Causes of clear nasal discharge:
Allergies
The most common reason for a dog to have a ‘runnier-than-usual’ nose is allergic rhinitis. Dogs with allergies may sneeze more than usual, or exhibit reverse-sneezing, but should otherwise be in good spirits. You may notice a seasonal pattern to this kind of runny nose, it may also come alongside other allergic symptoms like itchy eyes or itchy skin.
Airway Irritants
Irritants like second-hand smoke, incense, diffusers and perfumes can all be a cause of rhinitis and sneezing with clear discharge.
Viral Infections
Viral infections like parainfluenza can cause sneezing with clear discharge. This may be accompanied by other symptoms, including coughing.
Causes of ‘snotty’ or purulent nasal discharge:
Foreign Body
While it may seem unlikely, one of the more common causes of nasal discharge in dogs is having a foreign object stuck up there (and no, we’re not talking about crayons or marbles like you might see in another certain species).
Common culprits are things like blades of grass or plant awns. Discharge may be clear to start, but often progresses to thicker or purulent discharge. Excessive sneezing is a classic concurrent symptom as they try to expel the object.
Bacterial Infection
Primary bacterial nasal infections are uncommon in dogs, but sometimes we will see green snotty discharge in the nose of dogs suffering from ‘kennel cough’ (otherwise known as Canine Infectious Respiratory Disease Complex; most hospitals offer a Bordetella vaccine that prevents this condition) or other upper respiratory infections. This is especially common in brachycephalic breeds like bulldogs and pugs.
Tooth Root Abscess
An infected tooth root can present as one-sided, nasal discharge in dogs. Other symptoms include swelling on that side of the face and decreased appetite or drooling.
Fungal Infection
A surprisingly common cause of nasal discharge in dogs is fungal infections. A fungus called Aspergillus is the most common culprit; it causes chronic thick discharge that is both mucoid and purulent (green). Some dogs will have bloody discharge. This discharge can be unilateral (only one nostril) or bilateral (both nostrils).
Causes of bloody nasal discharge:
Trauma
As one might expect, sufficient trauma to the nose or muzzle can cause nose bleeds in dogs just like it does in humans. This is a relatively uncommon occurrence.
Tumor
Unfortunately, tumors are one of the most common causes of bloody nasal discharge in dogs. This discharge can be blood tinged or frank blood like in a nose bleed.
Dogs with nasal tumors will often show other signs like excessive sneezing. It is most common in older, dolichocephalic (long-nosed) dogs like collies. Nasal tumors can also cause clear or mucoid discharge. The discharge is most commonly unilateral (one-sided).
Clotting Disorder
Disorders that affect the ability of blood to clot can present with nose bleeds as a symptom. These bleeds can be dramatic in some cases. Examples of clotting disorders include hemophilia and von Willebrand’s disease.
Certain rat poisons will also inhibit clotting factors and can lead to major bleeds including from the nose. This type of bleeding will affect both nostrils. In these cases you may also see bleeding from other parts of the body.
Parasitic Infestation
Nasal mites are a fairly uncommon cause of nasal discharge, but are seen more commonly in certain regions (like Scandinavia). Nasal discharge, sneezing, and pawing at the face and nose are the most common symptoms.
