ISSN 2398-2977      

Equine asthma


Synonym(s): Recurrent airway obstruction (RAO), Chronic obstructive pulmonary disease (COPD), Heaves, Broken wind, Chronic hypersensitivity bronchitis, Chronic hypersensitivity bronchiolitis (CB), Chronic alveolar emphysema, Small airway disease (SAD), Allergic airway disease


  • Common, chronic, progressive lower respiratory disease syndrome of horses and ponies.
  • Equine asthma is a relatively new term for recurrent airway obstruction (RAO), often applied to both young and older horses with lower airway disease characterized by inflammation and accumulation of mucus, however, in younger horses there is not measurable evidence of airway obstruction.
  • Cause: chronic exposure to inhaled dust particles containing allergens, eg moldy hay, thermo-actinomycetes.
  • Signs: poor performance, chronic coughing and increased expiratory effort, poor stable/barn environment.
  • Diagnosis: clinical signs and history; bronchoalveolar lavage or inhalation challenge.
  • Treatment: palliative only; change environment - remove allergens and improve management; control inflammation - corticosteroids; increase airway diameter - bronchodilators.
  • Prognosis: clinical signs usually reversible on removal of allergens; susceptible horses may develop hyper-reactivity.
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  • Exposure of susceptible horses to hay and dusts rich in organic material containing spores of fungi and thermophilic bacteria.
  • Over 50 species of moulds have been identified in stable air.
  • Combination of genetic predisposition, environment and disease history.
  • May be a final common pathway for structural damage to the bronchopulmonary system.
  • An earlier bronchopulmonary infection may be an initiating factor (often history of febrile respiratory episode).
  • Most allergenic bacteria include Aspergillus fumigatus, Faenia rectivirgola and Thermoactinomycetes vulgaris.

Environmental factors

  • Poorly cured hay (high levels of thermo-actinomycetes - see below).
  • Dusty feeds.
  • Poor ventilation.
  • Prolonged stabling.
  • Bacteria.
  • Endotoxins.
  • Animal debris - dander, hair, urine, feces.
  • Plant material.
  • Insect debris, eg forage mites Lepidoglyphus destructor.
  • Fungal debris, eg spores, hyphae, mycotoxins, sporangia.


  • Pathogenic mechanism based upon hypersensitivity.
  • Micropolyspora faeni/Faeni rectivirgula: thermophilic actinomycete.
  • Aspergillus fumigatus Aspergillus spp - fungal spores.
  • Wet hay and straw Hay: moldy(>20% moisture) overheat → contamination with thermotolerant fungi and actinomycetes.
  • Water content at baling is main determinant of heating and microbial development in hay.

Stable ventilation is less important than removing the source of dust.

Predisposing factors


  • Genetic susceptibility to inhaled allergens.
  • Poor stable management:
    • Inadequate clearance of wet bedding (humid environment).
    • Poor ventilation.
    • Poor quality bedding and hay.
  • Changes in bedding or hay.



  • Precise immunological mechanisms not yet determined.
  • Types 1 (immediate), 3 (arthus) and 4 (delayed) reactions implicated.
  • Repeated/chronic exposure to inhaled spores → deposit on mucosa → primary irritation; inhaled endotoxin may also have a primary irritant effect or potentiate the effects of allergen inhalation.


  • Airway reactivity:
    • Susceptible horses develop severe airway obstruction in response to apparently minor stimuli → hyper-reactive airways.
    • Airway hyper-reactivity is non-specific - occurs in response to exposure to anything, including inflammatory mediators, that might cause bronchospasm.
    • Reactivity lessens when exposure to agents is avoided.
    • Exposure for 7 h to a dusty environment → hyper-reactivity persists for <72 h.
  • Inflammation →:
    • Neutrophils invade airways.
    • Release of proinflammatory mediators, eg histamine → smooth muscle contraction, increased airway sensitivity.
    • Leukotrienes → bronchospasm.
    • Reduced availability of inhibitory mediators, eg prostaglandin E2.
    • Inactivation of nitric oxide, a neurotransmittor in the iNANC nervous system.
    • Increased mucous production and secretion + increased bronchial blood flow and vascular permeability → edema and plugging of airways with mucous.
  • Lung function → :
    • Airway obstruction → increased pulmonary resistance + decreased compliance → increased respiratory effort to achieve necessary tidal volume.
    • Ventilation-perfusion mismatching and hypoxemia.

    Although the horse must increase its respiratory effort there is no evidence of hypoventilation expressed as an increase in PaCO2.

    • Increased respiratory drive + airway obstruction → 'heaves' (double expiratory effort).
    • Despite airway narrowing → increased peak tidal flow rates.

Structural changes

  • Clara cells (non-ciliated bronchiolar epithelial cells) undergo ultrastructural alteration even in mild disease leading to degeneration and loss of differentiation.
  • Metaplasia of goblet cells.
  • Focal changes in peribronchiolar alveoli including necrosis of Type I cells, fibrosis, transformation of Type II cells, emphysema and increased numbers of Kohn's pores.
  • Conducting airways show secondary non-specific changes including focal loss of ciliated cells.
  • Viral respiratory infection → mucosal ulceration and decreased mucociliary clearance → enhanced penetration of antigens across damaged epithelial surfaces in respiratory tract; damaged mucociliary clearance means enhanced antigen/allergen retention.
  • Chronic inflammation → mucosal and smooth muscle proliferate enhancing the ability of the airways to contract.
  • Although an immune-mediated basis for RAO has been supported by many studies, positive skin tests and serum precipitins to fungal and thermophilic actinomycete antigens probably reflect a history of exposure in normal and susceptible horses, rather than a specific susceptibility to RAO.


  • Chronic.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Pirie R S (2014) Recurrent airway obstruction: A review. Equine Vet J 46 (3), 276-288 PubMed.
  • Venugopal C et al (2013) Effect of potential therapeutic agents in reducing oxidative stress in pulmonary tissues of recurrent airway obstruction-affected and clinically healthy horses. Equine Vet J 45 (1), 80-84 PubMed.
  • Bertin F R, Ivester K M & Couetil L L (2011) Comparative efficacy of inhaled albuterol between two and-held delivery devices in horses with recurrent airway obstruction. Equine Vet J 43 (4), 393-398 PubMed.
  • Leclere M, Lefebvre-Lavoie J, Beauchamp G & Lavoie J P (2010) Efficacy of oral prednisolone and dexamethasone in horses with recurrent airway obstruction in the presence of continuous antigen exposure. Equine Vet J 42 (4), 316-321 PubMed.
  • Laumen E, Doherr M G & Gerber V (2010) Relationship of horse owner assessed respiratory signs index to characteristics of recurrent airway obstruction in two Warmblood families. Equine Vet J 42 (2), 142-148 PubMed.
  • Berndt A, Derksen F J, Venta P J et al (2009) Expression of toll-like receptor 2 mRNA in bronchial epithelial cells is not induced in RAO-affected horses. Equine Vet J 41 (1), 76-81 PubMed.
  • Lavoie J-P, Thompson D, Hamilton E et al (2008) Effects of a MAPK p38 inhibitor on lung function and airway inflammation in equine recurrent airway obstruction. Equine Vet J 40 (6), 577-583 PubMed.
  • Bakos Z (2008) Digital luminescence thoracic radiography in horses with recurrent airway obstruction. Vet Rec 162 (4), 122-124 PubMed.
  • Barrelet A (2007) Laboratory investigation of poor performance in horses: Part 2 - Investigation of respiratory disease. UK Vet 12 (1), 15-21 VetMedResource.
  • Camargo F C et al (2007) Trimetoquinol: bronchodilator effects in horses with heaves following aerolised and oral administration. Equine Vet J 39 (3), 215-220 PubMed.
  • Jost U et al (2007) A region on equine chromosome 13 is linked to recurrent airway obstruction in horses. Equine Vet J 39 (3), 236-241 PubMed.
  • Camargo F C et al (2006) Intravenous and intratracheal administration of trimetoquinol, a fast-acting short-lived bronchodilator in horses with 'heaves'. Equine Vet J 38 (6), 563-569 PubMed.
  • Cesarini C, Hamilton E, Picandet V & Lavoie J P (2006) Theophylline does not potentiate the effects of a low dose of dexamethasone in horses with recurrent airway obstruction. Equine Vet J 38 (6), 570-573 PubMed.
  • Deaton C M et al (2006) Comparison of the antioxidant status in tracheal and bronchoalveolar epithelial lining fluids in recurrent airway obstruction. Equine Vet J 38 (5), 417-422 PubMed.
  • Ghio A J, Mazan M R, Hoffman A M & Robinson N E (2006) Correlates between human lung injury after particle exposure and recurrent airway obstruction in the horse. Equine Vet J 38 (4), 362-367 PubMed.
  • Lavoie J-P et al (2006) Lack of clinical efficacy of a phosphodiesterase-4 inhibitor for treatment of heaves in horses. J Vet Intern Med 20 (1), 175-181 PubMed.
  • Laan T T J M, Bull S, Pirie S R, Fink-Gremmels J (2006) The role of alveolar macrophages in the pathogenesis of recurrent airway obstruction in horses. J Vet Intern Med 20 (1), 167-174 PubMed.
  • Simonen-Jokinen T et al (2005) Effect of composition and different fractions of hay dust suspension on inflammation in lungs of heaves-affected horses: MMP-9 and MMP-2 as indicators of tissue destruction. Equine Vet J 37 (5), 412-417 PubMed.
  • Deaton C M et al (2005) Antioxidant and inflammatory responses of healthy horses and horses affected by recurrent airway obstruction to inhaled ozone. Equine Vet J 37 (3), 243-249 PubMed.
  • Gerber V, Lindberg A, Berney C & Robinson N E (2004) Airway mucus in recurrent airway obstruction - short-term response to environmental challenge. J Vet Intern Med 18 (1), 92-97 PubMed.
  • Rossdale P D et al (2004) Pulmonary leiomyosarcoma in a 13-year-old Thoroughbred stallion presenting as a differential diagnosis to recurrent airway obstruction. Equine Vet Educ 16 (1), 21-28 VetMedResource.
  • Herholz C, Straub R, Braendlin C et al (2003) Measurement of tidal breathing flow-volume loop indices in horses used for different sporting purposes with and without recurrent airway obstruction. Vet Rec 152 (10), 288-292 PubMed.
  • Kolm G, Zappe H, Schmid R et al (2003) Efficacy of Montelukast in the treatment of chronic obstructive disease in five horses. Vet Rec 152 (26), 804-806 PubMed.
  • van Den Hoven R, Zappe H, Zitterl-Eglseer K et al (2003) Study of the effect of Bronchipret on the lung function of five Austrian saddle horses suffering recurrent airway obstruction (heaves). Vet Rec 152 (18), 555-557 PubMed.
  • Picandet V, Leguillette R & Lavoie J P (2003) Comparison of efficacy and tolerability of isoflupredone and dexamethasone in the treatment of horses affected with recurrent airway obstruction ('heaves'). Equine Vet J 35 (4), 419-424 PubMed.
  • Aviza G A et al (2001) Outcome of horses diagnosed with and treated for heaves. Equine Vet Educ 13 (5), 243 VetMedResource.
  • Robinson N E et al (2001) Management of heaves. Equine Vet Educ 13 (5), 247 VetMedResource.
  • Brazil T J & McGorum B C (2001) Molecules and mediators of inflammation in equine heaves - mechanisms and markers of disease. Equine Vet J 33 (2), 113-115 PubMed.
  • Raulo S M et al (2001) MMP-9 as a marker of inflammation in tracheal epithelial lining fluid (TELF) and in bronchoalveolar fluid (BALF) of COPD horses. Equine Vet J 33 (2), 128-136 PubMed.
  • Pirie R S et al (2001) Pulmonary and systemic effects of inhaled endotoxin in control of heaves horses. Equine Vet J 33 (3), 311-318 PubMed.
  • Robinson N E (2001) International workshop on equine chronic airway disease. Equine Vet J 33 (1), 5-19 PubMed.
  • Mair T S & Derksen F J (2000) Chronic obstructive pulmonary disease - a review. Equine Vet Educ 12, 35-44 VetMedResource.
  • Duvivier D H, Votion D, Roberts C A et al (1999) Inhalation therapy of equine respiratory disorders. Equine Vet Educ 11 (3), 124-131 VetMedResource.
  • Duvivier D H, Bayly W M, Votion D et al (1999) Effects of inhaled dry powder ipratropium bromide on recovery from exercise of horses with COPD. Equine Vet J 31 (1), 20-24 PubMed.
  • Vandeput S et al (1998) Environmental control to maintain stabled COPD horses in clinical remission - effect on pulmonary function. Equine Vet J 30, 93-96 PubMed.
  • Duvivier D H et al (1997) Airway responses of horses with COPD to dry powder inhalation of ipratropium bromide. Vet J 154, 149-153 PubMed.
  • Votion D et al (1997) Aerosol deposition in equine lungs following ultrasonic nebulization versus jet aerosol delivery system. Equine Vet J 29, 388-393 PubMed.
  • Burrel M H et al (1996) Respiratory disease in Thoroughbred horses in training - the relationship between disease and viruses, bacteria and environment. Vet Rec 139, 308-313 PubMed.
  • Derksen F J (1996) Use of a hand-held, metered-dose aerosol delivery device to administer pirbuterol acetate to horses with 'heaves'. Equine Vet J 28, 306-310 PubMed.
  • Moore B R (1996) Lower respiratory tract disease. Vet Clin N Amer Eq Pract 12, 457-472 PubMed.
  • Robinson N E et al (1996) The pathogenesis of chronic obstructive pulmonary disease of horses. Br Vet J 152, 283-306 PubMed.
  • Dixon P M, Railton D I & McGorum B C (1995) Equine pulmonary disease - a case control study of 300 referred cases (Parts 1-4). Equine Vet J 27 (6), 416-439 PubMed Part 1 Part 2 Part 3 Part 4.
  • Mair T S (1995) Changing concepts of COPD. Equine Vet J 27 (6), 402-403 PubMed.
  • Erickson D F et al (1994) Clinical efficacy and safety of clenbuterol HCl when administered to effect in horses with chronic obstructive pulmonary disease (COPD). EquineVet J 26, 331-336 PubMed.
  • Tesarowski D B, Viel L, McDonell W N & Newhouse M T (1994) The rapid and effective administration of a b2 agonist to horses with heaves using a compact inhalation device and metered-dose inhalers. Canadian Vet J 35, 170-173 PubMed.
  • Woods P S A et al (1993) Airborne dust and aeroallergen concentration in a horse stable under two different management systems. Equine Vet J 25, 208-213 PubMed.
  • Bracher V et al (1991) An investigation into the incidence of chronic obstructive pulmonary disease (COPD) in random populations of Swiss horses. Equine Vet J 23 (2), 136-141 PubMed.
  • Mart E, Gerber H, Essich G et al (1991) The genetic basis of equine allergic diseases 1. Chronic hypersensitivity bronchitis. Equine Vet J 23 (6), 457-460 PubMed.
  • Kaup F J, Drommer W & Deegen E (1990) Ultrastructural findings in horses with chronic obstructive pulmonary disease (COPD) (Parts I and II). Equine Vet J 22 (5), 343-355 PubMed Part 1 Part 2.
  • Clarke A F (1987) A review of environmental and host factors in relation to equine respiratory disease. Equine Vet J 19 (5), 435-441 PubMed.
  • Webster A J F et al (1987) Air hygiene in stables 1 - Effects of stable design, ventilation and management on the concentration of respirable dust. Equine Vet J 19 (5), 448-453 PubMed.

Other sources of information

  • McGorum B (1998) Inflammation and COPD in Horses. In: Proc World Equine Airway Symposium (detailed pathophysiology of inflammatory process in COPD).
  • Robinson N E (1998) COPD - Past, Present and Future. In: Proc World Equine Airway Symposium.
  • Robinson N E (1997) Pharmacology of the Equine Tracheobronchial Tree. In: Proc 43rd Annual AAEP Conv. pp 95-105.
  • Robinson N E (1997) Pathogenesis and Management of Airway Disease. In: Proc 43rd Annial AAEP Conv. pp 106-115.

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