Blood in the stool means that blood has entered the digestive system somewhere and this is not normal. There are many reasons why we might find blood in a pet's stool and a range of ways it might appear. Also the significance of its appearance varies, so we will stick here to the important principles to be considered when a pet is affected.

Blood is not always obvious in the stool as it does not always appear red as we would expect.

Black or very dark tarry stools

Blood that enters the digestive system from the oesophagus (gullet) stomach or upper small intestine will be acted upon by the digestive enzymes (juices) and this turns red blood black.

Very dark or black (often tarry) stools can therefore indicate bleeding in the upper digestive tract. It is worth noting that this may also be caused by swallowing blood, for instance from licking a bleeding wound, or swallowing blood from a mouth injury or tooth problem.

We call this black stool “melena”.

If you suspect your pet has altered or dark blood in its stool then you should seek advice from your veterinary surgeon as soon as reasonably possible. 

Any blood in the stool associated with weakness or pale membranes should be treated as an emergency.

Most often we see blood in the stool as red. It may vary from small flecks and clots to a watery diarrhoea that seems to be almost all blood known as haemorrhagic diarrhoea.

Red blood in the stools will generally have originated in the lower part of the digestive system as it has not been subject to the digestive enzymes. 

In pets and in particular in dogs, blood in the stool is most commonly associated with conditions that will pass or are relatively easily resolved.

Colitis

This is a common finding in dogs and typically shows as diarrhoea with increased frequency and quite often unproductive straining between episodes of diarrhoea. The stool is often accompanied by mucus and often spots or streaks of blood. This is probably the most common situation in which we see blood in the stool. Similar symptoms are also seen in cats.

Every case of colitis can be a little different but is often associated with a change of diet or  unusual treat. Colitis in dogs can often be triggered by anything oily or fatty that they aren't accustomed to, but anything unusual that they are given, or find themselves, can trigger a diarrhoea that includes some blood in the stool.

Any time blood is seen in the stool either red or black you should seek veterinary advice, the majority of cases are mild and will pass or are easily managed but there are cases where it may be part of a more serious problem. You are advised to have your vet do a physical examination at the first reasonable opportunity if your pet also appears unwell, fails to respond to home care or you feel the condition is deteriorating.

If the bloody stool is associated with any other signs, for instance, weakness, vomiting, lethargy, unwillingness to eat then you should treat that with greater urgency.

Haemorrhagic diarrhoea

This can be very dramatic, pets may produce large quantities of very watery bloody diarrhoea very suddenly and this can weaken them rapidly and lead to dehydration and anaemia in a short space of time. Cases like this should be treated as an emergency.

The most serious causes of bloody diarrhoea, usually seen in very young animals (though not always) are caused by very nasty viruses.

Parvo virus in dogs and feline enteritis in cats.

These are disease we vaccinate puppies and kittens against, it is critical that puppies and kittens get their vaccinations at the earliest opportunity and you should be planning for this and checking which vaccinations, if any, have been given to the pet whilst still with the breeder and seeing a vaccination record to prove this.

Any bloody diarrhea in a puppy or kitten should be treated as an emergency if they appear remotely unwell.