Constant sneezing in dogs is a sign of nasal irritation, which can indicate the onset of an infection or just be the result of a multitude of scents in the air, according to WebMD. However, if mucus accompanies violent sneezing, the dog needs to be checked out by a veterinarian. Constant sneezing may also indicate a tumor or parasites in the nasal passage or a foreign substance in the eye.
WebMD states that constant sneezing in a dog can indicate some sort of respiratory infection. If this sneezing is accompanied by other discharge, such as coughing, fever or mucus from both the nose and the eyes, then the dog could have canine influenza.
Another possible reason for constant sneezing is the presence of a foreign substance in the dog's nose. If this is the case, co-existing symptoms may include a bloody nose and head shaking. WebMD advises treating any nosebleeds by applying cold and pressure to the nose, then consulting a veterinarian.
PetMD points out that constant sneezing in young dogs can sometimes be attributed to a cleft palate or bronchial infections. In older dogs, dental disease is also a likely culprit.