Horses have evolved to be free roaming grazing animals. Given the option to live this way they will spend about three quarters of their time with their heads down and will be continually on the move albeit slowly, as they search for the tastiest grass.
As a result of this evolutionary fact, the respiratory system of the horse is relatively inefficient at clearing pathogens and irritants away. As particles simply “drain” out under gravity with the head down, nature has not concentrated so much on the mucus clearing mechanism of fine hairs lining the airway that are enjoyed by other species.
Most horse owners in the modern world do not have the facilities or the time to allow their horse access to the plains and so we keep them in stables for at least part of the time, usually more in winter due to ground conditions. Unless the horses are really lucky they have relatively restricted grazing areas compared with their wild cousins. With the ever increasing value of competition horses it has become usual to have individual turn out paddocks to avoid conflict, but this also has the effect of reducing the amount of space available for an individual horse to move around whilst grazing. Horses roaming free will often jog as a group from one location to the next and even the small increase in respiratory effort involved increases the volume of air exchanged in the lungs.
The equine respiratory tract is susceptible to many infections such as strangles, which affects the upper airways and is similar to a human streptococcal throat but generally more serious. Influenza, herpes and many other non- specific viruses are diseases of the lower airways often with bacteria being involved as secondary invaders. The lower airways are defined as everything below the vocal cords- i.e. the trachea (wind pipe) bronchi and the lungs. When acute infection occurs as in influenza, the horse will usually have a raised temperature but whatever the cause, lower airway disease will manifest as increased nasal discharge, cough, exercise intolerance and increased respiratory effort. Whilst lower airway disease may be initially caused by infectious agents it is almost inevitably aggravated by the environment and on many occasions the environment is the only cause. The symptoms are due to increased mucus production and spasm in the smaller airways and are caused either by infectious agents or irritants and allergens in the horse’s environment. When symptoms persist for more than about a week or so the condition is described in medical terms as being chronic and the majority of respiratory conditions fall into this category.
“ Broken wind ” “Heaves” “COPD”(Chronic obstructive pulmonary disease) “RAD” (Recurrent airway Disease) and many other titles have been awarded to chronic equine airway disease, but recent research has identified two types which can be inter related: Inflammatory airway disease (IAD) , the body’s response to irritants in the environment and Recurrent Airway Obstruction (RAO) which it is now called Equine Asthma and is the result of an allergic response. The latter condition is defined as recurrent disease with clinical signs of airway obstruction which can be partially reversed by bronchodilators.
In both cases the clinical symptoms are the same; reduced exercise tolerance with longer recovery time, increased respiratory effort, whitish discharge and cough. The definitive diagnosis can be made by your veterinary surgeon from a tracheal wash which samples the mucus as does the more specialised technique of bronchoalveolar lavage. A sample of the cells is sent for laboratory analysis. It is clear that what starts as an inflammatory condition can easily develop into an allergic one as the horse become sensitised. Many horses will develop chronic airway disease following minor respiratory infections when the airways are already compromised. It makes sense therefore to be aware of the risks and take adequate precautions with respiratory health. I always would assume that any respiratory condition is likely to turn allergic. Prevention is better than cure!
Equine asthma is obviously the more severe of the two scenarios, but IAD can develop into equine asthma and there is clearly a degree of overlap between the two conditions. However inflammatory airway disease can and does exist in a sub clinical form where none of the obvious respiratory symptoms are shown.