ISSN 2398-2977      

Mild to moderate equine asthma


Synonym(s): MMEA, inflammatory airway disease (IAD), small airway disease, small airway inflammatory disease, lower respiratory tract inflammation, non-infectious inflammatory airway disease, bronchiolitis, allergic airway disease


  • Common disorder of young performance horses; also occurs in older performance horses.
  • Less severe form of airway inflammation than severe equine asthma Severe equine asthma.
  • Horses with mild to moderate equine asthma (MMEA) do not have obvious respiratory distress and usually have no systemic signs of illness, but show varying evidence of airway inflammation.
  • Etiology, pathophysiology and diagnosis of this disorder are currently controversial, and the term probably encompasses a number of different diseases that are associated with lower airway inflammation in young and older performance horses.
  • Cause: etiological agents include non-infectious (direct irritants and aero-allergens) and infectious (predominantly bacterial, but possibly viral) causes.
  • Signs: poor performance, coughing, nasal discharge.
  • Diagnosis: history and clinical signs; endoscopy of the trachea reveals increased mucous; cytological analysis of tracheal aspirate and/or bronchoalveolar lavage required for definitive diagnosis. 
  • Treatment: change environment to control exposure to dust and other airway irritants; control airway inflammation using inhaled or systemic corticosteroids; increase airway diameter using inhaled or systemic bronchodilators; appropriate antimicrobials if bacteria are involved.
  • Prognosis: good if appropriate treatment and control strategies implemented; development of chronic disease, and relationship to severe equine asthma, poorly understood.



  • Controversial.
  • Can be divided into non-infectious and infectious causes (there is debate whether both syndromes should be included within the term MMEA).
  • Non-infectious agents thought to cause lower airway inflammation include direct irritants and aeroallergens. Direct irritants include:
    • Dust.
    • Endotoxin.
    • Mites.
    • Ultrafine particles and air pollution. 
    • Blood from EIPH.
  • However, specific environmental agents associated with MMEA have not been identified except endotoxin.
  • The role of specific aeroallergens in MMEA, eg molds, mites, pollens, has not been elucidated. 
  • Infectious causes are predominantly bacterial Lung: pneumonia - bacterial, though less commonly parasitic and viral agents have also been shown to cause airway inflammation in young performance horses.
  • Bacteria associated with MMEA include, Streptococcus equi ss zooepidemicus Streptococcus spp, S. pneumoniae, members of the Pasteurellaceae (including Pasteurella spp Pasteurella multocida and Actinobacillus spp), Mycoplasma spp Mycoplasmas and ureaplasmas, and Bordatella bronchiseptica Bordetella bronchiseptica. Enterobacteriaceae Enterobacter / Aerobacter spp may become involved after antimicrobial therapy (superinfections). Anaerobes are rarely involved.
  • Equine respiratory viruses (equine influenza Equine influenza, equine herpes virus-4 and 1 Respiratory: EHV infection, equine rhinitis virus Rhinovirus infection A and B) are rare causes of acute lower airway inflammation in performance horses. Their relationship with chronic disease is poorly understood.

Predisposing factors


  • There may be a genetic predisposition to MMEA similar to severe equine asthma Severe equine asthma, but this has not been investigated.


  • A number of management and training practices predispose performance horses to development of lower airway inflammation.
  • Housing in stables with high exposure to airway irritants including dust, endotoxin, and aeroallergens (dusty hay, straw bedding, etc).
  • High intensity exercise, eg racing and training.
  • Transportation, especially long distance transportation.
  • Co-mingling with other horses.
  • Exercise induced pulmonary hemorrhage Lung: EIPH (exercise-induced pulmonary hemorrhage).


  • Controversial, especially the temporal relationship between potential etiological agents.
  • Airway inflammation may be regionalized and involve predominantly large airways (trachea +/- bronchi), predominantly small airways (bronchioles and alveoli) or both.
  • Inflammation involving different inflammatory cells, eg neutrophils, eosinophils, mast cells, or a mixture of these inflammatory cells, is currently included in the term MMEA.  However, different pathways may → airway inflammation involving different cell types.
  • MMEA involves airway inflammation, which may or may not → parenchymal disease, eg bacterial pneumonia. 
  • Acute exposure to airway irritants, eg endotoxin, or aeroallergens → airway inflammation → mild accumulation of mucus and inflammatory cells → subclinical disease.  
  • In some cases, accumulation of mucus in lower airways may predispose to bacterial colonization of the lower airways due to a decreased ability to clear bacteria → exacerbation of existing inflammation → plugging of airways with mucus and inflammatory cells → clinical disease and poor performance.
  • Primary bacterial infections with S. equi ss zooepidemicus and S. pneumoniae are also reported.
  • Chronic exposure to airway irritants may produce airway hyper-reactivity and airway plugging with mucus and inflammatory cells → bronchospasm → ventilation-perfusion mismatch → poor performance.
  • Relationship of MMEA in young performance horses with severe equine asthma in older horses is not known, but MMEA may be a precursor of Severe equine asthma in some circumstances.


  • Variable: both acute and chronic.
  • Chronological relationship of specific etiological agents is poorly understood.


  • Variation in prevalence of MMEA reported between stable complexes.
  • In general, stables with poorer ventilation have higher prevalence of MMEA and clinical signs are more prolonged in poor stable environments.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Gy C, Leclere M, Vargas A, Grimes C & Lavoie J P (2019) Investigation of blood biomarkers for the diagnosis of mild to moderate asthma in horses. J Vet Intern Med 33 (4), 1789-1795 PubMed.
  • Pirie R S (2017) Mild to moderate equine asthma: an overview. UK Vet Livestock MagOnlineLibrary.
  • Richard E A et al (2009) Influence of subclinical inflammatory airway disease on equine respiratory function evaluated by impulse oscillometry. Equine Vet J 41 (4), 384-389 PubMed.
  • Ramzan P H L, Parkin T D H & Shepherd M C (2008) Lower respiratory tract disease in Thoroughbred racehorses: Analysis of endoscopic data from a UK training yard. Equine Vet J 40 (1), 7-13 PubMed.
  • Barrelet A (2007) Laboratory investigation of poor performance in horses: Part 2 - Investigation of respiratory disease. UK Vet 12 (1), 15-21 VetMedResource.
  • Robinson N E, Karmaus W, Holcombe S J, Carr E A & Darksen F J (2006) Airway inflammation in Michigan pleasure horses: prevalence and risk factors. Equine Vet J 38 (4), 293-299 PubMed.
  • Mazan M R, Vin R & Hoffman A M (2005) Radiographic scoring lacks predictive value in inflammatory airway disease. Equine Vet J 37 (6), 541-545 PubMed.
  • Wood J L N et al (2005) Inflammatory airway disease, nasal discharge and respiratory infections in young British racehorses. Equine Vet J 37 (3), 232-235 PubMed.
  • Robinson N E et al (2003) Inflammatory airway disease: defining the syndrome. Conclusions of the Havemeyer WorkshopEquine Vet Educ 15 (2), 61-63 VetMedResource.
  • Christley R M et al (2001) A case-control study of respiratory disease in Thoroughbred racehorses in Sydney, Australia. Equine Vet J 33 (3), 256-264 PubMed.  
  • Holcombe S J et al (2001) Stabling is associated with airway inflammation in young Arabian horses.Equine Vet J 33 (3), 244-249 PubMed.
  • Chapman P S et al (2000) Retrospective study of the relationships between age, inflammation and the isolation of bacteria from the lower respiratory tract of thoroughbred horses. Vet Rec 14, 91-95 PubMed.
  • Raymond S L & Clarke A F (1998) Small airway disease and equine respiratory health. Aust Equine Vet 16 (1), 21-31 VetMedResource.
  • Viel L (1997) Small airway disease as a vanguard for chronic obstructive pulmonary disease. Vet Clin North Am Eq Pract 13, 549-560 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 VetMedResource.
  • 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.
  • 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.

Other sources of information

  • Newton J R (2004) Performance Effects of Lower Respiratory Tract Disease. In: Proc 43rd BEVA Congress. Equine Vet J Ltd, UK. pp 256.
  • Hodgson J L & Hodgson D R (2002) Inflammatory Airway Disease. In: Equine Respiratory diseases. Ed: Lekeux P. International Veterinary Information Services (, Ithaca, USA.
  • Hoffman A M (2002) Inflammatory Airway Diseases: Definitions and Diagnosis in the Performance Horse. In: Current Therapy in Equine Medicine 5. Ed: Robinson N E. W B Saunders Co, USA. pp 412-417.  
  • Hodgson J L & Hodgson D R (2002) Tracheal Aspirates: Indications, Technique, and Interpretation. In: Current Therapy in Equine Medicine 5. Ed: Robinson N E. W B Saunders Co, USA. pp 401-406.
  • Viel L & Hewson J (2002) Bronchoalveolar Lavage. In: Current Therapy in Equine Medicine 5. Ed: Robinson N E. W B Saunders Co, USA. pp 407-411.
  • Coeutil L L (2002) Aerosol Medications for the Management of Inflammatory Airway Disease (IAD). In: Proc Am Col Vet Int Med. pp 716-718.
  • Mazan M R (2002) Inflammatory Airway Disease Current Knowledge. In: Proc Am Col Vet Int Med. pp 707-709.
  • Rush BR (1999) Inflammatory Airway Disease. In: Proc 38th BEVA Congress. pp 144-145.
  • Newton J R & Wood J L N (1999) Summary of a Case Control Study of Acute Respiratory Disease in Young Thoroughbred Racehorses. In: Proc 38th BEVA Congress. pp 190-191.

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