Why Does My Horse Cough?

The equine athlete and non-athlete are often plagued with coughing, which can inhibit quality of life and reduce performance. Coughing can be sporadic, changing in intensity as the environment changes or it can be continuous, becoming worse or more prominent at the time of exertion as seen during competition.  Some coughs even get worse with certain seasons, becoming progressive over time. So, what makes them cough and what can we do about it?