Colitis

Colitis refers to inflammation of the colon (large intestine). It is one of the most common causes of diarrhoea in dogs. The large intestine's primary function is absorbing water from waste - when inflamed, this function is impaired, resulting in frequent, urgent defecation of small volumes often containing mucus or fresh blood.

Colitis can be acute (a single episode) or chronic (recurring or persistent). Acute colitis in an otherwise healthy dog frequently resolves within a few days with supportive management. Recurring or chronic colitis warrants investigation to find the underlying cause.

Recognising colitis vs. small intestinal diarrhoea

Understanding which part of the gut is involved helps guide diagnosis:

| Feature | Large intestinal (colitis) | Small intestinal | | ----------- | -------------------------- | ----------------------------- | | Frequency | High - many small episodes | Lower - fewer, larger volumes | | Straining | Common | Uncommon | | Mucus | Often present | Uncommon | | Fresh blood | Occasionally | Uncommon | | Weight loss | Uncommon | Common if chronic | | Vomiting | Less common | More common |

Common causes

• Dietary indiscretion: Eating something that doesn't agree with the gut - garbage, a new food introduced too quickly, rich treats
• Stress: Anxiety and psychological stress directly affects gut motility and mucosal integrity ("stress colitis" is well-recognised)
• Parasites: Giardia and Trichuris (whipworm) in particular
• Bacterial infection: Campylobacter, Salmonella, Clostridium
• Underlying IBD: Chronic colitis that doesn't resolve may be part of IBD (see IBD guide)
• Histiocytic ulcerative colitis: A specific form in Boxers and Bulldogs caused by an invasive E. coli infection; treated with enrofloxacin

What to log

For each colitis episode, record:

• Date of onset and resolution
• Stool characteristics: Frequency per day, consistency, presence of mucus, blood (bright red, dark, or streaky)
• Straining: Note whether the dog is squatting repeatedly with nothing coming out
• Appetite and water intake: Normal, reduced, or absent
• Any likely trigger: Dietary change, treat, new environment, stressful event, boarding
• Duration: How many days before stools normalised

Over time, a log of multiple episodes allows you to identify patterns - if colitis consistently follows stressful events, or consistently follows a specific treat or food, the trigger becomes visible.

Treatment

Acute colitis: Often self-resolving with a brief period of bland food (boiled chicken and rice, or a commercial GI diet), rest, and ensuring the dog stays hydrated. Your vet may prescribe metronidazole (an antibiotic with anti-inflammatory properties in the gut) or a probiotic.

Recurrent colitis:

• Dietary management - a high-fibre or hydrolysed diet often helps; increased dietary fibre can regulate large bowel transit
• Parasitic treatment if indicated
• Investigation for underlying IBD if no other cause is found
• Stress management for dogs with clearly stress-triggered episodes

When to contact your vet

Contact your vet soon if:

• Blood in the stool is significant (more than a streak) or the dog is distressed
• The dog is not eating, lethargic, or showing signs of abdominal pain
• Diarrhoea has not improved within 48 hours of bland diet management
• Episodes are recurring (more than three times per year)

Go to the emergency vet if:

• Large volumes of fresh blood in stool
• Collapse or severe lethargy

Frequently Asked Questions

My dog has bloody diarrhoea - is that always serious? A few streaks of bright red blood in an otherwise alert dog with normal appetite is usually haemorrhagic colitis - uncomfortable but typically self-limiting. A large amount of blood, a dog that seems unwell, or blood accompanied by vomiting warrants same-day vet assessment. Dark, tarry stools suggest upper GI bleeding - more serious.

Can stress really cause colitis? Yes. The gut-brain axis is well-documented in dogs. Fireworks, vet visits, travel, house moves, a new pet or person in the household - these stressors can directly trigger colitis episodes. If you can identify a stress trigger and it's recurring, discuss with your vet whether management (anti-anxiety measures, dietary support) is appropriate.

My dog's colitis seems to come back around the same time each year. Why? Seasonal patterns in colitis may relate to dietary indiscretion (eating grass, berries, or found food during walks), parasites (whipworm has seasonal patterns), or environmental stress. Log the month, what was different, and look for a consistent trigger.

Is a probiotic helpful for colitis? Some evidence supports their use in acute and chronic colitis. They don't work as a stand-alone treatment for established colitis but may shorten episode duration and support recovery. Use a veterinary-specific product containing appropriate strains.

Can my dog's colitis be caused by their regular food? Yes - food sensitivity or allergy can manifest as large bowel diarrhoea. If the colitis is recurring and no other cause is found, a dietary elimination trial is worth discussing with your vet.