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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Seasonally recurring progressive lower airway disease. Exposure to inhaled particulates in pasture during hot months elicits inflammation, mucus hypersecretion, and bronchoconstriction. Available treatments are palliative. Removal from the pasture environment is necessary. SPA-EA is similar to asthma associated with barn dust, but differs in its specific association with pasture exposure during hot humid conditions.

Pathophysiology!!navigator!!

  • SPA-EA exhibits key facets of human asthma including airway hyperresponsiveness, airway inflammation, and reversible bronchoconstriction. Exacerbations are elicited by particulates inhaled while grazing, especially during high heat and humidity. Mold spores and grass pollen are suspected triggering agents.
  • Airway obstruction results from bronchoconstriction, mucus hypersecretion, decreased mucociliary clearance, and inflammatory exudate. Disease chronicity elicits progressive airway remodeling that limits reversibility of airway obstruction. Profound ventilation–perfusion inequalities and hypoxemia are common

Systems Affected!!navigator!!

Respiratory

Genetics!!navigator!!

Uncharacterized, likely a complex polygenic trait.

Incidence/Prevalence!!navigator!!

Unknown; unpublished survey reported 5% prevalence in Louisiana.

Geographic Distribution!!navigator!!

  • Prevalent in the southeastern USA
  • More recently described in the UK
  • Anecdotal reports in other places

Signalment!!navigator!!

  • Overrepresentation of Quarter Horse-type breeds and ponies may reflect regional disparities in breed distribution and extensive pasture exposure
  • Mature horses—12 ± 6 years
  • No sex predilection

Signs!!navigator!!

  • Clinical exacerbations predictably occur during hot humid conditions, worsen annually, and remit during cooler months
  • Initial signs are exercise intolerance and occasional cough
  • With progression, prominent signs include hyperpnea, dyspnea, increased expiratory effort, flared nostrils, and coughing
  • Vital signs may be increased, especially respiratory rate. Mildly increased (<39°C (<102.5°F)) body temperature is not uncommon; higher temperatures suggest alternate or intercurrent disease
  • Severely affected horses stand with the neck extended, become anorexic, and may become emaciated. They can develop a “heaves line” (hypertrophy of the external abdominal oblique muscles)
  • Thoracic auscultation reveals increased bronchovesicular sounds, expiratory wheezes (sometimes inspiratory), and crackles. In mild cases, auscultation is abnormal only during forced breathing

Causes!!navigator!!

Airborne particulates inhaled during grazing in hot humid conditions are incriminated.

Risk Factors!!navigator!!

  • Extensive pasture exposure (>12 h/day) increases disease risk in the southeastern USA.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Viral, bacterial, fungal respiratory infections
  • Anhidrosis often occurs in similar (hot and humid) environmental conditions. Associated tachypnea can be mistakenly interpreted as a respiratory condition

CBC/Biochemistry/Urinalysis!!navigator!!

  • Mild neutrophilia is common
  • Mildly increased fibrinogen may occur

Other Laboratory Tests!!navigator!!

  • BALF cytology, the preferred diagnostic, identifies increased neutrophils (>10%, often >20%)
  • Tracheal aspirate identifies mucopurulent inflammation with nondegenerate neutrophils. Bacterial and fungal elements reflect impaired mucociliary clearance, not necessarily infection. Intracellular bacteria and bacterial growth increase likelihood of active infection
  • Arterial blood gases often identify hypoxemia (PaO2 <80 mmHg). Hypocapnia (PaCO2 <40 mmHg) progresses to hypercapnia (PaCO2 >40 mmHg) in severe disease
  • Transcutaneous lung biopsy carries a rare risk of fatal bleeding, outweighing its diagnostic utility

Imaging!!navigator!!

  • Tracheal endoscopy reveals mucopurulent exudate. Carina thickening with disease chronicity
  • Thoracic radiography may reveal increased bronchointerstitial pattern

Other Diagnostic Procedures!!navigator!!

  • Tentative diagnosis from historical, signalment, and clinical findings is confirmed by neutrophilic BALF inflammation, exclusion of other diseases, and improvement following environmental modification
  • Airway hyperresponsiveness to methacholine bronchoprovocation is identified in exacerbation and remission

Pathologic Findings!!navigator!!

  • Chronicity causes airway remodeling with increases in airway smooth muscle mass, peribronchiolar inflammatory infiltrate, fibrosis, intraluminal mucus and inflammatory cells, epithelial hyperplasia with goblet cell hyperplasia/metaplasia, peribronchiolar fibers of the elastic network, and complex cellular disorganization of the terminal bronchiole (terminal bronchiolar remodeling)
  • Areas of alveolar overinflation (air trapping) are common; true emphysema is rare

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

Remove the inciting cause. Cases with overt distress or that improve slowly despite strict environmental management warrant therapeutic intervention to improve oxygen delivery, reverse airway obstruction due to bronchoconstriction and mucus accumulation, and limit airway inflammation.

Appropriate Health Care!!navigator!!

In- or outpatient medical management.

Nursing Care!!navigator!!

  • Exacerbations are reversible with adequate quarantine from the pasture environment in a dust-free indoor environment during warm months of the year. Severely affected horses are stalled on rubber mats without bedding, or using low-dust bedding (pellets, shredded cardboard). Less severe disease may initially be managed on pasture that is cut very short (necessitating an alternate complete diet)
  • During seasonal disease exacerbation, reexposure to inciting pasture elicits disease within days

Activity!!navigator!!

Activity is dictated by disease severity and response to therapy; horses in clinical remission may be exercised normally.

Diet!!navigator!!

  • To minimize dust, a complete pelleted feed that addresses 100% of the forage requirement is preferred, even during cool season remission
  • Cubed forage or haylage are preferred to hay (also during remission). When hay is fed, it should be fully submersed in water immediately prior to feeding
  • Round hay bales should not be offered, even during remission

Client Education!!navigator!!

  • During clinical remission, affected horses should be kept in a low-dust environment when stabled, and pasture should be relatively short (a few centimeters high), necessitating an alternate complete diet
  • Disease recurs predictably in successive years, allowing removal of affected horses from offending pasture prior to seasonal disease onset

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

Medications recommended for treating heaves are indicated for treating SPA-EA.

Oxygen

Animals with profound dyspnea, hyperpnea should receive inhaled oxygen supplementation (≥10 L/min).

Corticosteroids

  • In severe cases, use parenteral dexamethasone 0.05 mg/kg every 24 h for 3 days; decrease dose and increase interval. Disease recrudescence after treatment should direct improved environmental management. Caution in horses with endocrinopathies
  • Inhaled formulations (nebulization, pMDIs) are administered using equine spacers and decrease systemic side effects
  • Effects of beclomethasone (beclomethasone) dipropionate (≥1 μg/kg every 12 h) or fluticasone propionate (4 μg/kg every 12 h) are delayed, but useful for chronic management

Bronchodilators

  • Bronchodilators may be life-saving. Long-term administration must be combined with corticosteroids and environmental control
  • Several inhaled β2-adrenergic agonists are effective and available as pMDI or for nebulization. Fast-acting/short-lasting—levalbuterol (levosalbutamol) (1.25 mg/horse (author's recommendation, higher than published dosage)), albuterol (salbutamol) (1–2 μg/kg)
  • Oral β2-adrenergic agonists (clenbuterol and albuterol syrup; albuterol tablets; terbutaline sulfate) have low efficacy
  • Due to the adverse gastrointestinal effects, systemic parasympatholytic bronchodilators should be limited to the short-acting muscarinic antagonist hyoscine butylbromide (Buscopan) at a dose of 0.3 mg/kg, IV, during asthmatic crisis
  • Ipratropium bromide pMDI (1–4 μg/kg) is effective but can excessively dry respiratory secretions

Expectorant, Mucolytic, and Mucokinetic Agents

  • Limited anecdotal reports of efficacy
  • β2-adrenergic agonists also improve mucociliary clearance

Contraindications, Possible Interactions!!navigator!!

  • Corticosteroids are contraindicated with infection
  • Oral mucokinetic agents that stimulate the gastropulmonary mucokinetic vagal reflex, such as iodides, can exacerbate bronchospasm

Alternative Drugs!!navigator!!

  • Horses with poor response to bronchodilators may benefit from IV magnesium sulfate administration (40 mg/kg given over 20 min). The effect is limited to the duration of the infusion. Concurrent doses have not been evaluated and have potential for toxicity
  • Mast cell stabilizers have been advocated for limiting seasonal exacerbations

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Monitor expiratory effort and pulmonary adventitial sounds
  • Some horses with extreme small airway obstruction fail to respond to inhaled bronchodilators due to the severe hypoxic vasoconstriction and bronchoconstriction; they require prolonged inhaled oxygen therapy for improvement. These horses typically present with decreased lung sounds, lack adventitious lung sounds such as wheezes and crackles, and have a paradoxical breathing pattern
  • Severely hypoxemic horses should be monitored by serial PaO2 measurements

Prevention/Avoidance!!navigator!!

Dust-free environment, away from pasture during the warm months of the year.

Possible Complications!!navigator!!

  • SPA-EA is debilitating and fatal in severe cases without proper environmental control and medical treatment
  • Chronic hypoxic vasoconstriction can lead to pulmonary hypertension and right heart failure

Expected Course and Prognosis!!navigator!!

  • Though episodic dyspnea is reversible with environmental control and medical therapy, complete isolation from offending pasture particulates is impractical. Thus, affected horses worsen annually at a rate that is influenced by their management
  • Neutrophilic airway inflammation often persists during seasonal disease remission
  • The onset of disease in horses with SPA-EA is specifically associated with pasture in the summer, and the signs of disease tend to worsen over subsequent seasons. Advance cases may eventually develop clinical signs in response to barn dust, reflecting a progressive airway hyper-responsiveness

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Pregnancy/Fertility/Breeding!!navigator!!

Fetal growth retardation and death may occur with hypoxic mares.

Synonyms!!navigator!!

  • Summer pasture heaves
  • Summer pasture-associated equine asthma (SPA-EA)
  • Summer pasture-associated recurrent airway obstruction

Abbreviations!!navigator!!

  • BALF = bronchoalveolar lavage fluid
  • PaCO2 = partial pressure of carbon dioxide in arterial blood
  • PaO2 = partial pressure of oxygen in arterial blood
  • pMDI = pressurized metered dose inhaler
  • SPA-EA = summer pasture-associated equine asthma

Suggested Reading ⬆ ⬇

Beadle RE. Summer pasture-associated obstructive pulmonary disease In: Robinson NE, ed. Current Therapy in Equine Medicine. New York, NY: WB Saunders, 1983:512–516.

Costa LRR, Seajorn TL, Moore RM, et al. Correlation of clinical score, intrapleural pressure, cytologic findings of bronchoalveolar fluid, and histopathologic lesions of pulmonary tissue in horses with summer pasture associated recurrent airway obstruction. Am J Vet Res 2000;61:167–173.

Costa LRR, Johnson JR, Baur ME, Beadle RE. Temporal clinical exacerbation of summer pasture associated recurrent airway obstruction and relationship with climate and aeroallergens in horses. Am J Vet Res 2006;67:1635–1642.

Author(s) ⬆

Authors: Lais R.R. Costa and Cyprianna Swiderski

Consulting Editors: Daniel Jean and Mathilde Leclère