Rhinitis is the term for nasal inflammation in a dog and often causes a runny nose. Most common causes of rhinitis are harmless and don't affect the dog's quality of life. However, some causes are more serious and require attention from a veterinarian.
Allergies are the most common cause of nasal discharge. Your dog may have seasonal allergies, such as those to pollen or grass, or have environmental allergies, such as those to dust or cigarette smoke. Generally, discharge due to allergies is clear in colour.
Respiratory infections also are common causes of runny noses in dogs. The most common respiratory infection is kennel cough. Bacterial, viral or fungal infections often cause yellow or green discharge and need to be treated by a veterinarian.
Dogs explore the world with their noses. Occasionally an object will become lodged in the nose, causing irritation and inflammation. Often the nasal discharge will be mixed with blood from the irritated nasal membranes.
Nasal tumours are rarer than allergies or infections but will cause nasal discharge. Almost always, the discharge will be mixed with blood. A dog with a bloody nose should always be evaluated by a veterinarian.
Dogs who are excited will often have a clear discharge from their nose. This discharge generally goes away quickly and is not a medical concern. However, if your dog's nose continues to run after the excitement has subsided, consider a medical cause.