Q: My horse had a virus over a year ago and has fully recovered, but every so often, he’ll cough as we warm up. I soak his hay, his bedding is not dusty and he is turned out for eight hours a day, so any idea why he is coughing?
Deidre Carson answers:
It is difficult to simplify the reasons why horses cough. In the normal airway, there are cells which ‘clean up’ any foreign material and others which produce the normal secretions that keep the airway moist. There are also very fine ‘hairs’ on the cells that help to sweep any normal and abnormal material upwards towards the pharynx (throat) where it would usually then be swallowed.
Any condition which increases or alters the production of the normal secretions (particularly mucus) stimulates an increased number of cells in the airway or damages the clearance mechanisms can result in coughing. For example, a viral or bacterial infection will damage the normal tissues, increase the amount of mucus, increase the number of inflammatory cells, and may reduce the ability of the airway to clear the increased debris from all levels of the lung and trachea.
This stimulates the cough reflex as the body attempts to remove the irritating material. When the infection has cleared, the airway can remain sensitive to inhaled material, such as dust or fungal spores even when the horse does not have an allergy. This sensitivity can persist for many months if the infection was severe.
Similarly, an allergy stimulates an inflammatory response with increased mucus and reactive cells. When the allergy is treated, by removing the source of the allergen (substance causing the allergy), and reducing the allergic response with medication, the airway remains poised to react to any slight insult in the future. This can make the airway very sensitive to even the tiniest amount of the allergen. Other less likely causes of your horses cough could be a heart condition or (even more unlikely) lungworm, which occurs in horses kept with donkeys.
As your horse coughs only infrequently and occasionally, I would not be too concerned, but it is advisable to continue with ‘dust-free’ management as much as you can. If the frequency or nature of the cough changes, you should ask you vet to investigate. A tracheal wash can be performed to look at the cell number and types present in the larger airways, and do determine the level of mucus present. This information can be used to help with a diagnosis and treatment choice.