Equis ISSN 2398-2977

Lung: inflammatory airway disease

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

Contributor(s): Prof David Hodgson, Jennifer Hodgson, Jacquelyn McCann, Nicola Menzies-Gow

Introduction

  • Common disorder of young performance horses; also occurs in older performance horses.
  • Less severe form of airway inflammation than equine asthma Equine asthma.
  • Horses with IAD 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 infectious (direct irritants and aero-allergens) and infectious (predominantly bacterial) causes.
  • Signs: poor performance, coughing, nasal discharge.
  • Diagnosis: history and clinical signs; endoscopy of the trachea revbeals 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 inhaled corticosteroids; increase airway diameter bronchodilators; appropriate antimicrobials if bacteria are involved.
  • Prognosis: good if appropriate treatment and control strategies implemented; development of chronic disease, and relationship to equine asthma, poorly understood.

Pathogenesis

Etiology

  • Controversial.
  • Can be divided into non-infectious and infectious causes (there is debate whether both syndromes should be included within the term IAD).
  • 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 IAD have not been identified except endotoxin.
  • The role of specific aeroallergens in IAD, 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 IAD include,Streptococcus equi ss zooepidemicus  Streptococcus spp  ,S. pneumoniae, members of the Pasteurellaceae (includingPasteurellaspp   Pasteurella multocida  andActinobacillusspp),Mycoplasmaspp   Mycoplasmas and ureaplasmas  , andBordatella 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

General

  • There may be a genetic predisposition to IAD similar to equine asthma Equine asthma, but this has not been investigated.

Specific

  • A number of management and training practices predispose racehorses 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)  .

Pathophysiology

  • 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 term IAD.  However, different pathways may   →    airway inflammation involving different cell types.
  • IAD 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 withS. equi ss zooepidemicusandS. pneumoniaeare 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 IAD in young performance horses with equine asthma in older horses is not known, but IAD may be a precursor of heaves in some circumstances.

Timecourse

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

Epidemiology

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

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • 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 (www.ivis.org), 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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