Infection in the maxillary sinus occurs frequently in the horse. It is possible to have primary maxillary sinusitis often caused by Streptococcus equi, however the most common cause of sinus infection is extension of infection from a diseased tooth or as an aftermath to tooth repulsion. The involved tooth can be a split tooth, an open infundibulum caused by a congenital defect, and in older horses, paradontal disease.