Chronic Cough

Coughing is an important airway defence mechanism, but chronic cough (here defined as of > 2 months duration) can be deleterious: it may cause bronchial muscular hypertrophy and can exhaust affected animals, causing for instance decreased feed intake in chronically coughing foals. While a chronic cough is always abnormal, it is important to keep in mind that many mild and even some severe respiratory diseases may result in no or very little coughing. In the following, the mechanism, stimuli, clinical approach and treatment of chronic cough are briefly described.

Pathophysiological mechanisms
Coughing, a sudden explosive expulsion of air through the glottis, is a non-specific reaction to an irritant stimulus, be it mechanical, inflammatory or, as in probably most if not all chronic cases, a combination of both. Inflammation, however, will also make the airways hyperreactive, so that otherwise normal innocuous stimuli will be enough to cause coughing: the threshold is lowered.
The cough reflex itself has barely, if at all, been studied in horses. In general, reflex cough is initiated by activation of a subset of afferent sensory nerves by inhaled, aspirated or locally produced substances. There are important species differences (some animals, e.g. mice and rats do not have a cough reflex at all), however, so we must extrapolate with caution: in the guinea pig, the long debated existence of “cough receptors” has recently been substantiated (Mazzone, 2005). The cough receptor is described as a low threshold extrapulmonary mechanosensor, which mediates the primary cough reflex. In contrast, other receptors and pathways, such as the intrapulmonary mechanosensors SAR (slowly adapting airway mechanoreceptor; primary reflex effect: Hering-Breuer) and RAR (rapidly adapting airway mechanoreceptor; primary reflex effect: Tachypnea) as well as the intra- and extrapulmonary (C-Fibers and Aδ-Fibers) chemosensors (nociceptors) appear to have secondary modulating effects on coughing.
There is evidence that the cough receptors are located primarily in the extrapulmonary airways, i.e. larynx, trachea and large bronchi. Thus, irritant stimuli proximal (nose; will cause snorting) and distal (the very large surface area of the small bronchi, bronchioles and the alveoli) to this region will not directly result in a cough, but may well influence coughing through an inflammatory reaction and stimulation of other reflex pathways. Specifically, chronic cough associated with airway obstruction or chronic irritation may involve the recruitment of RAR and/or chemosensors.
The cough cycle is composed of four phases, deep inspiration; compression of the air against the closed glottis; explosive expression after sudden opening of the glottis; followed by relaxation.  [...]