Colitis in Cats: Symptoms, Causes & Treatment (2026)

What Is Colitis?

Colitis is inflammation of the colon — the final section of the large intestine. The colon's primary job is absorbing water from digested material and forming solid stool. When it becomes inflamed, this process is disrupted: the colon contracts more frequently, absorbs water poorly, and secretes mucus as a protective response. The result is urgent, frequent defecation with loose or liquid stool that often contains mucus or bright red blood.

Colitis in cats falls into two categories:

• Acute colitis: Sudden onset, often after a stressor or dietary trigger. Most cases resolve within 3–5 days with or without treatment.
• Chronic colitis: Recurring or persistent over weeks to months. Requires investigation to find the underlying cause.

Understanding whether the problem is in the large bowel (colon) or the small bowel is the first and most important step — because the causes, diagnostic approach, and treatment differ significantly between them.

Large Bowel vs. Small Bowel Disease

The character of the diarrhea tells you which part of the GI tract is involved. This distinction guides the entire workup.

Some cats have both — mixed large and small bowel disease — particularly with IBD or triaditis, where inflammation spans multiple GI segments simultaneously.

Signs of Colitis in Cats

• Frequent trips to the litter box, often producing little or nothing
• Straining and vocalizing during defecation
• Fresh bright red blood coating or mixed into the stool (hematochezia)
• Mucus in or on the stool — clear, white, or yellowish
• Loose or semi-formed stool rather than fully liquid diarrhea
• Urgency — accidents outside the litter box despite being house-trained
• Intermittent vomiting
• Reduced appetite during acute episodes
• Normal or near-normal weight (distinguishes colitis from small bowel disease)

Causes of Colitis in Cats

Stress (Idiopathic / Stress Colitis)

Stress is the single most common trigger for acute colitis in cats. Cats are highly sensitive to environmental changes, and the colon is particularly responsive to stress hormones — changes in colonic motility, secretion, and mucosal permeability occur rapidly. Common triggers include:

• Introduction of a new pet or person to the household
• Moving to a new home
• Boarding or a vet visit
• Construction or loud noise nearby
• Change in litter brand, litter box location, or feeding schedule
• Owner absence or change in routine

Stress colitis typically resolves within 3–5 days. Recurring episodes suggest the stressor is ongoing or the cat has underlying anxiety that benefits from environmental modification and sometimes pharmaceutical support.

Dietary Causes

• Sudden diet change: Abrupt transitions disrupt the colonic microbiome. Always transition gradually over 7–10 days.
• Food allergy or intolerance: An adverse reaction to a dietary protein — most commonly chicken, beef, fish, or dairy — can cause chronic colitis alongside or without skin signs.
• Dietary indiscretion: Eating something unusual, especially high-fat or unfamiliar food.

Parasites

Several parasites specifically target the large bowel in cats:

• Tritrichomonas foetus: A protozoan parasite that is the most important cause of chronic large bowel diarrhea in young cats, particularly those from multi-cat households or catteries. It is not detected on standard fecal flotation — requires PCR testing of fresh stool. Treatment is ronidazole (not metronidazole, which is ineffective against T. foetus).
• Giardia: Can affect the large bowel as well as the small intestine. Standard fecal antigen tests or PCR detect it.
• Cryptosporidium: Particularly in immunocompromised cats or kittens. Requires acid-fast staining or PCR to diagnose.

Inflammatory Bowel Disease (IBD)

IBD in cats can involve the colon, producing colitis as part of a broader immune-mediated GI condition. Colitis-predominant IBD in cats shows the large bowel pattern — straining, blood, mucus — but the inflammation extends into the colonic wall rather than the small intestine. Definitive diagnosis requires colonoscopy with biopsy; treatment is immunosuppression with prednisolone.

Bacterial Infections

Salmonella, Campylobacter, and Clostridium perfringens can all cause acute colitis with bloody diarrhea and systemic signs. Cats that hunt, eat raw meat, or have exposure to other animals are at higher risk. Bacterial colitis often causes systemic illness (fever, lethargy, loss of appetite) alongside the GI signs.

Systemic Disease

Large bowel signs can also stem from systemic conditions:

• Hyperthyroidism: Excess thyroid hormone increases colonic motility, causing diarrhea and urgency. A T4 is essential in any senior cat with GI signs.
• Chronic kidney disease: Uremic toxins can disrupt normal colonic function.
• Colonic neoplasia: Large bowel tumors — particularly adenocarcinoma and lymphoma — can cause chronic colitis signs in older cats. Usually associated with progressive weight loss and deterioration not explained by simpler causes.

What Bloodwork Shows

In simple, acute colitis, bloodwork is often completely normal. The colon does not absorb nutrients, so colitis does not cause low albumin or cobalamin deficiency the way small bowel disease does. The value of bloodwork is ruling out systemic triggers, not confirming colonic inflammation.

Key additions for any senior cat with colitis:

• T4 (thyroid): Elevated in hyperthyroidism — the most commonly missed systemic cause of diarrhea in senior cats.
• BUN, creatinine, SDMA: Assess kidney function — CKD is common in older cats and can contribute to GI signs.
• CBC: Elevated white cells suggest infection or significant inflammation; anemia may indicate chronic blood loss.
• Albumin: Normal in pure colitis (colon doesn't absorb protein). Low albumin in a cat with apparent colitis suggests concurrent small bowel involvement.

Fecal testing is often more informative than bloodwork for colitis: standard fecal flotation for common parasites, fecal antigen or PCR for Giardia, and PCR for Tritrichomonas foetus (fresh, refrigerated sample sent to a specialized lab) if T. foetus is suspected. Fecal culture can identify bacterial pathogens.

Diagnosing the Cause

Treatment

Dietary Management

Diet is central to managing most forms of colitis in cats:

• High-fiber diet: Soluble and insoluble fiber normalize colonic motility and feed beneficial bacteria. Prescription GI diets (Hill's w/d, Royal Canin GI Fiber Response) are commonly used. Adding plain canned pumpkin (1–2 teaspoons daily) as a fiber source can help mild cases.
• Hydrolyzed or novel protein diet: Used when food allergy or intolerance is suspected. Must be strictly maintained — no treats or flavored medications that contain the excluded protein.
• Highly digestible low-fiber diet: Used for some cats where high fiber worsens signs. The right fiber level varies between individuals.

Medications

Environmental Management for Stress Colitis

When stress is the identified or suspected cause, environmental modification is essential alongside any short-term medication:

• Maintain a consistent routine — same feeding times, same litter box locations
• Provide sufficient litter boxes: one per cat plus one extra, in separate quiet locations
• Offer hiding spaces, vertical territory (cat trees, shelving), and window perches
• Use synthetic feline facial pheromone diffusers (Feliway) to reduce baseline anxiety
• Minimize exposure to the identified stressor where possible
• For multi-cat households: ensure each cat has its own resources (food bowls, water stations, litter boxes, sleeping spots)

Tritrichomonas Foetus: The Most Missed Cause

Tritrichomonas foetus is a protozoan parasite that lives in the large intestine of cats and is the most underdiagnosed cause of chronic large bowel diarrhea — particularly in younger cats or those from catteries, shelters, or multi-cat homes.

The diarrhea caused by T. foetus is classic large bowel in character: frequent, small, malodorous stools with mucus and often fresh blood, persistent straining, and perineal irritation. The cat is otherwise often bright and eating normally — the GI signs are the main complaint.

Standard fecal flotation does not detect T. foetus. PCR testing of a fresh, unrefrigerated sample sent to a specialized laboratory is required. The test must be repeated on fresh stool (organism dies rapidly after defecation) and should be run at a laboratory with proven sensitivity for this organism.

Treatment is ronidazole — metronidazole does not reliably eliminate T. foetus and should not be used as a substitute. Ronidazole must be dosed carefully (at most 30 mg/kg once daily for 14 days) as higher doses cause neurological toxicity. Clinical signs may improve before the organism is fully eliminated — repeat PCR testing 4 weeks after treatment confirms clearance.

Prognosis

Acute stress colitis has an excellent prognosis — most episodes resolve within a few days. Recurrence is common in anxious cats or those in unstable environments.

Dietary-responsive colitis carries an excellent long-term prognosis with appropriate diet management and strict adherence.

Tritrichomonas foetus — most cats eventually achieve spontaneous resolution over months to years, even without treatment, as their immune system controls the organism. However, treatment with ronidazole significantly shortens the course and reduces transmission risk. Prognosis for full recovery is good.

IBD-related colitis requires ongoing management — most cats are controlled with diet and prednisolone, though periodic flares occur. Regular monitoring of response and medication adjustment is needed long-term.

Large bowel lymphoma in cats (when it occurs) is typically large cell — more aggressive than the small cell form that affects the small intestine. Prognosis is more guarded and requires oncology consultation.