Equine asthma: managing the environment

The term equine asthma is used to describe nonseptic lower airway inflammation under a broad phenotypic definition that allows for multiple, potentially overlapping, subcategories of disease (Pirie et al, 2016). This umbrella term is designed to cover the diversity in aetiology, immunology, pathophysiology, genetics and clinical severity undoubtedly underpinning the disease syndrome; it is not a continuum. Currently we are restricted to two categories, mild to moderate equine asthma and severe equine asthma, these being predominantly differentiated by the presence at rest of increased respiratory effort in the latter (Couëtil et al, 2016). Mild to moderate equine asthma has previously been called inflammatory airway disease (IAD), while severe equine asthma has been known as chronic obstructive pulmonary disease (COPD), recurrent airway obstruction (RAO) and ‘heaves’.

Severe equine asthma's clinical signs, ranging from poor performance to coughing and expiratory dyspnoea, can be attributed to lower airway inflammation induced by immunological responses, including hypersensitivity, to airborne organic dust. A myriad of different agents present in a horse's stabled environment can contribute to the aetiology of this disease, with moulds having a principle role along with endotoxins (Pirie et al, 2003; Pirie, 2014). Horses with severe equine asthma have increased airway reactivity, thus non-specific stimuli such as other particulates and irritant gases, for example ammonia, may exacerbate or prolong clinical signs (Pirie, 2014). Severe equine asthma is generally associated with stabling, which is unavoidable in many situations or climates, especially during winter. However, not all severe equine asthma is associated with housing, as recognised in the subset of equine pasture asthma (formerly summer pasture associated obstructive pulmonary disease), where inciting agents have been suggested to be pollens and fungal spores in association with heat and humidity (Costa et al, 2006; Bullone et al, 2016). The management changes discussed in this article will not be of direct benefit to such horses, but removal to a low-dust housed environment is a standard recommendation.