Changes

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 accumulation.<ref name="one">Robinson, N E (2001) Recurrent Airway Obstruction. In '''Equine Respiratory Diseases''', ''IVIS''</ref> This causes a cough, nasal discharge, and respiratory compromise. When exposure to the allergen is eliminated, obstruction and clinical signs resolve or attenuate. However, animals are susceptible to recurrent episodes of airway obstruction throughout their lives, so management is essential. The diseases is more likely to be [[Type III Hypersensitivity|type III hypersensitivity]] reaction.<ref name="Medicine and Surgery">McGorum BC, Dixon PM, Robinson NE (2007) '''Equine respiratory medicine and surgery''', ''Elsevier''</ref>

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 accumulation.<ref name="one">Robinson, N E (2001) Recurrent Airway Obstruction. In '''Equine Respiratory Diseases''', ''IVIS''</ref> This causes a cough, nasal discharge, and respiratory compromise. When exposure to the allergen is eliminated, obstruction and clinical signs resolve or attenuate. However, animals are susceptible to recurrent episodes of airway obstruction throughout their lives, so management is essential. The diseases is more likely to be [[Type III Hypersensitivity|type III hypersensitivity]] reaction.<ref name="Medicine and Surgery">McGorum BC, Dixon PM, Robinson NE (2007) '''Equine respiratory medicine and surgery''', ''Elsevier''</ref>

Since disease requires exposure to organic dust, recurrent airway obstruction occurs in stabled horses that are 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 lives.<ref name="two">Robinson N E et al (1995) '''The pathogenesis of chronic obstructive pulmonary disease of horses'''. ''The British Veterinary Journal'', 152, 283-306</ref> [[Summer Pasture-Associated Recurrent Airway Obstruction|Summer pasture-associated obstructive pulmonary disease (SPAOPD)]] 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 factors.<ref name="one">

Since disease requires exposure to organic dust, recurrent airway obstruction occurs in stabled horses that are 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 lives.<ref name="two">Robinson N E et al (1995) '''The pathogenesis of chronic obstructive pulmonary disease of horses'''. ''The British Veterinary Journal'', 152, 283-306</ref> [[Summer Pasture-Associated Recurrent Airway Obstruction|Summer pasture-associated obstructive pulmonary disease (SPAOPD)]] 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 factors.<ref name="one" />

==Pathogenesis==

==Pathogenesis==

When a horse is moved from pasture to a stable, the hay it is fed and the straw it is bedded on harbour organic dusts. These dusts contain components which are capable of causing inflammation of the lungs, such as specific allergens, endotoxin, moulds and small particulate matter<ref name="one">. Although small particles and endotoxin are known to cause pulmonary inflammation, there is evidence to suggest that there is an allergic component to recurrent airway obstruction. For example, bronchoalveolar lavage fluid in RAO has been shown to have increased levels of IgE specific for various moulds³, and the cytokine response appears to be skewed towards TH2.⁴ Both of these facts are suggestive of an allergic mechanism.

When a horse is moved from pasture to a stable, the hay it is fed and the straw it is bedded on harbour organic dusts. These dusts contain components which are capable of causing inflammation of the lungs, such as specific allergens, endotoxin, moulds and small particulate matter<ref name="one" />. Although small particles and endotoxin are known to cause pulmonary inflammation, there is evidence to suggest that there is an allergic component to recurrent airway obstruction. For example, bronchoalveolar lavage fluid in RAO has been shown to have increased levels of IgE specific for various moulds³, and the cytokine response appears to be skewed towards TH2.⁴ Both of these facts are suggestive of an allergic mechanism.

On exposure to dust, neutrophils accumulate in the lung and quickly invade the lumen of the airway¹. Airway obstruction then develops due to several mechanisms. Mucus becomes more viscous and accumulates in the lumen, and bronchospasm is initiated by the actions of inflammatory mediators on airway smooth muscle and cholinergic nerves⁵. Oedema of the airway wall also contributes to narrowing, and in horses suffering chronic disease, the wall remodels to give mucus metaplasia, smooth muscle hypertrophy and peribronchial fibrosis^{1, 6}.

On exposure to dust, neutrophils accumulate in the lung and quickly invade the lumen of the airway¹. Airway obstruction then develops due to several mechanisms. Mucus becomes more viscous and accumulates in the lumen, and bronchospasm is initiated by the actions of inflammatory mediators on airway smooth muscle and cholinergic nerves⁵. Oedema of the airway wall also contributes to narrowing, and in horses suffering chronic disease, the wall remodels to give mucus metaplasia, smooth muscle hypertrophy and peribronchial fibrosis^{1, 6}.