Most forms of constipation in dogs are mild, but severe chronic constipation can be related to hypertrophy or overstretching of the muscles in the large intestine. This condition, called megacolon, can reduce intestinal motility and make it difficult for a dog to pass feces. Megacolon is rare in dogs. In some cases, it is the result of an inherited abnormality, but more often it develops from chronic, untreated forms of constipation. If fecal matter remains too long in the colon, it becomes dry and impacted and may cause permanent damage to the smooth muscles of the intestinal tract. This will make it even harder for the dog to defecate normally, and usually leads to a lifelong problem with constipation. Dietary indiscretion, lack of fluids, bowel obstruction, or a neuroglial condition that limits muscular contractions can all cause constipation. If untreated, these conditions can develop into obstipation, a form of constipation in which the colon is severely impacted. The origin of megacolon is not fully understood, so it’s not known what degree of impaction leads to permanent muscular damage. Even if the condition is not directly inherited, there may be a genetic tendency toward the disease. Megacolon is rare in dogs, but once it has developed, treatment is difficult and the dog may need to stay on a strict diet to avoid recurrence.

Expansion and weakening of the large intestine can be present with severe forms of constipation. Veterinarians call this condition megacolon. The muscles that normally contract to pass feces become permanently weakened and the dog may have continuing problems with constipation.

Symptoms will be bowel related at first, but over time failure to adequately pass stool can lead to systemic problems. See a veterinarian if you notice any of the following signs.

There are three types of constipation.

Intraluminal  This is due to a problem inside the colon. Often the cause is fecal matter that is difficult to pass because it contains indigestible material such as fur or bone. Low fluid intake and intestinal tumors can also cause intraluminal constipation. This is the most common type of constipation in dogs.

Extraluminal  This is due to colon compression, often from other organs in the body. Enlarged glands, a poorly healed pelvic fracture, or a stricture in the colon can cause extraluminal constipation.

Neurological  This neuromuscular deficiency in the colon can also cause constipation. Megacolon is included in this type, as well as other conditions that affect neurological control such as hypothyroidism, hypokalemia, or hypercalcemia.

Megacolon can be congenital or acquired. Congenital forms are not well documented in dogs. Acquired conditions may be related to some of the following factors.

The veterinarian will be able to tell that your dog is severely impacted on a physical examination. The colon is usually hard and distended. A rectal exam will be necessary to check for a tumor or stricture that could be blocking the passage of stool. A history of constipation often indicates the possibility of megacolon, so the veterinarian will need to know about prior problems as well as how they were treated. Injuries, recent surgeries, drug treatment, or any other known condition could all be relevant.

Abdominal x-rays are usually ordered for severe constipation. With megacolon, the colon will appear enlarged and impacted. Bloodwork and urine tests will be needed to evaluate the overall health of your dog and check for any other issues that could be causing or contributing to the problem. A colonoscopy could also be ordered to check for lesions or tumors in the colon, but in many cases the impacted feces will have to be cleaned out before this is possible.

Many different medications may be prescribed for constipation, including laxatives, stool softeners, suppositories, and enemas. Most dogs with severely impacted bowels do not respond to these type of treatment and the feces often has to be flushed out manually with enemas and warm water. The dog will be anesthetized during this process. Dehydrated dogs may need intravenous fluids and electrolytes to make their system stable enough for anesthetic.

Once the impacted feces is removed, the veterinarian will try to ensure the problem doesn’t recur. Diet change is often recommended; either high or low fiber diets may be effective depending on your dog’s condition. Regular exercise is recommended to stimulate the bowels. The veterinarian may also suggest daily laxatives or stool softeners. Prokinetic drugs, especially cisapride, are often prescribed to encourage intestinal motility.

If your dog has recurring problems, surgery is sometimes an option. A sub-total colectomy or a total colectomy can remove all or part of the colon, either connecting the small intestine to the rectum, or retaining the ileocolic junction. This surgery has been more effective at treating cats with megacolon than dogs, but in some cases your veterinarian might recommend it. There will be a three week recovery period after surgery and the dog may have loose bowels for several months. A low residue diet is usually prescribed for life and the veterinarian will need to monitor your dog for any post-surgery metabolic problems.

Many cases of megacolon can be prevented by treating constipation promptly. If the condition disrupting colon motility is identified and treated before megacolon develops, your dog will have a good chance of recovery. In general, a healthy diet and adequate exercise can reduce the likelihood that severe constipation will occur. You should call a veterinarian if you believe it has been at least 48 hours since your dog had a bowel movement.