Dogs sneeze for mostly the same reasons that people sneeze. Sneezing is when air is suddenly expelled through the nasal passages or the mouth so a dog can do this just as you can, even though his mouth and nose are shaped differently. The body is typically trying to get rid of something irritating through the nose or mouth. The irritation can be caused by an allergy, something foreign that shouldn’t be in the nose (for instance, a blade of grass), an infection, or in some cases by a tumor.
Allergies. A dog that sneezes because of an allergy usually has some kind of seasonal allergy. He may have an allergy to pollen or certain plants, for example. Weeds, trees, and other plants can produce their particular pollen for a few weeks and then they are no longer a problem. A dog will sneeze when pollen from what he’s allergic to is in the air. A dog that has this kind of allergy will often show other allergy symptoms such as watering eyes, scratching at his skin, and chewing on his paws. Most dogs remain active and healthy otherwise. They will return to normal once the pollen that bothers them is gone.
Foreign Bodies. Dogs can also sneeze if they have a foreign body in their nasal passage. This happens more frequently than you might think. A dog is much lower to the ground than humans are and dogs use their noses to smell everything. It’s not unusual for a dog to inhale a small object that can get stuck. Dogs can even get objects as big as sticks stuck up their noses. Sneezing is the body’s way of trying to expel the object. If your dog is having sneezing fits and especially if he is producing some blood from his nose when sneezing, he should see a vet. He might have something lodged in his nose that needs to be removed.
Infections. Dogs can get infections that lead to sneezing. Some of them can be serious. For instance, your dog could have an abscessed tooth that drains into his nasal passage. Or a dog can have a sinus infection. These are also infections that directly involve the nose. These infections can become chronic and they can produce pus or a bloody discharge. If your dog is sneezing and he has this kind of discharge, be sure to take him to the vet.
Tumors. Another reason dogs sneeze is if they have a tumor affecting their nasal passages. This sometimes happens in older dogs. Intranasal tumors are not very common in dogs but they can occur. A dog with this kind of tumor will sneeze occasionally and produce a bloody discharge from one size of the nose. The sneezing will become more frequent as the tumor grows. This kind of tumor is often malignant and it is difficult to treat, but there are some methods of treatment.
Any time your dog has a bloody nose you should take him to see a vet. Bloody noses are not uncommon in people but they can be serious with dogs, so let your vet check it.