First let me say I am not a vet and I am not giving medical advise.
I have noticed a lot of threads about horses with coughs, runny noses, and all around allergy/cold symptoms.
I tried Antihist for a couple months, a little improvement but not enough. Switched to Cough Free and her cough is mostly gone. Nose not as runny.
So if there is no fever to alert infection, it's worth trying. Smells horrid, so do not smell it, it may need to be mixed in feed with applesauce or such for awhile. This stuff is awesome though.
Farnam - Your partner in horse care
$14 at fleet farm for a 32 day supply. Once again I'm not saying it will cure anything but it worked wonders for me and could be worth a try.