CONDITION

Constipation and Megacolon in Cats

Why this matters now

Constipation and megacolon is one of the most common gastrointestinal conditions encountered in cats, particularly affecting middle-aged to older individuals. The condition may begin as episodic constipation that initially responds to dietary modification or occasional medical intervention, but in some cats progresses to a state of chronic, recurrent obstipation where the colon becomes permanently dilated and loses its ability to generate effective peristaltic contractions. Certain breeds, particularly Manx and other tailless or short-tailed breeds, may have a predisposition related to sacral spinal abnormalities that affect colonic innervation. Male cats appear to be affected more frequently than females, though the condition can develop in any individual.

The progression from episodic constipation to established megacolon typically occurs over months to years, though the rate varies considerably between individuals. Early episodes of constipation may respond well to dietary fibre supplementation, increased water intake, and occasional laxative use. As the condition progresses, episodes may become more frequent, require more aggressive intervention to resolve, and leave shorter intervals of normal defecation between episodes. With each episode of significant faecal retention and colonic distension, the colonic smooth muscle becomes increasingly stretched and damaged, progressively losing its contractile capacity. Eventually, the colon may become so dilated and atonic that it can no longer move faecal material through effectively, establishing the state of megacolon. At this advanced stage, the condition becomes refractory to medical management alone, and some cats may require surgical intervention to maintain quality of life.

Signals & patterns

Early signals

Occasional straining in the litter tray

The cat may spend longer than usual in the litter tray, adopting a posturing position with visible effort but producing smaller, harder stools than normal. These episodes may be intermittent, separated by periods of apparently normal defecation.

Slightly reduced stool frequency

Rather than the typical daily or twice-daily bowel movements, the cat may begin to skip days between defecations. The stools produced after these longer intervals may be larger and firmer than usual.

Small, hard faecal pellets

The character of the stools may change before the frequency decreases, with normal-sized, well-formed stools being replaced by smaller, harder, drier pellets that reflect prolonged water absorption in the colon.

Occasional vomiting

Some cats may vomit intermittently during episodes of constipation, as faecal retention can trigger a vagal reflex or produce generalised nausea. The vomiting may not be immediately connected to the defecation difficulty.

Later signals

Prolonged absence of bowel movements

The cat may go three or more days without producing any stool, despite frequent visits to the litter tray. Productive defecation becomes increasingly rare, and when stools are passed, they may be very hard, dry, and small.

Visible straining and discomfort

The cat may cry or vocalise while attempting to defecate, spend extended periods in the litter tray, and may begin to associate the litter tray with discomfort, potentially leading to elimination outside the tray.

Palpable abdominal mass

In advanced cases, the retained faecal mass within the distended colon may be palpable through the abdominal wall, feeling like a firm, tubular structure in the caudal abdomen. Some owners may notice that the cat's belly feels firmer than usual.

Progressive weight loss and reduced appetite

As the condition becomes more established, the chronic discomfort and abdominal fullness may progressively reduce appetite. Weight loss may develop as food intake decreases and the cat becomes less active due to persistent discomfort.

Click to read about the biological mechanisms

How this is usually investigated

Investigation of constipation and megacolon in cats involves characterising the severity of faecal retention, assessing colonic structure, and identifying any underlying causes that may be contributing to impaired colonic function.

Physical examination and abdominal palpation

Purpose: The colon can often be palpated through the abdominal wall in cats, allowing assessment of the volume and consistency of retained faecal material, the degree of colonic distension, and the presence of any pain response. The perineal region and rectum may also be assessed for masses, strictures, or pelvic abnormalities.
Considerations: Abdominal palpation in constipated cats requires care, as the distended colon may be uncomfortable and excessive pressure can cause discomfort. The degree of faecal retention palpated does not always correlate with the severity of the underlying motility disorder.

Abdominal radiography

Purpose: Radiographs provide objective assessment of the volume of faecal material within the colon, the degree of colonic distension, and the presence of any pelvic abnormalities that might contribute to obstruction. Serial radiographs can track response to treatment and identify progressive colonic dilation.
Considerations: Radiographs show the current state but do not directly assess colonic motility. A moderately distended colon may represent a recoverable state with adequate motility, or may reflect early megacolon with failing muscle function. The distinction often requires clinical assessment of the response to evacuation.

Blood work and metabolic screening

Purpose: To identify systemic conditions that may contribute to constipation, including dehydration, electrolyte abnormalities (particularly hypokalaemia and hypercalcaemia), hypothyroidism, and renal disease. These metabolic factors can impair colonic motility and may be correctable.
Considerations: Metabolic screening is particularly important in cats presenting with constipation for the first time or in those where the condition has changed in character, as identifying and correcting an underlying metabolic cause may resolve or improve the constipation.

Rectal examination

Purpose: To assess the pelvic canal diameter, identify rectal masses, strictures, or polyps, and evaluate the character of faecal material within the rectum. Narrowing of the pelvic canal from previous fractures is a recognisable cause of obstructive constipation.
Considerations: Rectal examination in cats requires sedation or anaesthesia due to the small size of the patient and the discomfort associated with the procedure. It provides important information about the distal colon and rectum that cannot be obtained from external examination alone.

Response to evacuation

Purpose: The colonic response to manual evacuation under anaesthesia — whether it can contract and function normally once emptied, or whether it remains dilated and atonic — provides important information about the reversibility of the condition. This therapeutic trial helps distinguish recoverable constipation from established megacolon.
Considerations: If the colon returns to near-normal diameter after evacuation and the cat resumes normal defecation with medical support, the prognosis for medical management is more favourable. If the colon remains dilated and the cat rapidly re-accumulates faeces despite medical therapy, this suggests established megacolon with irreversible smooth muscle dysfunction.

Options & trade-offs

Management of feline constipation ranges from dietary and lifestyle modifications for mild cases to surgical intervention for cats with established megacolon. The approach is typically escalated based on the severity and responsiveness of the condition.

Dietary modification and hydration

Increasing dietary moisture content through wet food, adding water to food, or encouraging water intake through fountains can help maintain faecal hydration. Dietary fibre supplementation — either soluble fibre (such as psyllium) or insoluble fibre — may help promote colonic motility and stool passage in some cats.

Trade-offs: Dietary approaches are non-invasive and may be sufficient for mild or early constipation. However, the optimal type and amount of fibre varies between individuals, and some cats with established megacolon do not respond to dietary modification alone. Excessive fibre in a cat with poor colonic motility may worsen distension rather than promote movement.

Osmotic and stool-softening laxatives

Lactulose is the most commonly used osmotic laxative in cats, drawing water into the colonic lumen to soften the faecal material and stimulate motility. Polyethylene glycol (PEG 3350) is an alternative osmotic laxative. These agents help maintain faecal hydration and promote passage.

Trade-offs: Osmotic laxatives require ongoing daily administration and dose adjustment. Too little may be ineffective, while too much can cause diarrhoea, dehydration, and electrolyte disturbances. Long-term compliance requires the cat to accept the medication, which can be challenging. Lactulose may cause flatulence and abdominal cramping in some cats.

Prokinetic medications

Drugs that stimulate colonic smooth muscle contraction, such as cisapride, may help restore or augment colonic motility in cats with impaired but not completely absent muscle function. Cisapride acts on serotonin receptors in the enteric nervous system to enhance coordinated peristalsis.

Trade-offs: Prokinetics are most effective when colonic smooth muscle retains some contractile capacity. In cats with advanced megacolon and complete smooth muscle replacement by fibrosis, prokinetics may have limited benefit. Cisapride availability varies by region and may require compounding pharmacy preparation.

Manual evacuation under anaesthesia

For cats with significant faecal impaction, manual evacuation under general anaesthesia allows the colon to be emptied through a combination of enemas and digital extraction. This provides immediate relief and allows assessment of colonic recovery potential.

Trade-offs: Manual evacuation requires general anaesthesia with its associated considerations. The procedure provides temporary relief but does not address the underlying motility disorder. Some cats require repeated evacuations at intervals of weeks to months if medical management is insufficient to maintain regular defecation.

Subtotal colectomy

Surgical removal of the majority of the colon is considered when medical management has failed and the cat's quality of life is significantly impacted by chronic obstipation. The procedure removes the non-functional dilated colon, allowing the small intestine to connect more directly to the remaining distal colon and rectum.

Trade-offs: Subtotal colectomy is a major surgical procedure with a recovery period of several weeks. Most cats experience soft or liquid stools for weeks to months after surgery as the remaining bowel adapts, and some may have chronically soft stools long-term. However, most cats that undergo the procedure experience significant improvement in quality of life compared to their pre-surgical state of chronic obstipation. Complications can include anastomotic dehiscence, though this is uncommon with experienced surgical technique.

Common misconceptions

Misconception:

"Constipation in cats is always caused by insufficient fibre or water intake."

Reality:

While dehydration and diet can contribute to constipation, the majority of feline megacolon cases are classified as idiopathic, meaning the primary defect is in the colonic smooth muscle itself rather than in dietary factors. Other causes include pelvic canal narrowing from previous fractures, neurological conditions affecting colonic innervation, and metabolic disorders such as hypokalaemia or hypercalcaemia. Simply increasing fibre or water may not be sufficient if the underlying issue is colonic dysmotility.

Misconception:

"If a cat strains in the litter tray, it must be constipated."

Reality:

Straining in the litter tray — technically termed tenesmus — can indicate either constipation (straining to defecate) or urinary obstruction (straining to urinate), and the postures adopted can appear very similar. Urinary obstruction, particularly in male cats, is a condition requiring prompt attention. Distinguishing between these two causes of straining is important, and checking whether the cat is producing urine and whether there is evidence of recent bowel movements can help differentiate them.

Misconception:

"Once a cat develops megacolon, surgery is the only option."

Reality:

Not all cats with colonic dilation require surgery. The distinction between severe constipation with recoverable colonic function and true end-stage megacolon is important. Many cats can be managed medically with a combination of dietary modification, laxatives, prokinetics, and periodic evacuation for extended periods. Surgery is typically considered when medical management no longer maintains adequate quality of life, but this threshold varies between individual cats.

Monitoring litter tray output — both frequency and character of stools — provides the most direct way to track colonic function over time. Noting how often the cat defecates, the consistency and size of the stools produced, and whether straining is observed helps identify trends before they become critical episodes. Maintaining hydration through wet food, water fountains, and multiple water stations throughout the home supports colonic function. If a cat has experienced constipation episodes, understanding the pattern — how frequently they occur, what seems to trigger them, and how they respond to intervention — helps characterise the trajectory of the condition.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS