Constipation and megacolon in cats

Symptoms an owner can spot at home

• Straining in the litter box, sometimes with vocalising, and producing little or nothing
• Small, hard, dry pellets of stool, or no stool for more than 48 hours
• Repeated trips in and out of the box
• Liquid stool passing around a hard mass, which owners often misread as diarrhoea
• Reduced appetite, vomiting, hiding, or a hunched posture

When to see a vet

• Straining in the box with nothing produced, this can equally be urinary obstruction, which kills male cats within a day
• No stool passed in more than 48 to 72 hours
• Vomiting, appetite loss, or lethargy alongside straining
• Blood on the stool or around the anus
• Recurring episodes, repeat constipation is a workup, not a home-remedy loop

What it is

Faeces sitting too long in the colon lose water, harden, and become progressively harder to pass. Constipation is the mild end. Obstipation is intractable constipation the cat cannot resolve on its own. Megacolon is the end stage, where the colonic smooth muscle is permanently stretched and can no longer generate effective contractions; at that point the change is not reversible and management becomes lifelong.

The causes cluster in a few groups worth knowing, because they change what you do at home. Dehydration, including the subclinical dehydration of early chronic kidney disease, is the most common. Pain is second and badly under-appreciated: an arthritic cat that cannot comfortably posture, or that must jump to reach a box, starts avoiding defecation. Then come obstructive causes such as a healed pelvic fracture narrowing the canal, neurological disease, and idiopathic megacolon, which accounts for the majority of true megacolon cases.

Litter box aversion belongs on this list too. A box that is too small, too dirty, in a high-traffic spot, or too far away teaches a cat to hold, and holding is how constipation becomes a habit.

How vets diagnose it

Abdominal palpation and radiographs confirm faecal loading and grade colonic diameter, which is what separates constipation from established megacolon. The workup then looks for why: bloodwork and urinalysis for kidney disease, dehydration, and electrolyte abnormalities including hypokalaemia and hypercalcaemia; thyroid testing where indicated; pelvic imaging for old fractures; and an orthopaedic assessment, since feline osteoarthritis is widespread and easily missed.

Treatment overview

This is editorial overview, not a treatment plan; your vet builds the plan. Acute episodes are treated with rehydration, usually intravenous or subcutaneous fluids, and gentle evacuation under veterinary supervision. Ongoing management combines an osmotic laxative, a prokinetic where the colon still contracts, dietary fibre or moisture strategy, and treatment of the underlying driver.

Two safety points are absolute. Never give a human enema product to a cat; phosphate-containing enemas cause fatal electrolyte derangements in cats. And do not treat repeated episodes at home with over-the-counter laxatives while the underlying cause goes uninvestigated. Cats with refractory megacolon that fail medical management may be candidates for subtotal colectomy, which has good long-term outcomes in appropriately selected patients.

What owners can do

• Raise water intake first. Wet food is the highest-leverage change available: canned diets carry roughly 70 to 80% moisture against under 10% in dry kibble.
• Add a circulating fountain and multiple wide, shallow bowls away from food. Cats drink more from moving water and dislike whisker contact with narrow bowls.
• Size the box properly: length at least 1.5 times the cat's body length, nose to tail base. Most commercial boxes are too small for an adult cat.
• Use a low-entry box for senior or arthritic cats. A 15 cm lip is a real obstacle to a cat with hip or spinal pain.
• Follow the n+1 rule: one box per cat plus one, on every floor of the home, away from noise and appliances.
• Scoop daily, minimum. A soiled box is a documented driver of holding behaviour.
• Keep the cat lean and treat osteoarthritis. Pain-free posturing is a prerequisite for normal defecation.
• Track defecation frequency on a calendar. In a covered, multi-cat home this is the only way to notice a two-day gap.