Generally put, a dog should not be leaking anything from anywhere. Tell that to the parents of the droolers, right? Drooling, within what is normal for your dog, is an exception.
A dog’s nose is usually moist–for a reason.
Dog noses secrete a thin layer of mucus from membranes within the nose which helps to absorb scent chemicals.
On top of that, besides their footpads, dogs also secrete sweat from the surface of their noses. And the base of the nose is the drainage for superfluous tears.
There are plenty of natural reasons for a wet nose.
What about a runny nose, though?
Like any other cavities exposed to the outer world, the nasal passages are lined with mucosa. I explained what mucus does in the article on mucus in the stool. The purpose of the mucus is to trap foreign particles and infectious agents; it makes me think of the Cinderella story where the prince had put glue on the steps so she couldn’t escape.
A runny nose then is generally a result of increased production of mucus.
Unless it is chronic or comes with other symptoms to hint you might be having a problem on your hands, a small amount of clear discharge is usually not too worrisome.
A dog’s nose will temporarily get runny for many of the same reasons ours do–exposure to dust, high winds, strong fragrances–anything irritating to delicate nasal tissues. If my dog’s nose kept constantly running, though, I would not dismiss that.
Cloudy, smelly, yellow or green discharge is a different story.
I have seasonal allergies, and my nose can run like a faucet. In dogs, though, allergies usually present themselves through skin problems. I would be very slow to dismiss my dog’s nasal discharge due to allergies. It does happen, but it’s not as common as you’d think. Infection is a much more likely culprit, mainly if the discharge is anything but clear and odorless.
Nasal infections can be caused by bacteria, fungi, and/or viruses. For example, viral disease distemper can cause sticky, yellow nasal discharge.
Dental disease and a runny nose?
You won’t find it so weird once you think about it. Infections can spread. Just because it starts at the gums doesn’t mean it will stay there. These bacteria can spread into the nasal cavities and even systemically, such as to the heart.
Other potential causes of nasal discharge include mites, foreign objects, polyps, pneumonia, immune-mediated disorders, trauma, and tumors. Problems outside the respiratory system that can lead to a runny nose include swallowing disorders, facial nerve damage, or a digestive tract disease.
When you see pus, make a fuss.
If your dog’s nasal discharge is cloudy, yellow, green, smelly, or with blood in it. Do see a vet. Particularly with nose bleeds, I would be highly concerned about nasal cancer or a severe infection.
If the discharge is clear and doesn’t smell, I would see a vet if it was excessive or chronic.