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Severe Equine Asthma- how can current research help us understand the specific causes and pathophysiology?
Severe equine asthma (SEA) is a performance-limiting allergic airways disease, similar to human asthma, that affects genetically predisposed horses when exposed to allergens associated with the stabling environment. Fourteen percent of the UK horse population suffer with SEA, resulting in coughing, nasal discharge and respiratory difficulty, as a result of lower airway inflammation, bronchoconstriction and mucus production. The immunological basis of SEA is still unclear; however, several studies have indicated the role of an antibody, known as immunoglobulin E (IgE)(Künzle et al., 2006). When this antibody binds with an allergen in the affected horse, a range of inflammatory causing molecules are released, such as histamine. Ergo, reducing allergen exposure in horses with SEA is paramount to successful treatment (Jackson et al., 2000).
The predominant source of these airborne allergens in the stabling environment results from the dust portion of forage, which can contain a range of fungi, bacteria, pollen and arthropods. When the affected horse is removed from this environment a period of remission will result. Markham first associated SEA with the stabling environment in 1656, providing initial advice to sprinkle hay with water. Since then, our knowledge of hay treatments to limit allergen exposure has greatly improved. Classically, haylage or soaked hay was fed to affected horses, however, these options do not sufficiently reduce respirable particles long term, and the later increases bacteria levels. More recently, high temperature steam treating hay has been shown to reduce the levels of respirable particles, as well as, living mould and bacteria, therefore reducing allergen-interaction (Moore-Colyer et al., 2016). Further studies demonstrated horses fed dry hay had significantly increased clinical scores when compared with horses fed steamed hay (Blumerich et al., 2012).
A range of clinical techniques have been utilised to establish the causal allergens associated with SEA, most successfully the analysis of allergen-specific IgE. The assessment of IgE levels in serum and bronchoalveolar lavage fluid has suggested the role of several fungi and bacteria, most notably Aspergillus fumigatus, Saccharopolyspora rectivirgula and Thermoactinomyces vulgaris (Morán and Folch, 2011).
Recent developmental work in multiple extract microarrays, in a project partly funded by Haygain, permitted the evaluation of IgE-allergen interactions on a wider scale. Microarray technology has enabled the profiling of IgE against 146 proteins (extracts, pure and recombinant), identifying sensitization in SEA horses to range of fungi, arthropods, pollens and bacteria previously untested (White et al., 2017). Thermal-processing of microorganisms has previously been shown to result in exterior alterations, preventing IgE recognition, and therefore the associated allergic response. Expanding this initial work, the authors aim to use the developed microarray technology to evaluate the effect high temperature hay steaming has on IgE-allergen interaction (White, pers. com.). This study will provide new data on the aetiology and pathophysiology of SEA in the horse, and provide management/treatment recommendations improving the health, welfare and performance of affected horses.
Künzle, F, Gerber, V, Van Der Haegen, A, Wampfler, B, Straub, R and Marti, E (2006) IgE-bearing cells in bronchoalveolar lavage fluid and allergen-specific IgE levels in sera from RAO-affected horses. J. Vet. Med. 54, 40-47.
Jackson, CA, Berney, C, Jefcoat, AM and Robinson, NE (2000) Environment and prednisone interactions in the treatment of recurrent airway obstruction (heaves). Equine vet. J. 32, 432-438.
Moore-Colyer, MJS, Taylor, JLE and James, R (2016) The effect of steaming and soaking on the respirable particle, bacteria, mould and nutrient content in hay for horses. J Equine Vet Sci, 39, 62-68
Blumerich, C.A., Buechner-Maxwell, V.A., Scrratt, W.K., Wilson, K.E., Ricco, C., Becvarova, I., Hodgson, J. and Were, S. (2012) Comparison of airway response of Recurrent Airway Obstruction affected horses fed steamed versus non-steamed hay. Proceedings of the Annual ACVIM Conference, 2012.
Morán G, Folch H (2011) Recurrent airway obstruction in horses – an allergic inflammation: a review. Vet Med-Czech 56:1–13
White S, Moore-Colyer M, Coüetil L, Hannant D, Richard EA, Marti E and Alcocer M (2017) Development of a multiple protein extract microarray for profiling allergen-specific immunoglobulin E in horses with severe asthma. Proceedings of the World Equine Airways Symposium, 2017.