Summer Pasture–Associated Recurrent Airway Obstruction
Basic Information 
Clinical Presentation
Physical Exam Findings
• Prominent signs are labored expiratory effort, flared nostrils and coughing.
• Affected horses are alert but anxious because of respiratory difficulty.
• Vital signs are often increased, especially respiratory rate and mildly increased body temperature.
• Affected horses develop a “heave line” (hypertrophy of the external abdominal oblique muscles).
• In mild cases, thoracic auscultation reveals increased bronchovesicular sounds at rest; wheezes and expiratory crackles are evident during forced breathing.
• Thoracic auscultation of severely affected horses at rest reveals wheezes, generally expiratory (sometimes inspiratory), and expiratory crackles. Wheezes may be audible without stethoscope.
• In severe cases, affected horses have a stance with the head and neck extended forward, increased respiratory rate, flared nostrils, and/or end-expiratory effort. These horses are dehydrated and anorexic, undergo weight loss, and may become emaciated.
• Appearance of clinical signs is seasonal and predictable based on time of the year if the animal is kept in the same environment.
Etiology and Pathophysiology
• Exposure to inhaled particulates present in pasture during warm months of the year leads to inflammation and obstruction of lower airways.
• During clinical exacerbation, affected horses develop airway obstruction (from excessive production of viscous mucopurulent secretion, decreased mucociliary clearance, and bronchoconstriction) resulting in expiratory effort and coughing and leading to ventilation/perfusion inequalities and hypoxemia.
Diagnosis 
Initial Database
• Complete blood count and fibrinogen: Normal or mild leukocytosis, mild mature neutrophilia, mild hyperfibrinogenemia
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