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. Persistent large bowel inflammation can affect quality of life. Weight loss and vomiting are uncommon when disease is confined to the colon, so either one widens the list of possible causes. The pattern of episodes may provide clues about potential contributing factors.
Signals & patterns
Early signals
Increased frequency of defaecation
Affected animals may need to defaecate 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 defaecate
The animal may strain during or after passing faeces, sometimes remaining in position longer than necessary. In cats, repeated straining in the litter tray with little or nothing produced can instead be attempts to pass urine through a blocked urethra, which occurs almost exclusively in males and which owners sometimes mistake for constipation. The two cannot be told apart reliably from posture alone, and a blocked urethra can be fatal within 2 to 3 days without treatment.
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.
Discomfort when passing faeces
Some animals show pain when defaecating. Flatulence is reported with some causes, such as Tritrichomonas foetus infection in cats.
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 tried first. 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"
Streaks or spots of fresh red blood on the stool of an animal that is otherwise bright and eating often come from the inflamed colon lining and may resolve as inflammation subsides. The amount of blood and the animal's general state change the picture. Profuse bloody diarrhoea, often likened to raspberry jam, with vomiting, lethargy or loss of appetite is the pattern of acute haemorrhagic diarrhoea syndrome in dogs, in which fluid loss can cause shock before dehydration is recognisable. Parvovirus, mainly in unvaccinated or incompletely vaccinated dogs aged 6 weeks to 6 months, also causes bloody diarrhoea with vomiting and lethargy. How much fluid an animal has lost cannot be judged at home from its appearance.
"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 point higher up, while mucus and straining point towards 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.
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.
Giardiasis
Giardia is a protozoal parasite that lives in the small intestine. Affected animals usually pass soft, pale, poorly formed faeces that contain mucus, and blood is usually not present. Because mucus in the stool is also a feature of colitis, faecal testing for Giardia is often part of the investigation of persistent diarrhoea.
Acute Haemorrhagic Diarrhoea Syndrome (AHDS)
Acute haemorrhagic diarrhoea syndrome causes sudden, profuse bloody diarrhoea in dogs, commonly with vomiting and lethargy. It is one of the conditions separated from colitis when blood appears in the stool.
Urethral Obstruction
In cats, straining to pass urine through a blocked urethra can look like straining to pass faeces, so it is considered when a cat strains in the litter tray and produces little.
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: 9 October 2026 · Dr Alastair Greenway MRCVS