Sneezing in dogs most commonly occurs due to the inhalation of some irritant of the upper airway, usually dust, pollen or powders, and lasts until the nasal passages are clear again. However, sneezing can also be symptomatic of infectious diseases, obstructions or allergies, as stated by Vetstreet.
Though allergic reactions are more common among humans, dogs can also react badly to pollen or other allergens and start sneezing uncontrollably, usually due to nasal inflammation, references Vetstreet. Short-nosed breeds like pugs and bulldogs often sneeze and snort in order to clear their upper airways and nasal passageways, the irritation caused by excess tissue. Individual dogs vary in sensitivity to allergens, but a sudden onset of sneezing signals often signals an obstruction of some kind ranging in severity from a foreign body to a tumor.
If sneezing begins slowly and shows no signs of decreasing, Vetstreet cautions that it could be the manifestation of an infectious disease like kennel cough or distemper, or a more serious illness that affects the upper respiratory system. Although sneezing is usually harmless, nasal discharge consisting of mucus, blood or other materials is a serious sign of a more urgent medical condition. Sneezing in conjunction with other symptoms such as shortness of breath, difficulty breathing, poor appetite and behavior changes may also indicate a more severe illness.