Changes

When a horse with a history of RAO 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 matter¹. 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 moulds^haliwell,  and the cytokine response appaears to be skewed towards TH2^lavoi: both of these facts are suggestive of an allergic mechanism.

When a horse with a history of RAO 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 matter¹. 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 moulds^haliwell,  and the cytokine response appaears to be skewed towards TH2^lavoi: 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. Aiway obstruction then develops due to several mechanisms. Mucus becomes more viscous and accumulates in the luman, and bronchospasm is initiated by the actions of inflammatory mediators on airway smooth muscle and cholinergic nerves^Olszewski. 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^allen.

On exposure to dust, neutrophils accumulate in the lung and quickly invade the lumen of the airway¹. Aiway obstruction then develops due to several mechanisms. Mucus becomes more viscous and accumulates in the luman, and bronchospasm is initiated by the actions of inflammatory mediators on airway smooth muscle and cholinergic nerves^Olszewski. 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, allen}.

==Signalment==

==Signalment==