Summer Pasture-Associated Recurrent Airway Obstruction
(SPA-RAO)
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Also known as: Summer Pasture-Associated Obstructive Pulmonary Disease, Summer Pasture-Associated Heaves
Description
Summer Pasture-Associated Recurrent Airway Obstruction (SPA-RAO) is an inflammatory condition of the lower airways affecting pasture-kept horses. This is in contrast to Recurrent Airway Obstruction that affects horses that are kept in a stabled environment. SPA-RAO is characterised by airway inflammation, increased airway mucus production, reduced mucociliary clearance and bronchoconstriction in response to allergen exposure. The pathogenesis of the disease is largely unknown but suggested hypotheses include inhaled pollens or outdoor moulds, or ingestion of a pasture-derived pneumotoxin. It occurs most commonly in the southern states of America but has also been reported in the United Kingdom.
Signalment
SPA-RAO generally affects adult horses only and the average age of onset is 9 years of age. There is no reported sex predilection but Quarter Horses and Appaloosas are more prone to developing the condition.
Clinical signs
Clinical signs are similar to those associated with RAO but occur in horses kept out at pasture during the summer months, usually for more than twelve hours a day. Signs can include exercise intolerance, productive cough, dyspnoea, increased expiratory effort, nasal discharge and flared nostrils. Lung field auscultation reveals fine crackles and wheezes. In severe cases, wheezes may be audible without a stethoscope. Pyrexia may be a feature if a secondary bacterial infection of the airways has occurred. Affected horses often stand with their neck arched and elbows abducted and an abdominal 'heave' line may be visible due to hypertrophy of the external abdominal oblique muscles.
Diagnosis
Diagnosis is usually based on the characteristic and seasonal appearance of clinical signs combined with a history of exposure to pasture during late spring or summer. Endoscopy may be used in order to examine the airways for the presence of inflammation or mucus. A sample of mucus can be obtained via bronchoalveolar lavage which often yields copious mucopurulent material. Cytology performed on the sample usually reveals >25% non-degenerate neutrophils confirming the presence of lower airway inflammation. Curschmann's spirals may also be observed, representing inspissated mucus in the obstructed lower airways. Thoracic radiology is not commonly performed but often reveals an increased bronchointerstitial pattern.
Treatment and management
Control of environmental exposure is essential in horses affected with SPA-RAO. Affected horses should only be allowed access to pasture during late autumn, winter and early spring. They should ideally be kept in a dust-free environment such as a clean stable with rubber matting and no bedding. If hay appears to act as a trigger to the horse, a complete pelleted diet can be fed or alternatively hay should be thoroughly soaked. Alfalfa hay has been previously associated with less respiratory problems than grass hay and may be a preferable source of roughage for affected horses. Horses with severely compromised respiratory function should be rested until their condition has improved.
In some cases, the response to environmental management may be significant and further treatment may not be required. In more severe cases, drug therapy may be required to suffiently manage the horse's condition. Corticosteroids (e.g. dexamethasone)are used in order to reduce airway inflammation and can be administered via nebuslisation using an inhaler in order to produce the maximum concentration of the drug in the respiratory tract. Minimal systemic side effects are induced with this mode of administration. This treatment can be combined with a bronchodilator such as clenbuterol to reduce smooth muscle contraction in the lower airways. Antibiotics may be required if seconday bacterial infection is present.
References
- Lavoie, J. P., Hinchcliff, K. W. (2009) Blackwell's Five-Minute Veterinary Consult: Equine John Wiley and Sons
- McGorum, B. C., Edward Robinson, N., Dixon, P. M., Schumacher, J. (2007) Equine respiratory medicine and surgery Elsevier Health Sciences
- Orsini, J. A., Divers, T. (2007) Equine Emergencies: Treatment and Procedures Elsevier Health Sciences