Megacolon is a colon that has stretched so wide, and lost so much of its muscle power, that it can no longer push stool out. It is far more common in cats than in dogs, and in most cats no cause is ever found. To an owner it looks like constipation that keeps coming back and gets harder to fix each time.
The good news is that your vet can step up from fluids, diet and medicine to surgery, and many cats live comfortably once it is managed. Catching it early makes every step easier.
One warning first. If your cat is squatting and straining in the litter box with nothing coming out, especially a male cat, call your vet now. Straining to pee looks almost the same as straining to poop, and a blocked bladder is an emergency.
What is megacolon in cats?
The colon is the last stretch of the large intestine. It pulls water out of stool, stores it, and squeezes it toward the rectum with muscle contractions run by nerves from the lower spinal cord.
In megacolon, those contractions fail. The Merck Veterinary Manual describes hypomotility (weak, sluggish movement) plus "persistent, severe dilation of the large intestine." Stool piles up, the colon keeps drawing water out of it, and the mass turns hard and dry. When a cat can no longer pass that mass at all, vets call it obstipation.
So megacolon is more than a bad case of constipation. It is a change in the colon itself. On an X-ray, vets use a simple yardstick: a colon wider than about one and a half times the length of the fifth lumbar vertebra (one of the backbones just in front of the pelvis) points to megacolon.
How constipation turns into megacolon
There are two roads to the same place.
- Constipation stretches the colon. International Cat Care explains that repeated bouts of constipation and obstipation can make the colon swell and lose its ability to contract. Texas A&M's veterinary college describes megacolon as what follows prolonged, untreated constipation, once the colon is permanently enlarged.
- The colon fails first. In idiopathic megacolon, the muscle and nerve problem in the colon is the starting point, and nobody can say why it began. Constipation is the result, not the cause.
Either way, time matters. Merck notes that signs lasting longer than six months have been linked to irreversible changes in the colon. Dr. Sue Lim of Texas A&M puts it plainly: "It's easier to correct the condition when caught early, while the colon still has good function." A cat that keeps getting constipated needs a proper workup while the colon still works well.
What causes megacolon in cats?
- Idiopathic megacolon (about 60%). The most common form, according to Merck: the colon stops working and no reason turns up. The percentages here come from a veterinary surgeon's review for the World Small Animal Veterinary Association (WSAVA).
- A narrowed pelvic canal (about 20%). A pelvic fracture that heals out of line, often after a road accident, can squeeze the passage the colon runs through. Masses and strictures (scar narrowing) can do the same. Merck notes this kind may be correctable if it is found early.
- Nerve damage (about 5% each for nerve injury and the Manx deformity). Spinal cord injuries, tail-pull injuries and the spinal deformity of Manx cats can cut the nerve supply that makes the colon contract.
Merck and Texas A&M list other things that make constipation, and so a stretched colon, more likely: dehydration (including the chronic dehydration of kidney disease), low potassium, some medicines such as opioid pain relievers, and a cat holding stool in because of stress, pain or a dirty litter box. Older and overweight cats and cats that have been constipated before are at higher risk.
Signs of megacolon in cats
They are the signs of constipation, only longer lasting and more stubborn:
- straining in the litter box, with repeated attempts and little or nothing produced
- crying or seeming uncomfortable while trying to go
- few stools, or small, hard, dry ones
- small amounts of liquid stool or smears around the tail, which look like diarrhea but are fluid leaking past the hard mass
- a belly that looks swollen or feels tense
- vomiting, a poor appetite and weight loss
- lethargy, hiding and dehydration
Lim describes a sign that fools people: "Eventually, the cat may stop trying to defecate altogether." A quiet litter box is not always good news.
When to call your vet
We built these tiers from the sources below to help you decide quickly. If your cat already has megacolon, your vet's plan comes first.
Call now, day or night:
- straining with no urine coming out and no fresh urine clumps in the litter, especially in a male cat (this can be a urinary blockage, which is life-threatening)
- repeated vomiting, a painful or swollen belly, weakness or collapse
Call today:
- no stool within 48 to 72 hours of the last one, or any discomfort before that
- known megacolon: no stool in the time your vet set, or sooner if your cat is straining or vomiting
- straining to poop, crying in the box, or liquid smears
- known megacolon: the usual diet and medicines have stopped working
Mention at your next visit:
- stools slowly getting smaller or harder over weeks
- gradual weight loss
- questions about diet, water intake or the litter box setup
How vets diagnose megacolon
Diagnosis starts with your story: how long this has been going on, the last normal stool, any past fall or car accident, medicines and diet. Merck notes that most cats with megacolon have a history of constipation. Then come the checks:
- Belly exam: a stretched colon full of firm stool is usually easy to feel.
- Rectal and neurological exams: a narrowed pelvic canal, masses, strictures, and the nerves of the spine and tail.
- X-rays: these confirm megacolon, show how wide the colon is, and reveal old pelvic fractures.
- Blood and urine tests: dehydration, kidney problems and low potassium.
- Ultrasound or colonoscopy in some cats, to look for masses or a stricture.
Treatment: what your vet will do
Merck describes medical treatment for megacolon as palliative: it keeps stool moving and the cat comfortable, but it does not repair the colon. Surgery comes in when it stops working.
Clearing the backlog
Fluids come first, because a cat with a packed colon is often dehydrated. Then the vet removes the stool with enemas or, when the mass is too big and hard, by hand while the cat is sedated or under general anesthesia. It can take two or three sessions over as many days (Merck), so many cats stay in the hospital.
Keeping things moving
Once the colon is empty, your vet builds a plan for the long haul. It may include:
- a stool softener or an osmotic laxative, which holds water in the stool
- a prokinetic medicine, which stimulates whatever colon movement is left (Merck notes these are less likely to help long-standing megacolon)
- a diet chosen by your vet, often a prescription food, and more moisture through wet food
- sometimes probiotics
Your veterinarian chooses each product and dose and adjusts them over time.
Surgery
When a narrowed pelvis is the cause and it is caught within about six months, surgery to widen the pelvic canal may be enough, according to Merck. When medicines stop working, the standard operation is a subtotal colectomy, which removes most of the colon. Merck calls it the procedure of choice.
Why enemas and laxatives stay with your vet
This is the part we most want owners to remember: please don't give your cat an enema or a human laxative at home.
- Merck warns that "phosphate-containing enemas must be avoided in cats" because they can cause severe electrolyte imbalances and life-threatening high phosphate levels.
- "Administering an enema to a cat is a veterinary job and should not be attempted at home," says Liz Bales, VMD, in PetMD. Some enemas made for people are very toxic to cats.
- According to Merck, laxatives can make dehydration worse, and stimulant laxatives are dangerous when the bowel is blocked. A cat with megacolon can be both.
- Fiber added to a dehydrated cat's diet can make the impaction worse, Merck warns. Ask your vet before adding pumpkin or any fiber product.
What you can safely do is support your vet's plan: fresh water in several spots, the diet your vet recommends, clean litter boxes that are easy to reach, medicines exactly as prescribed, and a note of the date of each stool so you spot a slowdown early. Our guide on how to help a cat with constipation covers the daily habits.
Can a cat live without a colon?
Yes. After a subtotal colectomy, expect soft or loose stool for up to about two months, Merck says, and some cats never go back to fully normal stool. Cats also tend to go more often. The muscle ring at the anus stays in place, so cats keep control of their bowels.
What is the outlook for a cat with megacolon?
It depends on how long the colon has been stretched and how it responds to treatment.
- With medical care. "Most cats can manage well with medical treatments if caught early," says Lim. Expect a lifelong plan with rechecks, because constipation tends to come back.
- After surgery. International Cat Care calls the long-term results of subtotal colectomy "very good," and the WSAVA review calls them excellent. It is still major surgery: Merck notes that very thin cats are at greater risk in the days after the operation, loose stool is common for a while, and in one study constipation came back in 35% of cats.
- End-stage megacolon. After long stretching, the damage is permanent and medicines stop working, so surgery is usually the next step.
Megacolon in kittens and young cats
Megacolon is mainly a problem of middle-aged and older cats, though VCA Animal Hospitals notes it has been seen in cats as young as three or four years. Merck mentions that a congenital (present from birth) cause of obstipation has been suggested.
Constipation in a kitten can have other causes. One rare example from Merck: kittens born with an underactive thyroid can be constipated, small for their age and lethargic. Your vet can test for it.
A kitten that strains, cries, has a swollen belly or stops eating needs a vet the same day.
Questions to ask your vet
- Is this megacolon or constipation, and what did the X-rays show?
- Is there a cause we can fix, such as a narrowed pelvis?
- What should normal stool look like for my cat now?
- How long without a stool should I wait before I call you?
- At what point would you suggest surgery, and who would do it?
Originally written by Akosua Kumi Nyarko. Updated by the I Love Veterinary editorial team.
Sources
- Merck Veterinary Manual: Megacolon in Cats
- Merck Veterinary Manual: Constipation, Obstipation, and Megacolon in Small Animals
- Merck Veterinary Manual: Hypothyroidism in Animals
- International Cat Care: Constipation in cats
- Texas A&M College of Veterinary Medicine: Tackling Tummy Troubles: Helping Your Cat Overcome Megacolon
- Coomer AR, WSAVA World Congress Proceedings 2013: Current Treatment for Megacolon and Other Colonic Conditions
- PetMD: How To Treat Constipation in Cats
- VCA Animal Hospitals: Megacolon in Cats
This article is for general information and is not a substitute for advice from your veterinarian.
