Dogs can sneeze for a number of reasons, including allergies, infections and foreign bodies. Older dogs may also sneeze if they have a tumor growth somewhere in their nasal cavity.
Allergies may cause a dog to sneeze if they are allergic to grass or tree pollen. Sneezing in dogs during allergy season is usually limited, although the dog may display watering eyes, chewing on his feet and scratching his sides. It's possible but not common for a dog to be allergic to rug fibers or cigarette smoke.
Infections can also induce sneezing in dogs, including infections of the nose or sinuses, or an infection from a tooth or its root that drains into the nose. Sneezing can also be caused by a cold virus; dogs can have that condition at any time of the year. Some cases can turn into a long-term chronic condition, usually accompanied by a blood discharge from the nose.
If a dog ever has a bloody nose or the sneezing is causing the dog to be uncomfortable, the owner should seek treatment with a veterinarian. A bloody nose in a dog accompanied by sneezing may be caused by a tumor, especially if the dog is older.