Recurrent Airway Obstruction

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

Do not confuse with: summer pasture associated obstructive pulmonary disease.

Description

Recurrent airway obstruction (RAO) is an inflammatory, obstructive respiratory disease of horses. Disease is induced by the exposure of susceptible animals to organic dust, which gives neutrophil influx to the airways, bronchospasm and mucus accumulation¹. This causes a cough, nasal discharge, and respiratory difficulty. When exposure to the allergen is eliminated, obstruction and clinical signs resolove or attenuate. However, animals are susceptible to reccurent episodes of airway obstruction throughout their lives, and so management is essential.

Since disease required exposure to organic dust, recurrent airway obstruction occurs in stabled horses that are, for example, bedded on straw and fed hay. The condition is therefore most prevalent in the northern hemisphere because horses tend to be stabled for large parts of their lives². Summer pasture-associated obstructive pulmonary disease (SPAOD) is a similar condition that occurs when horses kept on pasture are sensitive to environmental allergens, and may be considered the same disease as RAO but with different inciting factors¹.

Pathogenesis

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

Signalment

RAO usually becomes apparent in middle-aged and older horses, and persists for life^allen. There is some evidence that there may be a genetic component to the disease, as many horses do not suffer RAO when they are housed in environments that can provoke clinical disease in others^marti.

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)

References

1. Robinson, N E (2001) Recurrent Airway Obstruction. In Equine Respiratory Diseases, IVIS.
2. Robinson N E et al (1995) The pathogenesis of chronic obstructive pulmonary disease of horses. The British Veterinary Journal, 152, 283-306.
3. Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
4. Allen, K and Franklin, S (2007) RAO and IAD: respiratory disease in horses revisited. In Practice, 29(2), 76-82.
5. Marti, E et al (1991) The genetic basis of equine allergic diseases 1. Chronic hypersensitivity bronchitis. The Equine Veterinary Journal, 23, 457-460.
6. McGorum, B et al (2007) Equine respiratory medicine and surgery, Elsevier Health Sciences.
7. Durham, A (2001) Update on therapeutics for obstructive pulmonary diseases in horses. In Practice, 23(8), 474-481.