You have not mentioned whether your horse has any other symptoms which are common for your horse to experience alongside a cough, such as nasal discharge, watery eyes and fever, so let’s assume your horse is not suffering from these. In simple terms, there are two kinds of coughs in horses – coughing at rest, and exercise-induced cough. While it sounds like your horse has an exercise-induced cough, it means that your horse must not ever cough, or cough only rarely in the stable.

Heaves (Chronic Obstructive Pulmonary Disease) is a respiratory disease which often results in decreased exercise tolerance, difficulty breathing and abnormal sounds (from where it’s common name comes from). These symptoms are as a result of narrowing and inflammation of the airways of the lungs, which is usually caused by mold (fungus) or dust. Thus, in nearly all cases a change of environment will prevent these symptoms.

Heaves can be confused with other lung disorders, such as pneumonia (parasitic or bacterial), cancer, diaphragmatic hernia and others. However, in nearly all other causes of the same symptoms the horse will have a much more marked change in behavior, such as depression, loss of appetite or extreme pain. In addition, horses who have Heaves often have nasal discharge, sometimes only evident when they are bowing their head – such as when eating, and you can often hear a wheezing sound when you place your ear close to their nose.

It is very rare for horses to cough during exercise, even though some might display a short cough at the start of exercise, and it is generally a sign of a health problem. It is imperative that your horse has a thorough veterinary examination to identify the true cause of the coughing, including a radiograph and blood count.