lasts longer than a few days or is particularly severe (with the dog showing other signs of illness such as lethargy, vomiting, or a fever) can indicate a more serious problem.

Diarrhea refers to any condition that results in an increased fluid component to the stool. The condition can originate in either the small or large intestine (also called the colon) and, depending on the origin, the characteristics of the stool are different. Small intestinal diarrhea is characterized by large volumes of watery stool and can be accompanied by the other signs of illness described above. In contrast, dogs with large intestinal diarrhea produce small amounts of stool and have increased urgency. These dogs may also have blood and/or mucus in their feces. Large intestinal diarrhea is often referred to as colitis, which means inflammation of the colon.

Colitis is caused by any one of a variety of injuries that usually involve the innermost lining of the colon (called the mucosa). The mucosa, as the name suggests, has numerous mucus glands that protect the colon from injury and lubricate the passage of feces. The main functions of the colon are to absorb water and store feces until the animal defecates. In dogs with colitis, water is not effectively absorbed, and the ability of the colon to store feces is impaired. Excessive amounts of mucus, and even blood, are often passed with the feces of dogs affected with colitis because of damage to the protective mucosa lining.

Inflammation in the colon can decrease the movement of the colon. The colon normally moves the feces slowly back and forth, enabling maximum absorption of fluids. When this movement is abnormally slowed, the contents pass too quickly through the colon and diarrhea ensues.

There are many different causes of canine colitis. Diet, parasites, bacterial infections, and even stress are among the more common causes of colitis in dogs. Fiber-responsive colitis describes large bowel diarrhea that resolves by adding fiber to the diet. In some instances, hypersensitivity or allergic reactions to certain components in the diet can cause a disease called inflammatory bowel disease (IBD) of the colon.

Whipworms, another common cause, are parasites that inhabit the large bowel and attach to the mucosa. These worms interfere with the colon's ability to absorb water, and thus cause diarrhea.

Certain bacteria can also cause colitis. These bacteria normally live in the colon but, when conditions are right for these bacteria to overgrow, colitis develops. A species of bacteria called Clostridium perfringens is an extremely common cause of colitis in dogs. C. perfringens causes inflammation of the colon by producing a toxic substance called an enterotoxin. This toxin acts directly on the colon mucosa, causes the escape of fluid and salts (also called electrolytes), and results in decreased movement, which produces diarrhea.

In general, colitis is not difficult to diagnose, because the clinical symptoms are very specific for large bowel inflammation. As mentioned above, those symptoms include straining to defecate, production of scant amounts of watery feces that may contain mucus and/or blood, and increased urgency to defecate. The causes of colitis, however, can be more challenging to uncover. Your veterinarian may recommend trial diets with increased fiber or hypoallergenic diets to identify fiber-responsive colitis or IBD. A stool examination may reveal eggs of whipworms, indicating the need for a deworming agent.

Sometimes it's necessary for your veterinarian to see the inside of the colon, directly, by using an instrument called an endoscope. He or she can then take small biopsies of the colon for microscopic examination. Colitis caused by C. perfringens is a diagnostic challenge because these bacteria are normal inhabitants of the colon. Thus, determining if the bacteria are truly the cause of the colitis is difficult. Often, veterinarians must administer antibiotics based on their clinical suspicion of C. perfringens overgrowth.

Morris Animal Foundation is supporting a study to uncover a more definitive way to diagnose C. perfringens infections that commonly cause colitis in dogs. Dr. Stanley Marks of the University of California-Davis' School of Veterinary Medicine is working to design a "better mousetrap" for catching C. perfringens in the act of causing disease. In the study entitled, "Diagnosis and Epidemiology of Disease Caused by Enterotoxigenic Clostridium Perfringens Type A in Dogs," Dr. Marks and his colleagues are working to establish a variety of tests that can be used together to more accurately diagnose this disease.

These tests include counting spores produced by the organisms in feces and measuring the amount of enterotoxin present in the feces. This should decrease the number of dogs that receive antibiotics unnecessarily, and will also help veterinarians uncover the actual cause of colitis in many patients. As veterinarians are better able to rule out C. perfringens as the causative agent in dogs with colitis, they will also be able to more rapidly identify the true cause and provide these patients with appropriate therapy.