Its is a respiratory disease im affraid......none curable but very preventable and to some extent managable. Its usually found in badly ventilated barns where dusty conditions agrivate the horses wind pipe and lungs, and cause an alergic reaction (similar to asthma I suppose?) Bad mouldy/dusty hay is also a culprit although the exact cause isnt proven clinically.
You may have heard of its technical term COPD?? That is more regularly recognised as heaves is an old fashioned term that many don't recognise.
Heaves or COPD is definitely manageable depending on the severity of it. We have a horse who has COPD. Although he can no longer run barrels or do events he is highly usable. My daughter uses him 1-2 days a week (10 hour days) for her Mounted Patrol job. (She uses another horse the rest of the week and tries not to use him 2 days in a row) When we first got him, he was really bad. He was on steroids for about 6 months. (We didn't know he had it when we bought him) We water down his hay to cut down on dust. He is not stalled if at all possible. We limit any loping on him (although he still loves to do it.) He hasn't been on meds for over 3 years now. Basically we keep a close eye on him and if he is displaying any symptoms he gets a short "vacation" out in the pasture. If the symptoms get worse (they haven't) then he goes to the vet. One other thing. It can be harder to keep weight on a horse with COPD. Although he should be a bit thinner anyway to make it easier on him. You should just barely be able to see his ribs.