Noticing that your dog has bloody diarrhoea can be alarming. However, bloody diarrhoea does not always indicate a serious health problem. In fact visible blood in the stool is often less of a concern than "invisible" blood, which makes the stool turn black. There are many causes for bloody diarrhoea. It should always be immediately checked by your vet, but it may not be serious.
Two types of blood may be seen in the stool. The most obvious is bright red blood, often in diarrhoea or mixed with mucous. This is called hematochezia. The visible blood usually originates in the lower intestines, usually from parasites or a viral infection. Melena is blood that originates in the stomach and upper intestines. Because it has been digested, it colours the stool black, and can look like tar. Note that some foods, like liver, can also turn the stool black and tarry and is not necessarily anything to worry about.
However, the presence of any blood or suspected blood in the dog's droppings warrants an immediate vet visit.
Puppies and Parvovirus.
Parvovirus (parvoviral enteritis) is a highly fatal, very contagious viral disease. Dogs of any age can succumb to parvo, but it's most common in puppies. Many adult dogs have built an immunity, either from vaccinations or from mild exposure. Because parvo can live in the environment for months, puppies should be vaccinated against it.
One of the classic signs of parvovirus is bloody, extremely smelly diarrhoea. It is usually accompanied by lethargy, fever and an unwillingness to eat or drink. This is a medical emergency, especially for puppies, who can die quickly without treatment.
Dogs are often not very smart about what they eat--many vets call this "garbage gut." Luckily, dogs often vomit with ease and can hork up some foreign substances and inappropriate objects before they travel further down the gastrointestinal tract. If they don't, swallowed objects, harmful foods and rat poison may irritate or lacerate the stomach or intestines to the point of bleeding and bloody diarrhoea.
Never induce vomiting in a dog without first getting advice from a vet. Sharp or caustic things can do more damage coming back up. Treatment may range from medication to improving intestinal motility and soothing mucous membranes, to surgery to remove an obstruction.
Intestinal parasites, from hookworms to coccidia, can cause bloody diarrhoea. Again these are more common in puppies. Take a fecal sample and the dog to the vet. The sample can be checked for parasites, and the treatment is usually as simple as a single dose of wormer.
There are several, thankfully less common, causes for bloody diarrhoea. Bacterial infections such as salmonella, hemorrhagic gastroenteritis and E. coli will usually cause blood in the stool. Internal bleeding may be due to cancerous tumours in the spleen or stomach. Ulcerative colitis is a chronic condition, causing stomach upset and diarrhoea.
Treatment will depend on the condition. Never give your dog a human antidiarrhoeal medication without first asking your veterinarian. Some human medications are harmful to dogs, and if the diarrhoea is the body's attempt to rid itself of bacteria or a virus, preventing the diarrhoea can make the dog worse, not better.