Nasal discharge is not an uncommon finding in horses, and most of the time it is due to an infection somewhere in the respiratory system.  However, if the nasal discharge does not respond to treatment with antibiotics and continues for a period of time, it is possible the infection could be in one of the sinus cavities, especially if the discharge is from only one nostril.  And even though the sinuses are also part of the respiratory system, sinus disease can be a difficult problem to diagnose and treat. 

Sinus infection can develop secondary to a bacterial infection of the respiratory tract and although antibiotics can decrease the drainage, it generally will not resolve the problem.  In most cases, the sinus must be opened surgically and flushed with sterile fluid to cure the infection. Sinus infections can also occur because of tooth root infections.  The roots of the cheek teeth in the back of the horse’s mouth are directly adjacent to the sinus cavities, and infection of these teeth can cause the sinus to become infected.  In most cases to treat the infection it is necessary to remove the infected tooth, which can require general anesthesia and opening the sinus.  There are also masses that can develop in the nasal cavity called ethmoidal hematomas that can affect the sinus, as can cancerous masses.  Another cause of nasal discharge from a sinus is a sinus cyst, a cystic structure that produces a large amount of fluid in the sinus but is not infected.  Treatment of a sinus cyst involves removing a piece of bone over the sinus and dissecting out the cyst.  If you have a horse who has a nasal discharge for a long period of time, a sinus problem is the most likely source.