Some of these obstructions are quite subtle and the horse must be running very hard before the problems are apparent. It is a very exciting area of investigation. In the lower airway I can summarise by saying that in horses as in humans we've come to the realisation that most lower airway obstructions are a result of inflammation. This inflammation is frequently environmental in origin or environmental coupled with a viral infection. Most investigations up to date have been on ways to develop bronchodilators as methods of treatment, the next phase will be the development of better anti-inflammatory drugs for treatment of airway disease.
Bronchodilators effectively relieve bronchospasm.
Mucolytics are widely used but we don't know their efficacy or even if there's an advantage to using them. As for antibiotics, we don't know the incidence of bacterial infection or even its importance in horses with airway disease. In a mature horse, bacterial infection is not a terribly important cause of bronchial disease. In fact in all species, the bronchi seem to be rather resistant to bacterial infection. The big difference between Australia and and the northern hemisphere is in the incidence in non-infectious respiratory disease.