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Coughing is an important problem in horses and is an sign of lower airway disease. It may be caused by a variety  of infectious or environmental factors. Racehorses differ from the general horse population in a number of ways which may be relevant. In racing Thoroughbreds, coughing is often associated with inflammatory airway disease - a syndrome characterised by  mucopurulent tracheal discharge containing neutrophils.

A recent study by Dr Christley and his colleagues examined whether coughing was a useful indicator of lower airway inflammation in racing Thoroughbreds. They also aimed to evaluate  a number of potential risk factors that might influence coughing in that group of horses.
Coughing in Thoroughbred racehorses
The study was based in training yards around Sydney, Australia. They compared one hundred coughing horses with 148 control animals. Upper and lower airways were examined endoscopically. Tracheal fluids were collected. The  examination was scored for naso-pharyngeal mucus, pharyngeal lymphoid hyperplasia and tracheal mucus. Tracheal fluids were collected.  Smears were made and the cells present were classified   ( alveolar macrophages, haemosiderophages, lymphocytes, neutrophils and giant cells).

A questionnaire was completed for each horse in the study, recording details of age, stage of training, the time since it was last transported, and the time since it had last raced.

They found that:

 there was a greater risk  of coughing occurring in younger horses. "The decreased risk with increasing age suggests that horses develop immunity or tolerance to some of the causative agents, or that some component causes are more prevalent in young horses" said Christley.

 the risk of coughing decreased as training progressed suggesting that early training is a period of higher risk. This applied even to horses that had been trained before. They suggested that the reason for this might be that any immunity that the horses had acquired previously had been short lived. It might also be that there is a loss of tolerance to inhaled irritants following a period without being exposed. Another suggestion was that exercise training actually enhances immunity.

 coughing was a relatively specific indicator of lower airway inflammation.
Written by Mark Andrews.

Inflammatory airway disease

For more details see: Coughing in Thoroughbred Racehorses: risk factors and tracheal endoscopic and cytological findings. R.M Christley, D.R. Hodgson, R.J. Rose, J.L. Hodgson, J.L.N. Wood and S.W.J. Reid.  
Veterinary Record, (2001) 148, 99-104