Generally the disease is triggered following inhalation of an allergen. In the stabled horse, dust, pollens, molds and endotoxins present in hay and/or straw are the main culprits. The horse that lives in a field and that develops the summer pasture associated form appears to be more sensitive to field pollens. It is the prolonged exposure to these triggers that is at the origin of the chronicity of the disease.


RAO is characterised by inflammation of the lower respiratory airways. The disease is characterised by an increase in inflammatory cells and mucus production and can lead to chronic changes in the lung structure such as thickening of the airways. Furthermore, constriction of the airways occurs as a consequence to the inflammation, limits the passage of air and prevents the horse from breathing normally. 




The common symptoms are :

  • A cough which can be accompanied by mucus production
  • Occasional nasal discharge
  • Dilated nostrils, and increase in breathing effort and rate
  • Exercise intolerance, especially at the faster gaits

Most often these symptoms are more evident when the horse sustains a mild to moderate effort. There is no accompanying fever.




Your vet will start with a thorough clinical examination of your horse to check his vital parameters and his general appearance and demeanour. He will then examine his respiratory system. Inspecting the nostrils may reveal some discharge. Your vet will also watch the horse breathe to evaluate the effort and rate. He will also listen to the lungs to identify if there are any abnormal sounds. Don’t worry if he decides to use a bag on your horse’s nostrils to do this. This technique enables him to evaluate your horse’s response to the increased effort need to breath and will also exacerbates any abnormal breathing sounds.


To confirm his diagnosis your vet may perform further diagnostic tests :


  • An endoscopy to examine the respiratory tract
  • A bronchoalveolar lavage that allows him to recover cells from the bronchi and lungs
  • Less frequently venous and arterial oxygen and CO2 dosing and/or other function test to evaluate ventilation of the lungs.




To avoid an RAO bout, it is essential to limit inhalation of environmental allergens. The horse that lives outside and is allergic to field pollens will have to live inside or have limited access to the field. If it is not possible to winter your horse outside then measures will have to be taken to limit dust and moulds in the stable. Here are a few examples:


  • Choose good quality hay
  • Dampen dusty hay for at least ½ hour before feeding it
  • Use a hay bag
  • Remove dust regularly from the stable (including the ceiling). Don’t forget to do this when the horse is not inside
  • Prefer a dust free type bedding 



If the symptoms are deemed sever enough, your vet may prescribe drugs aswell as management techniques. These will include anti-inflammatories and bronchodilators. If your horse is severely distressed these products can be injected directly in to the vein and can be folloed up with oral or inhaled forms over a prolonged period of time.

In parallel or in prevention it is possible to provide nutritional support with ingredients that meet increased demands upon the respiratory system. There are products available that have been made specifically for horses that live in a dusty or pollen rich environment and that will provide suitable plants. This is for example the case of nigella (also called ‘black cumin’) whose efficacy no longer needs demonstrating.

Please consult your vet for further veterinary advise.



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Respiratory airway obstruction commonly still called heaves is considered today to be a form of equine asthma.


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