Mucus in Equine Lower Airway Disease

How Should Excessive Mucus Accumulation in the Airways Be Interpreted?
When Is Excessive Mucus Accumulation Observed? In classical antiquity, respiration was thought to be a cooling mechanism for the blood, and nasal mucus a discharge from the brain. The credit of pointing out the pathophysiological significance of the mucous apparatus to Western medicine belongs to René Théophile Hyacinthe Laennec (1819): Excess respiratory secretions became recognized as one of the cardinal signs and central problems in many respiratory diseases of humans. Healthy respiratory epithelium is covered by a thin (5-20 microns) mucus layer. For the purpose of this paper any (endoscopically) visible mucus accumulation is defined as excessive. Although excessive airway secretions are a hallmark of recurrent airway obstruction (RAO or heaves) (Robinson et al., 1996; Workshop), they are a characteristic but non-specific sign, associated with inflammation in various equine airway diseases of both non-infectious and infectious origin (Dixon et al., 1995). The amount of the observed mucus varies greatly between individuals, and in order to determine its significance a way to reliably quantify respiratory secretions must be found.

How Can the Clinical Significance of Mucous Accumulation Be Assessed? Excessive airway secretions (sputum) and their unfavourable rheological properties are associated with the severity (morbidity and mortality) of human airway diseases. So far, however, there is no definitive solution to the problem of assessment and quantification of “mucus”, and the question of which parameters are clinically relevant and of practical use remains unanswered. Excess secretions predispose humans to pulmonary bacterial infections, leading to exacerbations of cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD). When horses are prevented from lowering their heads for a prolonged period of time, mucus pooling with excessive bacterial growth occurs in the distal trachea (Raidal et al., 1996). This may increase the risk of bacterial pleuro-pneumonia in immunocompromised animals, and there is evidence that inflammatory lower airway disease in young racehorses in the UK is associated with bacterial colonisation (Chapman et al., 2000). Exacerbations associated with bacterial infections are not a clinical problem recognized in RAO-affected horses, however (Robinson et al., 1996; Workshop), even in individuals which have very large accumulations of mucus. Excessive mucus may be a direct cause of bronchial obstruction, however, and can amplify the lumen-narrowing effect of bronchoconstriction. In RAO, maximal bronchodilation has an inconsistent effect on compliance (Robinson et al., 1996), and resolves increased resistance (Broadstone et al., 1988) and uneven ventilatory distribution only partially (Rush et al., 1999). These remaining deficits may be due to mucus accumulation and airway wall thickening. These two components cannot be separated since we presently lack a reliable way to quantify airway secretions.  [...]