Built within the skull are bone encapsulated air pockets known as sinuses. The sinuses moisten inhaled air and trap particles within secreted mucous. Horses with sinus discomfort develop a nasal discharge on the affected side. Sinus problems occur with bacterial and viral infections (e.g.: Equine Herpesvirus), allergies, cysts, trauma, and tooth problems; bleeding from the nasal passages suggests an ethmoid hematoma.