Recurrent Airway Obstruction

Also known as: RAO, chronic obstructive pulmonary disease, COPD.

Description

Recurrent airway obstruction (RAO) is a common, performance-limiting, allergic respiratory disease of horses characterized by chronic cough, nasal discharge, and respiratory difficulty. Episodes of airway obstruction are observed when susceptible horses are stabled, bedded on straw, and fed hay, whereas elimination of these inciting factors results in remission or attenuation of clinical signs. The pathophysiology involves small airway inflammation (neutrophilic), mucus production, and bronchoconstriction in response to allergen exposure.

Recurrent airway obstruction (RAO) is an inflammatory, obstructive airway disease that becomes clinically evident in middle-aged horses. The disease, also known as heaves, is most prevalent in the northern hemisphere where horses are stabled for large parts of their lives and are fed hay [1]. A similar syndrome, summer pasture-associated obstructive pulmonary disease (SPAOD) occurs in the southeastern United States, Britain and California in horses that are kept on pasture when the weather is warm and humid [2,3]. Evidence to date suggests that the two syndromes are the same disease but with different initiating factors. Recurrent airway obstruction was formally known as chronic obstructive pulmonary disease (COPD). However, because of differences between equine and human COPD, a recent workshop recommended that the terms RAO or heaves, rather than COPD, be used for the horse disease [4]. Recurrent airway obstruction is initiated by the inhalation of organic dusts. The most common source of such dusts is hay and bedding [5-7]. Summer pasture-associated obstructive pulmonary disease is most likely a result of inhalation of organic dusts occurring in pastures in hot humid climates [2]. Organic dusts contain a variety of components that can initiate lung inflammation. These include specific allergens, endotoxin, components of molds such as beta-glucan, and small particulates. Elevated levels of specific IgE in bronchoalveolar lavage fluid (BALF) [8] favor the hypothesis that RAO is initiated by an allergic response to thermophilic molds and actinomycetes such as Faeni rectivirgula. A similar increase in specific IgE could not be documented in horses with SPAOD [9]. More recent studies examining the cytokine responses in both RAO and SPAOD lean toward a TH2 (allergic) immune response [10]. However, it has proved impossible to induce the heaves syndrome simply by administration of molds to susceptible horses [11-13]. Hay and stable dust contains endotoxin [7] and it is known from work in other species that administration of endotoxin into the airways can also induce many of the changes typical of RAO such as neutrophilic inflammation and mucus hypersecretion [14]. Small particles and fibers landing on the surface of the epithelium also can initiate the release of pro-inflammatory cytokines such as IL-8 [15,16]. Because organic dusts contain such a mixture of materials, RAO and SPAOD probably are initiated by several of these factors acting in concert. A genetic susceptibility to these diseases is suggested by the observation that many horses are housed without apparent problems in environments that can provoke airway obstruction in a RAO susceptible horse. Evidence in support of such a genetic component does exist

Diagnosis

Pathology

COPD (Image sourced from Bristol Biomed Image Archive with permission)

COPD scanning electron micrograph (Image sourced from Bristol Biomed Image Archive with permission)