Megacolon in cats: symptoms, causes and treatment

Summary

What is megacolon?

Megacolon is a condition in which the large intestine, or colon, becomes abnormally dilated and loses much of its normal ability to contract and propel faeces towards the rectum.

As faecal material remains in the colon, more water is absorbed from it. The stool becomes increasingly dry, firm and difficult to pass, causing further stretching of the colon and worsening the problem.

Megacolon is most commonly seen in cats and is usually associated with chronic or recurrent constipation. In many affected cats, however, the underlying cause remains unknown; this is known as idiopathic megacolon.

Expert advice from Dr Felicia:

Megacolon is not uncommon in our feline patients sadly but catching it early, performing thorough diagnostics to address any secondary causes and being pro-active with therapy can help improve outcomes.

Constipation, obstipation and megacolon: what is the difference?

• Constipation is infrequent or difficult passage of faeces, which are usually dry and hard.
• Obstipation is severe, persistent constipation in which the cat is unable to pass the accumulated mass of hard faeces normally.
• Megacolon is a pathological enlargement and loss of function of the colon that causes recurrent constipation or obstipation.

Symptoms of megacolon in cats

Cats with megacolon may show:

• repeated straining in the litter tray
• passing very small amounts of hard, dry faeces
• failing to defecate normally
• frequent trips to the litter tray with little result
• painful defecation or vocalising while straining
• defecating outside the litter tray
• reduced appetite
• lethargy
• abdominal discomfort or a visibly distended abdomen
• vomiting
• weight loss
• dehydration.

Some cats may pass small amounts of liquid stool or mucus around the impacted faecal mass. This can sometimes look like diarrhoea even though the underlying problem is severe constipation.

Expert advice from Dr Felicia:

While most cases of feline megacolon are considered idiopathic, it is important to consider physical (renal disease, pain), behavioural (stress) and environmental conditions (poor litterbox hygiene) which may be perpetuating factors.

When should you see the vet?

A cat that is repeatedly straining, producing very little stool or has not defecated normally should be assessed by a veterinarian rather than simply treated at home.

Seek veterinary advice promptly if your cat also has vomiting, marked lethargy, loss of appetite, abdominal pain or dehydration.

Straining but no urine? Treat it as an emergency

Cats with a blocked urethra may repeatedly enter the litter tray, strain and vocalise in a way that owners mistake for constipation.

If you are unsure whether your cat is trying to urinate or defecate, particularly if your cat is male and is producing little or no urine, seek emergency veterinary care immediately. Complete urinary obstruction can become life-threatening very quickly.

What causes megacolon in cats?

In many cats, no specific underlying cause can be identified. These cases are referred to as idiopathic megacolon and are thought to involve dysfunction of the nerves and/or smooth muscle responsible for normal colonic movement.

Other recognised causes or contributing factors include:

• narrowing of the pelvic canal following a previous pelvic fracture
• spinal cord or pelvic nerve injury
• congenital abnormalities affecting the spine or nerves, including some abnormalities seen in Manx cats
• masses or strictures that interfere with passage of faeces
• prolonged or recurrent constipation
• conditions causing chronic dehydration
• certain medications that slow intestinal motility.

Older, overweight cats, cats with chronic kidney disease and cats with previous episodes of constipation are also more likely to experience constipation, which may contribute to progression of the problem.

How is megacolon in cats diagnosed?

Diagnosis usually starts with the cat’s history and a physical examination. The veterinarian may be able to feel a large, firm colon filled with retained faeces during abdominal palpation.

Abdominal x-rays

Abdominal radiographs are particularly useful because they can confirm that the colon is abnormally enlarged, show the extent of faecal impaction and help identify possible underlying causes such as previous pelvic fractures or narrowing of the pelvic canal.

Other tests

Depending on the individual cat, additional investigation may include blood and urine tests to assess hydration, electrolyte abnormalities, kidney disease or other medical conditions.

A rectal examination, neurological examination, ultrasound, contrast imaging or colonoscopy may occasionally be needed when an obstruction, mass, neurological problem or other underlying cause is suspected.

Treatment for megacolon in cats

Treatment depends on how severely the cat is constipated, whether the colon still has useful motility and whether an underlying cause can be corrected.

Expert advice from Dr Felicia:

Cats tend to show quite marked improvement after evacuation of the impacted faeces. Once re-hydration is complete and they are eating well, then they can proceed home and commence their outpatient medical management of megacolon.

Treating severe constipation and obstipation

Cats with significant obstipation may require hospitalisation.

Treatment can include:

• intravenous or subcutaneous fluids to correct dehydration
• veterinary enemas
• stool-softening medication
• manual removal of impacted faeces under sedation or general anaesthesia in severe cases.

Some cats require more than one attempt to completely clear the impacted material.

Important warning about enemas

Never give a human enema to a cat unless specifically instructed by your veterinarian. Phosphate-containing enemas can cause severe, potentially fatal electrolyte disturbances in cats. Enemas should only be given under veterinary direction.

Medication

Once the colon has been cleared and the cat is adequately hydrated, medications may be prescribed to help prevent further constipation.

These can include osmotic laxatives, which increase water content in the stool, and prokinetic medications, which stimulate colonic contractions where useful motility remains.

One commonly used prokinetic medication is cisapride, although cats with advanced, longstanding megacolon may respond poorly once normal colonic motility has been severely lost.

Expert advice from Dr Felicia:

If the flares are infrequent then it may be considered acceptable for an occasional enema to be required if the cat is doing well most of the time. For patients repeatedly requiring manual evacuation, surgery should be considered.

What should a cat with megacolon eat?

There is no single diet that suits every cat with constipation or megacolon. Cats with mild or early constipation and some remaining normal colonic motility may benefit from a diet containing additional fibre because fibre can help retain water in the stool and stimulate bowel movement.

However, once the colon is severely dilated and its ability to contract has deteriorated, adding more bulk can sometimes make the problem worse. In these cats, a highly digestible, low-residue diet that produces a smaller volume of faeces may be more appropriate.

Diet should therefore be selected according to the individual cat’s condition and response to treatment, with guidance from your veterinarian.

Surgery for megacolon

If medical management can no longer control the constipation, subtotal colectomy is generally the treatment of choice. This involves surgically removing most of the poorly functioning colon and reconnecting the remaining intestine.

Loose stool or diarrhoea is common after surgery while the gastrointestinal tract adapts and may continue for several weeks or occasionally longer.

A large multicentre study of cats undergoing subtotal colectomy for idiopathic megacolon found long survival times and a high level of owner satisfaction, although complications and recurrence of constipation can occur.

Expert advice from Dr Felicia:

Sub-total colectomy surgery can be life-changing for cats affected by megacolon that stops responding to medical therapy. Since a large portion of the colon is removed, there is around 5% risk of internal wound breakdown and infection post-surgery.

What is the prognosis for a cat with megacolon?

The outlook depends on how advanced the disease is and whether the underlying cause can be corrected. Cats with early or less severe disease may remain comfortable with dietary management, appropriate laxatives, prokinetic medication and regular monitoring. However, established idiopathic megacolon is often progressive, and medical therapy may eventually become ineffective.

For cats that require surgery, subtotal colectomy can provide very good long-term quality of life. Loose stools are common during the early recovery period and some cats experience recurrent constipation, but owner satisfaction following surgery is generally high.

Earlier recognition and treatment of recurrent constipation may help prevent prolonged colonic distension and irreversible loss of function.

Because it is difficult to predict the costs of veterinary care, it can help to have measures in place to help prepare for the unexpected. Pet insurance can help by covering a portion of the eligible vet bill if the unexpected does happen.

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Can megacolon be prevented?

Not every case can be prevented, particularly idiopathic megacolon.

However, recurrent constipation should be taken seriously and investigated rather than repeatedly managed at home.

Ways to support normal bowel function include:

• encouraging good hydration
• providing clean, easily accessible litter trays
• maintaining a healthy body weight
• managing arthritis or other painful conditions that may make defecation difficult
• treating kidney disease or other conditions that contribute to dehydration
• following your veterinarian’s dietary and medication recommendations.

Cats should always have easy access to litter trays. Older cats or cats with arthritis may benefit from lower-sided trays that are easier to enter.

In summary

Megacolon is much more than ordinary constipation. It occurs when the colon becomes persistently enlarged and loses its ability to move faeces normally, resulting in recurrent severe constipation or obstipation.

Repeated straining, hard or absent stools, reduced appetite, vomiting and abdominal discomfort should not be ignored. Early veterinary assessment can help identify constipation before the colon becomes severely and irreversibly affected.

Treatment may involve fluids, stool-softening medication, dietary changes and prokinetic drugs. Cats with advanced megacolon that no longer responds to medical management may require subtotal colectomy, which can provide a very good long-term quality of life for many affected cats.

And importantly, never assume that a straining cat is constipated. A cat—particularly a male cat—that is repeatedly straining but producing little or no urine may have a urinary blockage and needs emergency veterinary care.

Sources:

“Megacolon in Cats”, Merck Veterinary Manual, updated 2025. https://www.merckvetmanual.com/digestive-system/surgical-problems-of-the-gastrointestinal-tract-in-small-animals/megacolon-in-cats. Accessed 20 October 2026.

“Constipation, Obstipation, and Megacolon in Small Animals”, Merck Veterinary Manual, updated 2025. https://www.merckvetmanual.com/digestive-system/diseases-of-the-large-intestine-in-small-animals/constipation-obstipation-and-megacolon-in-small-animals. Accessed 20 October 2026.

Munif MR, Williams RW, Mumu TT. “Megacolon in cats: Current insights and future directions”, The Veterinary Journal, 2026. https://pubmed.ncbi.nlm.nih.gov/41354320/. Accessed 20 October 2026.

Grossman RM, et al. “Evaluation of outcomes following subtotal colectomy for the treatment of idiopathic megacolon in cats”, Journal of the American Veterinary Medical Association, 2021. https://pubmed.ncbi.nlm.nih.gov/34727062/. Accessed 20 October 2026.

“Constipation in Cats”, VCA Animal Hospitals. https://vcahospitals.com/all-pets-hospital/know-your-pet/constipation-in-cats. Accessed 20 October 2026.

“Urethral Obstruction in Small Animals”, Merck Veterinary Manual, updated 2025. https://www.merckvetmanual.com/urinary-system/urolithiasis-in-small-animals/urethral-obstruction-in-small-animals. Accessed 20 October 2026.

“What’s the scoop on your pet’s poop?”, PetSure Pet Health Monitor, 2025. https://petsure.com.au/?p=15873. Accessed 20 October 2026.