Page 1061 - Veterinary Immunology, 10th Edition

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FIG. 32.3 Chronic airway obstructive disease in a horse showing
mucus in the lumen, epithelial hyperplasia, smooth muscle
hypertrophy, and mildly increased numbers of submucosal
inflammatory cells. (Courtesy Dr. B. Porter.)

RAO occurs most obviously in horses that inhale large amounts
of organic dusts such as those generated in dusty stables. It includes
obstructive pulmonary disease seen in stabled horses and summer
pasture–associated obstructive pulmonary disease. It is defined as a
severe debilitating disease characterized by coughing and an

increased breathing effort due to bronchoconstriction, airway
hyperreactivity, neutrophil infiltration and mucus accumulation in
the airways. Characteristically, horses with RAO suffer from

labored breathing even while at rest.
RAO is probably a hypersensitivity disease associated with an
enhanced Th2 response. Thus biopsies show significant increases in
type 2 cytokines, IL-1β, IL-8, TNF-α, TGF-β1, as well as TLR4 and
NF-κB transcripts. Similar but less marked trends occur in IL-17

and IFN-γ. High concentrations of the chemokine CXCL8
(interleukin-8 [IL-8]) are found in the bronchoalveolar washings of
affected animals. Exposure of cultured equine bronchial epithelial

cell cultures to hay dust or lipopolysaccharide increases IL-8,
+
+
CXCL2, and IL-1β expression. The percentage of CD4 , Foxp3 Treg
cells increases in the bronchoalveolar washings of horses with RAO,
suggesting that these may influence the course of this disease.
Horses with RAO may show positive skin reactions to

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