Blunt trauma from the outside or injuries inside the nose can cause bleeding through the nostrils.
Bloody discharge can appear from both nostrils. Traumas usually produce pure blood but bloody tinged mucous is also possible. If there are no severe injuries or fractures to the skull, bleeding usually stops by itself before long.
Dogs can suffer injuries through accidents or trauma to the head. Fights with dogs or other animals are also common sources of trauma to the nose. Younger and active dogs are more frequently injured. Certain infectious diseases, i.e. borrheliosis or ehrlichiosis can result in nose bleeds.
Diagnosis is based on the animal's clinical history. In order to rule out other causes, an x-ray of the nasal cavity and the skull should be taken.
Discharged blood can be dabbed off with a small towel. In order to stop bleeding, a towel can also be damped in cold water and pressed gently across the back of the nose. If you are suspecting an accident of some kind, please consult a veterinary clinic.