Page 1060 - Veterinary Immunology, 10th Edition

Several forms of chronic obstructive respiratory disease are
VetBooks.ir recognized in horses and collectively called equine asthma. They

are most conveniently divided into two syndromes: recurrent
airway obstruction (RAO) or “Heaves” and inflammatory airway

disease (IAD).
RAO is characterized by chronic airway inflammation,
bronchoconstriction, and neutrophil infiltration. RAO is seen in
horses over 9 years of age. IAD is a milder condition seen in horses

of any age. It is associated with decreased performance and a
chronic, intermittent cough. Both are forms of chronic bronchiolitis
associated with exposure to molds and other allergens in dusty
stable air.

It has been recognized that asthma in humans consists of several
distinctly different diseases. Some are associated with high
eosinophil counts in bronchoalveolar washes, while others are
characterized by high neutrophil counts. Some human cases have

both neutrophils and eosinophils while others have neither. A
similar spectrum and complexity is seen in equine asthma (Chapter
30).

Recurrent airway obstruction.

RAO resembles severe asthma in humans. Affected animals have
respiratory distress episodes triggered by inhalation of airborne
allergens even while at rest. During these episodes, horses develop
airflow obstruction as a result of bronchospasm, increased mucus
production, airway hyperplasia, and airway hyperresponsiveness

(Fig. 32.3). Neutrophil recruitment is apparent by 4 to 6 hours after
antigen challenge and precedes the development of airway
obstruction. Th2 cytokines appear to contribute to the recruitment

and activation of these neutrophils and the release of extracellular
NETS (Chapter 5). During remissions, horses are clinically normal
and their airway function and bronchial cytology appear normal.

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