Recurrent airway obstruction (RAO) or “heaves”, the equine equivalent of human asthma, is a common cause of poor performance and a significant welfare concern.
Severely affected animals show signs such as dilated nostrils, nasal discharge, and excessive chest movement. A "heave" line may be present. They may be incapable of any athletic activity. Mild cases may appear to breathe normally and require further investigation to confirm the diagnosis. However, most cases fall somewhere between the two extremes.
The signs are caused by a combination of increased mucus production in the respiratory tract and spasm of the muscles surrounding the airways. This results in narrowing of the airways, making it more difficult for the horse to breathe.
Recent research, funded by the Horserace Betting Levy Board, has been looking at the properties of mucus and the sites of its production in the lungs of horses with RAO.
An essential component of mucus is the mucins – high molecular weight proteins, produced by specialised cells such as those lining the respiratory tract. Their most important characteristic is that they can form gels, and so provide lubrication, transport and a physical barrier.
It was these mucins that formed the subject of this study. A summary of the work, carried out by Adele Williams as part of her PhD studies at the University of Manchester, has been published online.
Dr Williams found no difference in composition of the mucus between healthy and RAO-affected horses; the RAO cases just had more of it. Neither did she find any difference in mucin properties between horses housed inside or maintained at pasture.
The research showed that mucins were produced in cells lining the airways and in deeper glands. There was great variation in the size and number of these cells in both normal and RAO-affected horses. However, the mucin-producing cells in the deeper glands were larger in horses with RAO than in healthy control animals.
In the future it may be possible to use cell cultures to study mucus production in the laboratory, and to investigate ways of modifying mucus production. As part of the current project, mucin-producing cells were grown successfully in the laboratory for a short time. However there were problems with bacterial and fungal overgrowth. The technique needs to be refined before it can be used for analysing and modifying mucus production in the search for a treatment for mucus overproduction in horses with recurrent airway obstruction.