Chronic inflammatory airway disease in horses has been known by many names over the past few decades, including heaves, chronic obstructive pulmonary disease (COPD), inflammatory airway disease (IAD) and recurrent airway obstruction (RAO) – with the variant of summer-pasture associated RAO (SPRAO). In 2016, the American College of Veterinary Internal Medicine (ACVIM) revised consensus statement on IAD in horses proposed a further change in terminology, combining IAD and RAO under the umbrella of ‘equine asthma syndrome’, with a differentiation into mild-moderate equine asthma (formerly IAD) and severe equine asthma (formerly RAO).1 This change in terminology was based on the realisation that chronic airway inflammation in horses is likely to be a collection of clinical and clinicopathological manifestations – or phenotypes – that may have different underlying pathophysiological mechanisms, similar to human asthma syndrome.1,2 This evolving thought process has highlighted the need to further investigate possible differences between equine asthma phenotypes, such as factors causing the predominant inflammatory cell type recruited into the airways (neutrophils, eosinophils or mast cells), the possible role of infectious agents and the airway microbiome, as well as genetic factors. The two most commonly used medications for equine asthma patients are corticosteroids and bronchodilators. Corticosteroids, administered either systemically or by aerosol, can rapidly improve lung function, but some horses require long-term administration in order to keep clinical signs of disease at bay. Aerosolised bronchodilators provide immediate relief of bronchospasm …