SYMPTOM
Bloody diarrhoea
Owners may notice streaks of bright red blood or mucus on or in the stool, loose stool with blood mixed through it, or profuse watery diarrhoea that looks like raspberry jam. Blood from higher in the gut is digested on the way through and tends to make the stool black and tarry rather than red. The amount of blood, how often the dog is passing stool, straining, vomiting and the dog's energy all change what the sign is likely to mean.
Infectious Enteritis (Bacterial And Viral)
Organisms such as Campylobacter, Salmonella, and canine parvovirus damage the intestinal lining directly, allowing blood and plasma proteins to leak into the gut and appear in the stool alongside mucus. Parvovirus is most often seen in unvaccinated puppies and produces profuse, often haemorrhagic diarrhoea with marked lethargy, while bacterial causes can affect dogs of any age, particularly after scavenging or contact with raw diets. Onset is typically rapid, over one to two days. A puppy with parvovirus can progress from quiet and off food to vomiting, profuse bloody diarrhoea and collapse from fluid loss and sepsis within a day or two, and survival is considerably higher with hospital treatment than without it. Parvovirus is shed in faeces and survives in the environment for months, so it passes readily to other unvaccinated dogs, though it does not infect people. Campylobacter and Salmonella can pass from dogs to people through contact with faeces, which matters most for young children, older people, pregnant women and anyone with a weakened immune system.
Intestinal Parasites
Whipworm (Trichuris vulpis) attaches to and irritates the lining of the large intestine and caecum, causing chronic or intermittent blood-streaked diarrhoea rather than the sudden severe episodes seen with bacterial or viral infection, while the hookworm usually found in dogs in the UK, Uncinaria stenocephala, is a poor blood feeder that tends to cause diarrhoea, weight loss and protein loss through the gut rather than anaemia or visible bleeding. Ancylostoma caninum, a blood-feeding hookworm that can cause anaemia and dark or bloody diarrhoea in puppies, is rare in the UK but can occur in dogs that have been imported or have travelled abroad. Dogs with regular access to parks or shared outdoor spaces are more frequently exposed, as whipworm eggs persist in soil for long periods. Weight loss and a fluctuating appetite often accompany the diarrhoea when parasite burden is high.
Dietary Indiscretion, Acute Colitis And Gastroenteritis
A sudden change of food, fatty scraps, or scavenged material can irritate the intestinal lining enough to cause transient blood streaking in an otherwise formed or loose stool. This is one of the more common and self-limiting explanations, particularly in dogs known to raid bins or eat items found on walks, and it typically settles within a few days without other systemic signs. Acute colitis, sometimes following stress such as kennelling, travel or a house move, gives a similar picture of frequent small stools containing fresh blood and mucus, with straining. The benign pattern is usually a few streaks of blood or mucus in a dog that is bright, eating, drinking and not vomiting. Profuse watery or jelly-like bloody diarrhoea, vomiting alongside it, a dog that is flat or reluctant to get up, or gums that look pale, grey or tacky, point away from a simple upset. Early parvovirus, particularly in a puppy or a dog that is not fully vaccinated, and early acute haemorrhagic diarrhoea syndrome can look mild for the first few hours, and the degree of fluid loss cannot be judged reliably at home.
Coagulopathy And Toxin-Related Bleeding
Anticoagulant rodenticide ingestion impairs the blood's clotting ability and can lead to visible blood in the stool alongside signs of bleeding elsewhere, such as bruising or bleeding gums. Signs of anticoagulant rodenticide poisoning usually appear three to seven days after the bait is eaten, once the body's stored clotting factors are used up, so a dog can look entirely well in the meantime. Much of the bleeding is often internal, into the chest, abdomen or joints, and may show as weakness, breathlessness or pale gums rather than visible blood. This generalised bleeding tendency is less typical of infectious causes, where bleeding usually stays confined to the gut lining. An overdose of anti-inflammatory medication such as ibuprofen damages the gut lining directly rather than causing a generalised bleeding tendency, and tends to cause vomiting, which may contain blood, and black tarry stools from ulceration higher in the gut. Dogs with access to garden sheds, farmland, or human medication cabinets carry some background risk.
Acute Haemorrhagic Diarrhoea Syndrome
This syndrome, previously called haemorrhagic gastroenteritis, is one of the more common causes of sudden, profuse bloody diarrhoea in adult dogs, and small and toy breeds are overrepresented. The stool is often watery and red, like raspberry jam, and vomiting often starts first, sometimes several hours before the diarrhoea. It has been linked to toxins from Clostridium perfringens that damage the intestinal lining, allowing large volumes of fluid and protein to pour into the gut. Dehydration and shock can develop within hours, and blood tests often show concentrated blood before this is obvious from the outside. Most dogs recover within a few days with intravenous fluid therapy, and the outlook is poorer when shock is already established before treatment begins.
Intussusception, Addison's Disease And Heatstroke
These are less common but carry weight when they occur. Intussusception, where one section of bowel telescopes into the next, is seen mainly in young dogs, often after a bout of enteritis or a heavy worm burden, and tends to cause vomiting, abdominal pain and small amounts of dark, jelly-like bloody stool. Addison's disease (hypoadrenocorticism) can cause a waxing and waning illness with vomiting, bloody diarrhoea, weakness and sudden collapse. Heatstroke damages the gut lining and blood clotting, and bloody diarrhoea after overheating, alongside heavy panting and weakness, reflects severe heat injury. Recurrent streaks of fresh blood over weeks in an older dog can also come from rectal polyps or tumours.
Why timing matters
Early observation
In the first hours, a dog with a mild dietary upset and a dog in the early stages of parvovirus or acute haemorrhagic diarrhoea syndrome can look similar. The difference tends to show in the volume and frequency of diarrhoea, vomiting, energy and gum colour, and blood tests can show fluid loss before it is visible at home.
Later presentation
Once large volumes of bloody fluid are being lost, dehydration and shock can develop within hours, particularly in puppies and small dogs. With anticoagulant rodenticide poisoning, blood in the stool may appear only after internal bleeding is already under way.
The same sign spans a wide range, from a single streak of blood in a normal stool that settles within a day, to situations where the timing of fluid therapy, clotting treatment or surgery shapes the outcome. Duration alone is a poor guide: the time-sensitive causes tend to show themselves through the volume of diarrhoea, vomiting, lethargy, the dog's age and vaccination status, and possible exposure to poisons, rather than through how long the blood has been present.
Conditions commonly associated
Canine Parvovirus
Parvovirus destroys the intestinal lining, causing characteristic profuse, often bloody, foul-smelling diarrhoea.
Campylobacteriosis
In more severe cases, particularly in young animals, Campylobacter can cause diarrhoea streaked with fresh blood or mucus. The infection can pass to people through contact with faeces.
Ibuprofen Toxicity
Loss of the normal protection of the stomach and intestinal lining allows ulceration and bleeding, which tends to show as black, tarry stool and sometimes as blood mixed through loose stool.
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Intestinal Intussusception
Blood or mucus in the stool, sometimes described as redcurrant-jelly in appearance, develops as the telescoped bowel becomes congested and its lining sheds.
Salmonellosis
Blood-streaked, watery stools can appear when this bacterial infection inflames and damages the lining of the intestine. Salmonella can pass to people through contact with faeces, and raw-fed dogs can shed it without appearing unwell. In Great Britain, Salmonella found in a sample from a dog is reportable to APHA.
Anticoagulant Rodenticide Toxicity
Anticoagulant rodenticides stop blood from clotting, so bleeding into the gut can appear as blood in the stool, usually three to seven days after the bait is eaten and often alongside internal bleeding elsewhere.
Colitis
Inflammation of the large intestine is a common cause of frequent small stools containing fresh blood and mucus, with straining, and acute cases linked to stress or diet often settle within a few days.
Intestinal Parasites
Whipworm irritates the lining of the large intestine and can cause chronic or intermittent diarrhoea streaked with fresh blood.
Addisons Disease in Dogs
Low production of adrenal hormones (cortisol and, in most dogs, aldosterone) can cause vomiting and bloody diarrhoea alongside weakness, and the illness can progress to sudden collapse.
Acute Haemorrhagic Diarrhoea Syndrome (AHDS)
AHDS causes sudden, profuse, often raspberry-jam bloody diarrhoea with vomiting in adult dogs, and the fluid loss can lead to shock within hours.
Inflammatory Bowel Disease in Dogs
Inflammatory bowel disease causes chronic diarrhoea that can come and go over weeks, and when the large intestine is involved the stool can carry fresh blood and mucus, with straining.
When to explore further
When the diarrhoea is profuse, watery or jelly-like with a lot of blood, as fluid loss on this scale can lead to shock within hours
When vomiting, a flat or collapsed demeanour, pale, grey or tacky gums, or a tense, painful belly accompany the blood
When the dog is a puppy or not fully vaccinated, where parvovirus is a possibility and fluid loss affects a small body quickly
When rat or mouse bait, human medication such as ibuprofen, or overheating could be involved, or when black tarry stools, bruising or bleeding elsewhere appear
When streaks of blood keep returning over weeks, a pattern that fits parasites, inflammatory bowel disease or a rectal growth more than a one-off upset
Details that tend to be useful in a veterinary conversation include whether the blood is bright red or the stool black and tarry, how much is passed and how often, whether vomiting, straining or lethargy are present, gum colour, vaccination and worming history, and any possible access to rat or mouse bait, medication, bins or scavenged food. A photograph of the stool can help. Because Campylobacter and Salmonella can pass to people, handwashing after handling faeces and keeping young children away from soiled areas limit spread within the household.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS