The term "stable cough" refers to a syndrome where horses start coughing when they're confined to a barn. The problem isn't related to respiratory infections, although it does put the horse at a higher risk of developing one. Nasal discharges from your horse are due to stable cough and characterized as more thin or frothy, as opposed to the thick, discolored discharges associated with an actual infection. The stable cough is dry and hacking.
More significant than the cough is why the horse coughs. The cough is caused by airway spasm and inflammation called recurrent airway obstruction (RAO). The terms "heaves" and "stable cough" may be used interchangeably or heaves reserved for the most severe cases. Once irreversible injury to the lungs has occurred, the changes begin to resemble chronic obstructive pulmonary disease in people (COPD).
The most common trigger of RAO is mold found in straw and hay. Even hay and straw that looks clean and smells sweet may contain some level of mold. Environmental molds growing on untreated wood or concrete areas may also contribute. Although this sounds like an allergy, a Swiss study found no increase in IgE, the allergy immunoglobin, in lung tissue samples from horses with RAO. Other studies have found increased levels of IgE specific for common molds.
All the studies describe increased numbers of mast cells, which can release histamine and are involved in tissue remodeling and scarring, as well as infiltration with lymphocytes and neutrophils suggesting that RAO is primarily an inflammatory problem. The airway irritation in RAO can also be worsened by ammonia from decomposing urine, dust/mites and any small particles suspending in the air.
Some people dismiss stable cough as a minor problem, as long as the horse isn't running a fever or acting sick. This is a mistake. Left uncorrected, RAO will progressively worsen and eventually cause permanent lung damage. RAO also has been found to predispose high performance horses to lung bleeding - EIPH (exercise induced pulmonary hemorrhage).