Recurrent airway obstruction in horses - an allergic inflammation: a review

Recurrent airway obstruction (RAO), also known as heaves, is a debilitating and incurable disease of the equine airway. Affected horses develop bronchoconstriction and neutrophilic airway inflammation as a result of exposure to specific airborne irritants and allergens such as hay mould and barn dust. Clinical signs of RAO include exercise intolerance, coughing, nostril flare and abdominal push related to respiratory effort. Evidence suggests that both the innate and acquired immune response contribute to the activation of inflammatory cells resulting in type I hypersensitivity and type III hypersensitivity reactions with an increased expression of Th1- and Th2-cytokines, chemokines, adhesion molecules, receptors and release of reactive oxygen species (ROS). The regulation of inflammatory gene expression in RAO-affected horses is dependent on the binding of transcription factors such as Nuclear factor-(kappa)B (NF-kB), activator protein-1 (AP-1) and the cyclic AMP response element binding protein (CREB) to the promoter region of target genes. In chronic disease an increased number of mucous-producing cells and increased amounts of stored mucins are observed in conjunction with other characteristics of airway tissue remodelling. In this review the findings related to the inflammatory and immunologic response in RAO-affected horses will be presented, and this information will be integrated into existing concepts of immunopathologic mechanisms.