The horse was slightly allergic when we bought him .
We did a lot of work keeping the stables dust free , excellent bedding , never mucked out when horse was in stables small bale haylage etc etc but horse still had slight persitant cough.
Put him on the ventilate , cough stopped we kept him on it all the time then in time we stopped it and gave it only from November to march.
I'm at the end of my first tub now & ordered a second. Started him on it early in an attempt to keep him in work this summer as he has a pollen allergy He ususally gets knocked the end of March or early April so will see. Definately doing something though as hes got alot of clear/white snot after work so its pushing something out.