Unlike the heart, or other muscles, equine lung function does not improve with training. This means that a horse’s breathing — his respiratory system — is always a limiting factor in his performance.
Equine asthma (EA), in a mild to moderate form, can compromise lung function and, in turn, affect athletic capacity.
The condition causes airway inflammation and mucus accumulation, which increases the resistance to airflow in the horse’s lungs. As a result, less air is inhaled with each breath and more energy is used to move the muscles of the chest. This impairs the exchange of oxygen and carbon dioxide — meaning that less oxygen-rich blood is delivered to the muscles to power the body during hard or fast exercise.
The more severe the airway inflammation, the greater the reduction in performance. Its significance will also depend on activity, because muscles demand less oxygen for lower intensity exercise.
Up to 80% of thoroughbred racehorses have airway cytology (the study of cells) evidence of mild-moderate EA, with increasing inflammatory cell numbers associated with reduced racing speed. The number of stabled sport horses affected may be even higher, although the association with athletic function is less clear and more individual.
As many as three-quarters of horses presented for high-speed treadmill investigation of poor performance have EA in its mild-moderate form.
Subtle signs
Aside from performance issues, there may be little obvious evidence of mild-moderate EA.
Most of the increased mucus transported up the trachea (windpipe) is swallowed, so only a small amount may be coughed up or appear as nasal discharge. Fewer than half of horses affected display a chronic intermittent cough, which is more common at the beginning of exercise.
Any signs of increased respiratory effort at rest would suggest the presence of severe EA. An exaggerated breathing effort during exercise may be apparent, however, or a prolonged increased breathing rate afterwards.
One prominent feature of mild-moderate EA is airway hyper-responsiveness — the development of bronchoconstriction (narrowed airways) and a cough during exposure to an allergenic environment, which in a normal horse would have no effect. The condition most commonly occurs in a stable setting, although a summer version also exists.
Typically, clinical signs will have been present for at least four weeks. A more variable history can also occur, however, with spontaneous improvement and worsening that reflects variation in environmental stimuli. Physical examination and bloodwork should be carried out to exclude any evidence of systemic infection.
The main diagnostic test for human asthma is forced exhalation — known as peak flow. After taking a deep breath in, you blow as fast as you can into a peak flow meter. This is repeated frequently and the results are interpreted based upon your age, gender and height.
With horses, it is impossible to confirm a diagnosis of mild-moderate EA using peak flow measurement outside of a research environment. Instead, a two-stage approach is used: airway endoscopy, to visualise and grade tracheal mucus, followed by fluid sampling from the trachea (a tracheal wash) or the lower airways (bronchoalveolar lavage).
Healthy horses have either no visible mucus and are graded 0/5, or only a few isolated specks and are graded 1/5. Poor performance can occur with a tracheal mucus score of 2/5 in racehorses and 3/5 in sport horses.
The fluid from the tracheal wash or bronchoalveolar lavage is then analysed for specific inflammatory cells, namely neutrophils, eosinophils and mast cells. In mild-moderate EA, bronchoalveolar lavage is the recommended sample: a neutrophil percentage of more than 6% in racehorses or 10% in sport horses is sufficient to affect performance.
Positive prognosis
Medical treatment revolves around control of airway inflammation using corticosteroid anti-inflammatory drugs. Alongside this, environmental modification is essential. Giving the horse corticosteroid therapy while keeping him in a dusty atmosphere will lead to improved clinical signs and lung function, but airway inflammation will persist and clinical disease will recur at the end of treatment.
In mild-moderate EA there is little bronchoconstriction at rest, so the routine use of bronchodilators (medicines to make breathing easier) is typically not necessary — although they may contribute to improved exercise tolerance during the early stages of corticosteroid treatment.
The most suitable treatment will be based upon clinical signs, exercise tolerance and the results of airway endoscopy and airway fluid sampling. The efficacy of this plan should be monitored by repeat airway fluid sampling, at a suitable interval.
The prognosis is excellent with appropriate treatment and a low-dust environment, since airway remodelling (structural change) does not seem to be a component of the condition. Mild moderate asthma is a reversible condition, but can recur if exposure to allergens recurs. However, progression to severe EA has not been shown.
Ref Horse & Hound; 26 September 2019
