Ask H&H: autumn cough

My 12-year-old 16hh mare has just started coughing. The cough is deep and rough. She coughs two or three times and then only occasionally thereafter. The same thing happened this time last year and the year before. Each time the vet prescribed ventipulmin and suggested a pollen allergy such as summer pasture associated obstructive pulmonary disease (SPAOPD). The cough usually clears up after the medication but returns the following year — why?

She is turned out 24/7. I had just started putting hay in the field, but it was extremely clean and I dampened it; she has picked at it for two or three weeks with no problems. She has no nasal discharge, is fit and healthy, had her vaccinations last month and eats well.