Blood in Dog Poo: Causes, What It Looks Like, and When to See a Vet

Blood in a dog's poo is a sign that something is irritating or damaging the gastrointestinal tract.

It can appear as bright red streaks on the surface of the stool, blood mixed through loose poo, or a dark, black, tarry consistency. The colour and volume of blood, combined with how your dog is behaving, are the most important factors in working out what to do next. 

If you've just noticed something concerning in your dog's stool and you're not sure what it means, this guide is a practical starting point.

For a complete guide to what your dog's poo is telling you, see our dog poo colour chart.

The most common causes are dietary indiscretion, colitis, and intestinal parasites — but some causes, including haemorrhagic gastroenteritis and parvovirus, require urgent care.

Dietary Causes

The most frequent scenario is a dog eating something rich, unusual, or inappropriate — a fatty meal, table scraps, or something scavenged on a walk. 

A sudden change in diet can also irritate the gut lining and trigger bleeding. In most of these cases, the blood is fresh and red, the volume is small, and the dog recovers quickly with dietary rest.

Colitis and Gut Irritation

Colitis — inflammation of the colon — is the most frequent underlying cause of bright red blood in dog poo. Stress colitis is particularly common, triggered by boarding, travel, or changes in routine. 

Dogs with colitis often produce small amounts of fresh blood alongside mucus, and they typically act normally otherwise. It is one of the more reassuring presentations, but still warrants a vet assessment.

Intestinal Parasites

Hookworms and whipworms can cause GI bleeding, particularly in puppies and dogs with regular outdoor access or exposure to other dogs. Hookworms ingest blood directly from intestinal vessels and inject an anticoagulant that causes continued bleeding at attachment sites. 

Whipworms trigger inflammatory and haemorrhagic reactions in the cecum and colon, producing fresh blood in the faeces of heavily infected dogs. Regular worming is the primary way to prevent parasite-related bleeding. Browse our dog worming treatments for vet-recommended options.

Haemorrhagic Gastroenteritis (HGE)

HGE is characterised by a sudden, large volume of bloody diarrhoea — often described clinically as resembling raspberry jam. It causes rapid dehydration and can become life-threatening within hours. Dogs with HGE may have seemed perfectly well beforehand, which is why volume and sudden onset are the key warning signs rather than the dog's demeanour.

Parvovirus

Parvovirus is the most serious cause of bloody diarrhoea in dogs, and it primarily affects unvaccinated puppies. 

The presentation includes profuse, foul-smelling bloody diarrhoea, vomiting, and rapid deterioration. Vaccination is the best way to protect your dog against parvovirus. If you have an unvaccinated puppy showing any of these signs, treat it as an emergency.

Other Causes

Anal gland problems, rectal polyps, foreign body ingestion, trauma, and certain medications — particularly NSAIDs — can all cause GI bleeding. 

In older dogs, tumours of the bowel are an uncommon but possible cause and should be investigated if no other explanation is found. Your vet will consider the full picture.

A dog that is bright, eating, and drinking despite blood in its poo is usually experiencing a lower GI issue such as colitis — but it still warrants a same-day vet call to rule out something more serious.

Acting normally is a reassuring sign, and in most cases it does point toward a milder cause such as colitis or dietary irritation. The key distinction is between a small amount of fresh red blood — which is more likely to be lower GI and less serious — and a large volume of blood, or any dark tarry stool, which suggests something more significant, regardless of how the dog is behaving.

Monitor appetite, energy level, and whether the bleeding is increasing or stable. A dog that eats, drinks, and remains active while showing a small streak of blood is a different clinical picture from a dog that stopped eating yesterday and is now passing blood today.

If the blood is increasing in volume, if vomiting starts, or if your dog becomes lethargic, the reassurance of "acting normal" no longer applies. Call your vet immediately.

Most dogs that develop blood in their poo recover well with prompt veterinary care. The most important steps are acting quickly when you notice it, not waiting to see if it resolves, and following your vet's guidance on diet and recovery. 

For most mild cases, a short period of dietary rest, a course of probiotics, and a follow-up call is all that's needed. If you're concerned about your dog right now, call your vet today.