Spots of blood in your dog’s poo can be a worrying thing to notice; could your dog have something sinister going on inside, or should you stop worrying? Today we will discuss the various diseases that cause blood in a dog’s stools, as well as how vets diagnose these diseases.  

Where Does the Blood Come From? 

Blood found in a dog’s stools can originate from any part of the gastrointestinal tract, from the mouth to the anus. It may also come from the respiratory system if a dog coughs blood up then swallows it, or even elsewhere (like a bleeding cut) if your dog licks it off. To help determine where the blood comes from, we can determine if a dog has haematochezia or melaena.  

Haematochezia means poo associated with fresh red blood. The blood has not been broken down in the GI tract, meaning it must originate from the lower intestinal tract (the large intestine onwards). Conversely, melaena refers to poo with digested blood in it. The blood has been broken down by the intestinal tract, making it look dark brown or black (sometimes referred to as coffee grounds). The stools can sometimes look ‘tarry’.  

With melaena, the blood must originate before the large intestine. In some diseases, both haematochezia and melaena can be present at the same time.  In many cases, haematochezia and melaena are seen alongside diarrhoea and other GI symptoms, though this is not always the case. Any bleeding that originates from the stomach or higher may also result in blood seen in the saliva, vomit or after eating.  

Causes of Bloody Poo 

Oral Cavity Bleeding 

Starting at the top of the GI tract, it is common for bleeding in the mouth to lead to melaena. In young dogs, especially those that like to chew, we would consider lacerations and abrasions from chewing to be a top cause. Sharp sticks, hard kibble, bones or bits of plastic can all damage the oral cavity and lead to bleeding. In some cases, these objects can become lodged in the mouth, causing chronic pain, bleeding and melaena. You can prevent this by ensuring your dog is monitored when chewing, and preventing them chewing anything firmer than what can be dented with a thumbnail.  

Periodontal disease is far too common in dogs, especially in older dogs or those with short faces (brachycephalic). Without routine dental care, bacteria builds-up in a dog’s mouth as plaque. Bacteria causes inflammation which leads to gingivitis, or inflammation of the gums. Inflamed gums easily bleed, causing melaena when the blood is swallowed. In extreme cases, periodontal disease can lead to erosion of the teeth and bone, causing further bleeding and pain. This level of periodontal disease must be addressed with periodontal surgery by a vet. Teeth and gums can also bleed if damaged by hard objects, such as those listed above.  

Finally, certain masses in the mouth that bleed can result in melaena if the blood if swallowed. These include inflammatory lumps, such as cysts, abscesses or immune responses, as well as cancers. Many cancers of the mouth are benign such as epulis, but some can be malignant such as melanomas, squamous cell carcinomas (SCCs), and fibrosarcomas. Some of these bleed due to tissue damage, while others get caught when a dog is chewing. Any bleeding lump in a dog’s mouth should be investigated by a vet.  

Airway Bleeding 

As we discussed above, in some cases a dog can have bleeding into the respiratory system. The fluid tends to make dogs cough, resulting in the blood being pushed into the mouth, where it is swallowed and can lead to melaena. Other respiratory signs, like changes to breathing, exercise intolerance or collapse, as well as a lack of other GI signs can help a vet diagnose respiratory-origin melaena.   

Many of the same diseases listed for the oral cavity apply to the respiratory system as well, including inhaled foreign bodies and cancers (there are many cancers that spread from other organs to the lungs). Dogs can also cough up blood due to trauma to the chest such as following a collision with a car, lungworm or due to chronic bronchitis.  

Oesophageal Bleeding 

Bleeding from the oesophagus will inevitably leak into the lower GI tract, become digested and result in melaena. As before, foreign bodies and tumours are likely causes if they cause oesophagitis (inflammation of the oesophagus) or bleed themselves. Oesophagitis and bleeding can also occur due to damage from swallowing caustic substances like acid-batteries, or due to vomiting. Some dogs are prone to vomiting/regurgitation as a result of body shape, congenital deformities or other chronic diseases. Brachycephalic dogs, like Pugs and French bulldogs are especially prone to regurgitation.   

Stomach Bleeding 

There are many disorders of the stomach that result in bleeding – severe bleeds will cause melaena. Gastritis, or stomach inflammation, is very common as a consequence of dietary indiscretion, parasites, toxins, viral or bacterial infections, foreign body irritation and cancer. This inflammation can lead to bleeding.  

Stomach ulcers are also not uncommon in dogs, and can be linked to damage to the stomach, overuse of NSAIDs or corticosteroids, and a number of other diseases – many ulcers require the use of antacids to suppress acid production. Non-gastric diseases, such as kidney, liver or adrenal gland diseases can lead to vomiting, gastritis and stomach ulceration, with associated melaena.  

Intestinal Bleeding 

Conditions of the small and large intestines are very common causes of bloody poo in dogs. Remember that bleeding from the small intestine will result in black melaena, while bleeding from the large intestine causes fresh red haematochezia. As expected, many of the conditions listed above for the rest of the GI tract can also cause bleeding in the intestines, such as foreign body blockages, inflammation (enteritis) and cancers like lymphoma. Conditions that cause constipation, such as dehydration, can lead to straining and bleeding too.  

Two important diseases owners should be aware of are Parvovirus infections, and haemorrhagic gastroenteritis (HGE). Though HGE tends to be less severe than Parvovirus, both result in severe bloody diarrhoea, dehydration, weight loss and even death if untreated – young puppies are especially vulnerable. In many cases, dogs must be hospitalised for supportive treatment to help their bodies recover. Thankfully, the more serious Parvovirus is rare due to an effective vaccine all dogs should receive regularly. Other intestinal infections, such as Campylobacter, Salmonella, Coronavirus and more can also cause bleeding, but often are not as severe.  

Finally, in some cases dogs can have chronic enteritis that results in bleeding. Sometimes these symptoms wax and wane, meaning often a dog seems normal between bouts of diarrhoea and bloody stools. Some of the most common causes are inflammatory bowel disease, hypoadrenocorticism (Addison’s disease), food allergies or hypersensitivities, or Giardia parasitic infection.  

Anal Bleeding 

As you may have guessed, most of the above diseases that cause bleeding elsewhere in the GI tract also cause bleeding from the anus. The only major addition is bleeding from the anal glands – these glands are located either side of the anus, and are responsible for scent marking faeces when a dog toilets. In some cases, these glands can become diseased which leads to bleeding that can smear onto poo. These conditions include sacculitis (inflammation), impaction (blockage), infections and cancer. 

How Do Vets Investigate Bloody Stools? 

Investigating melaena or haematochezia in dogs can be a long process, particularly if the disease is chronic or unusual. Different vets will approach cases in different ways. Here is a general approach that many vets might take in cases of bloody stools. 

First, all vets will start with a thorough history taking 

They’ll be asking questions like how long has your dog been passing blood, is the blood fresh or black, is there diarrhoea or constipation, are they eating normally, and so on. These help a vet narrow down the origin of the blood, and identify the next steps.  

All vets should then perform a thorough clinical examination 

This involves checking your pet from nose to tail, not just the intestinal tract. They will likely assess your dog’s demeanour, dental health, heart and respiratory rate, temperature, abdominal comfort, and more. These, again, help narrow down the organs involved in the disease and any associated symptoms like pain or dehydration.  

They’ll then consider if they have enough information to make a working diagnosis 

At this point, if further investigation is needed, your vet may recommend a number of tests. Basic blood testing is useful to assess your pet’s overall health, as well as if organs like the liver or kidneys are involved in disease. Signs of infection, parasites and inflammation can also be identified with these tests. More specific blood tests that identify increases or decreases in certain vitamins or enzymes can help diagnose conditions like IBD. Some vets might also test for infectious diseases via a blood test as well. Faecal sample is always useful in cases of bloody stools – by looking at poo under a microscope and culturing it, evidence of parasites, bacteria and viruses can be diagnosed.  

Many vets will consider imaging as a next step in cases of bloody stools 

Depending on what they are looking for, this may include radiography/X-ray, ultrasonography, or advanced imaging like CT or MRI. These techniques allow vets to look for changes in the size and shape of organs, identify inflammation or cancer, and check how the guts move in real time. Beyond this, in some cases of chronic disease, vets might want to biopsy the gastrointestinal tract; this can be done via surgery or endoscopically. Some diseases, like IBD, can only be definitively diagnosed in this way. 

Final Thoughts 

As you can hopefully see, there are a huge number of diseases that can cause a dog to have blood in their stools. While there are some very serious diseases that cause bloody stools, and blood should never be completely ignored, mild diseases such as gastroenteritis are much more likely, particularly in otherwise young healthy dogs. Please don’t panic if you spot blood, but do let your vet know as soon as you can.