CONDITION
Colitis
Colitis describes inflammation of the large intestine—the final portion of the digestive tract where water is absorbed and stool takes its formed shape. When this lining becomes irritated or inflamed, the normal rhythm of absorption and movement changes, often producing softer stool, visible mucus, or small amounts of fresh blood. The urge to pass stool may become more frequent, and some animals strain even when little is produced. Owners typically arrive at this page after noticing changes in their pet's bowel habits—diarrhoea that seems to come from the back passage rather than higher up, jelly-like mucus coating the stool, or repeated trips to the garden with little result. The pattern can appear suddenly or build over days, and in some cases it resolves on its own while in others it persists or returns. This page explores the signals that point towards large bowel involvement, the range of causes that can trigger inflammation in this part of the gut, the ways these causes are investigated, and the approaches used to settle symptoms and address underlying triggers where they can be identified.
Why this matters now
Colitis can occur at any age in dogs and cats, though certain forms may be more prevalent in specific life stages. Stress-related colitis often appears following changes in routine, boarding, or travel. Dietary indiscretion is a common trigger, particularly in dogs. The condition can present as isolated episodes or may recur intermittently over time.
Acute colitis often resolves within a few days with appropriate management. However, some animals develop chronic or recurrent colitis that requires ongoing attention to identify and address underlying triggers. Without intervention, persistent large bowel inflammation can affect quality of life and nutritional status. The pattern of episodes may provide clues about potential contributing factors.
Signals & patterns
Early signals
Increased frequency of defecation
Affected animals may need to defecate more frequently than usual, sometimes with a sense of urgency.
Small volumes of soft faeces
Rather than producing normal-sized stools, the animal passes frequent small amounts of loose or semi-formed material.
Mucus in faeces
The presence of visible mucus coating the stool or passed separately is a hallmark sign of large bowel involvement.
Straining to defecate
The animal may strain during or after passing faeces, sometimes remaining in position longer than necessary.
Later signals
Fresh blood in faeces
Bright red blood on the surface of stools or passed separately suggests inflammation or irritation of the lower bowel lining.
Accidents in house-trained animals
The urgency associated with colitis may lead to loss of normal toileting habits.
Flatulence and abdominal discomfort
Increased gas production and visible discomfort around the abdomen may accompany ongoing inflammation.
Click to read about the biological mechanisms
How this is usually investigated
Investigation of colitis often begins with distinguishing large bowel from small bowel disease based on clinical signs. Identifying potential triggers and ruling out specific causes helps guide management approaches.
Faecal examination
Faecal PCR panels
Blood tests
Colonoscopy with biopsies
Dietary elimination trial
Options & trade-offs
Management of colitis depends on whether the condition is acute or chronic and whether an underlying cause has been identified. Many cases of acute colitis resolve with supportive care, while chronic cases may require ongoing dietary and medical management.
Dietary modification
Highly digestible diets, fibre supplementation, or novel protein diets may help reduce colonic irritation and support normal bowel function.
Trade-offs: Often forms the foundation of management. May require trial of different approaches to find what works for the individual. Some diets are more expensive than standard foods.
Probiotic supplementation
Beneficial bacteria may help restore normal gut flora balance and reduce inflammation in some cases.
Trade-offs: Generally well-tolerated. Evidence of effectiveness varies. May take time to show benefits.
Antiparasitic treatment
If parasites are identified or suspected, targeted treatment can resolve the underlying cause.
Trade-offs: Usually straightforward when parasites are confirmed. Some treatments may need to be repeated.
Anti-inflammatory medications
Various medications can help reduce colonic inflammation, ranging from locally-acting drugs to systemic immunosuppressants for immune-mediated cases.
Trade-offs: Choice depends on severity and chronicity. Some medications have potential side effects with long-term use. May be needed indefinitely in some chronic cases.
Stress management
Environmental modifications and potentially anxiolytic support may help in stress-related cases.
Trade-offs: Addresses an often-overlooked trigger. May need to be combined with other approaches.
Common misconceptions
"Colitis and diarrhoea are the same thing"
Colitis specifically refers to large bowel inflammation with characteristic signs including frequent small stools, mucus, and straining. Diarrhoea can originate from various parts of the digestive system with different underlying causes.
"Blood in the stool always indicates something serious"
While blood should always be noted, fresh red blood in colitis often results from surface irritation of the inflamed colon lining and may resolve as inflammation subsides.
"Colitis only affects dogs that eat inappropriate things"
While dietary indiscretion is one trigger, colitis can result from stress, infections, parasites, food sensitivities, or underlying inflammatory conditions unrelated to what the animal has eaten.
Related conditions
Gastroenteritis in Dogs
Gastroenteritis describes inflammation affecting both the stomach and intestines, and when that inflammation concentrates primarily in the large bowel the pattern often overlaps with colitis. Distinguishing between the two can depend on the distribution of signs—vomiting and small bowel diarrhoea pointing higher up, mucus and straining pointing toward the colon.
Gastroenteritis in Cats
Gastroenteritis in cats can present with colitis when inflammation settles predominantly in the large intestine rather than higher in the digestive tract. The clinical picture may include mucoid diarrhoea and increased urgency, patterns also seen in colitis, and investigation often considers both possibilities.
Gallbladder Mucocele
Giardiasis is an intestinal infection that can trigger inflammation in the large bowel, producing diarrhoea with mucus and sometimes fresh blood—signs that overlap with colitis. In some cases, giardia is identified during investigation for persistent colonic symptoms.
Tritrichomonas Foetus in Cats
Tritrichomonas foetus is a protozoal organism that colonises the large intestine in cats and often produces chronic diarrhoea with mucus or blood, a presentation characteristic of colitis. It is one of several infectious triggers considered when colitis persists or recurs in younger cats, particularly those from multicat environments.
Intestinal Adenocarcinoma
Intestinal adenocarcinoma arising in the large bowel can produce signs that mimic inflammatory colitis, including diarrhoea with mucus or blood and increased frequency of defaecation. Distinguishing between chronic inflammation and neoplasia in the colon may require imaging and tissue sampling.
Keeping track of stool frequency, consistency, and any visible abnormalities can help identify patterns. Noting any dietary changes, stressful events, or environmental factors around the time symptoms appear may reveal potential triggers. Understanding that some animals may be predisposed to recurrent episodes can help with planning appropriate dietary and lifestyle adjustments.
Last reviewed: 31 July 2026 · Dr Alastair Greenway MRCVS