A vertical passage or opening between the mouth and nasal cavity is called an oronasal fistula. Dolichocephalic dog breeds are more likely to be affected with this condition, especially the Dachshund. These fistulas are caused by a diseased state of a tooth in the upper jaw. The most common location for an oronasal fistula is where the root of the fourth premolar on the upper jaw enters the palate. The occurrence of such a fistula will cause irritation of the nose, runny nose, inflammation of the sinuses, infection, and possibly pneumonia.