I'm going through this now as well. Started Friday. I came out today (Saturday) since I had heard second hand that my horse was coughing Friday night, to see how he sounded and felt. Temp is normal, eating normal, bright eyed and silly as always.... however; anything faster than a walk, and he's coughing and sneezing all over the place. Green/yellow mucus running out his nose.
I called my vet right away and she wants me to give him 72 hours on the SMZ's. If he's not significantly better within 72 hours, then vet will want to come out and be more aggressive. Hoping it's not something too serious.