Bit & Bridle

Equine RAO

Other Names

  • Heaves
  • Broken wind
  • Chronic airway reactivity
  • Equine asthma (severe form)
  • Chronic obstructive pulmonary disease (COPD) — older terminology
  • Chronic small airway disease

What is Equine RAO?

Recurrent Airway Obstruction (RAO) — now more commonly classified under the umbrella term equine asthma — is a chronic inflammatory respiratory disease characterised by bronchoconstriction, excess mucus production, and airway inflammation. In severe cases it can cause significant respiratory distress and substantially reduce athletic performance and quality of life.

RAO most commonly affects stabled horses over the age of seven, though younger horses can be affected. It is triggered by hypersensitivity to inhaled organic dusts, mould spores, and endotoxins, particularly those associated with hay and straw. A related condition, Summer Pasture-Associated Obstructive Pulmonary Disease (SPAOPD), affects horses at grass and is triggered by airborne pollens and moulds rather than stable dust.

The condition is considered the equine equivalent of asthma in humans — an episodic, reversible airway obstruction driven by inflammation and bronchoconstriction. With appropriate environmental management and medical treatment, most affected horses can be managed effectively. For related respiratory conditions, see our overview of Equine Nasal Discharge.

Causes

RAO results from a hypersensitivity response to inhaled antigens. The principal triggers in stabled horses are:

  • Mould spores — particularly those from Aspergillus fumigatus, Faenia rectivirgula, and Thermoactinomyces vulgaris, present in hay and straw
  • Organic dust — microscopic particles of plant material, feed, and bedding inhaled during normal stable activities
  • Endotoxins — lipopolysaccharide components of bacterial cell walls found in round bale hay are particularly potent triggers
  • Ammonia — from urine breakdown in poorly ventilated stables, which irritates and sensitises the airways

Horses with RAO show an exaggerated immune response to these antigens, resulting in persistent neutrophilic inflammation of the airways. Genetic factors contribute — certain horse families show higher susceptibility — suggesting a heritable component to airway hyper-reactivity.

Symptoms and Signs

Clinical signs range from subtle performance reduction to severe respiratory distress. In mild to moderate disease:

  • Occasional coughing, particularly at the start of exercise or when entering the stable
  • Mildly increased resting respiratory rate
  • Bilateral mucopurulent nasal discharge
  • Reduced exercise tolerance and poor performance
  • Mild to moderate flaring of the nostrils during exercise

In severe or advanced disease:

  • Pronounced respiratory effort at rest — visible heaving of the flanks with each breath
  • Development of a “heave line” — a hypertrophied line of muscle along the ventral abdomen from chronic respiratory effort
  • Marked nostril flaring even at rest
  • Prolonged, forced expiration — the horse may appear to push air out with a double effort
  • Audible wheezing or crackling sounds on thoracic auscultation
  • Weight loss and muscle wasting in chronic, poorly managed cases

Diagnosis

Diagnosis is based on the characteristic clinical presentation, history, and response to treatment, supported by the following investigations:

  • Rebreathing examination — a plastic bag briefly placed over the muzzle to force deeper breathing amplifies airway sounds, allowing assessment of crackles and wheezes
  • Bronchoalveolar lavage (BAL) — fluid is instilled and retrieved from the lower airways via a flexible endoscope or blind technique. Cytological analysis of the recovered cells is the most accurate way to confirm airway inflammation and classify its type (neutrophilic in RAO, eosinophilic in allergic forms).
  • Endoscopy — allows direct visualisation of the trachea and carina, assessment of mucus quantity and character, and identification of excess mucus pooling
  • Thoracic auscultation — using a stethoscope to identify abnormal breath sounds, especially at the periphery of the lung fields
  • Spirometry and flow-volume loops — functional tests of lung capacity available at referral centres

Treatment

Treatment is two-pronged: environmental management to remove the triggers, and medical therapy to control inflammation and bronchoconstriction.

Environmental management (most important):

  • Turn the horse out to pasture as much as possible — even partial turnout significantly reduces dust exposure
  • If stabling is necessary, maximise ventilation — open all windows, doors, and ridge vents
  • Replace hay with haylage, steamed hay (using a commercial hay steamer), or soaked hay — soaking for 30 minutes reduces spore counts significantly; steaming is more effective and maintains nutritional value
  • Replace straw bedding with dust-free alternatives such as rubber matting, paper bedding, or shavings from kiln-dried sources
  • Do not store hay or straw in the same airspace as the affected horse
  • Avoid mucking out while the horse is in the stable

Medical treatment:

  • Bronchodilators — inhaled or systemic clenbuterol relaxes airway smooth muscle and provides rapid symptom relief. Inhaled bronchodilators (salbutamol, ipratropium) via equine aerosol mask are increasingly used and offer targeted delivery with fewer systemic effects.
  • Corticosteroids — systemic dexamethasone or prednisolone is highly effective for acute exacerbations. Inhaled fluticasone or beclomethasone can provide effective long-term airway anti-inflammatory control with reduced systemic side effects.
  • Mucolytics — dembrexine (oral) or N-acetylcysteine can help reduce mucus viscosity and improve mucociliary clearance

The British Equine Veterinary Association provides up-to-date guidance on equine asthma management at beva.org.uk.

Prevention

The cornerstone of prevention is minimising dust and mould exposure in the horse’s environment:

  • Feed high-quality, low-dust forage — steamed or haylage where possible
  • Use dust-extracted or dust-free bedding
  • Maximise turnout, especially for horses with a known history of airway sensitivity
  • Ensure stables are well-ventilated — stagnant air dramatically increases mould and ammonia levels
  • Keep horses away from round bale hay sources, which consistently show high endotoxin and dust levels

Prognosis

RAO is a manageable but not curable condition. Horses with well-controlled environments and appropriate medical therapy can remain in full work for many years. However, without adequate dust control, the condition progresses and can ultimately result in permanent structural changes to the airways, making management increasingly difficult. Horses with severe, chronic RAO that are unresponsive to treatment or whose management cannot be optimised may face significant welfare concerns.

When to Call the Vet

  • Your horse develops a persistent cough, nasal discharge, or reduced exercise tolerance
  • Breathing appears laboured at rest, or the respiratory rate is elevated
  • You notice the characteristic “heave line” along the belly
  • An existing RAO case deteriorates despite environmental management
  • You wish to confirm the diagnosis with a BAL or endoscopy before starting long-term treatment

Frequently Asked Questions

Is RAO the same as COPD in horses?

The terms are related but the terminology has evolved. “COPD” was used historically in equine medicine, borrowed from human respiratory medicine, but is now considered inaccurate as equine RAO is largely reversible — unlike human COPD. The current preferred term is equine asthma, with RAO representing the severe end of the equine asthma spectrum.

Can a horse with RAO still compete?

Many horses with well-managed RAO compete successfully. The key is ensuring dust exposure is minimised in the weeks leading up to competition and that any necessary medical treatment is within competition rules. Certain bronchodilators and corticosteroids are prohibited under FEI and BEVA regulations — discuss treatment plans with your vet if your horse competes under rules.

Does soaking hay actually help with RAO?

Soaking hay for at least 30 minutes in cold water significantly reduces the number of respirable mould spores. However, steaming is considerably more effective — commercial hay steamers reduce fungal counts by over 99% while preserving nutritional value. Soaking can leach water-soluble carbohydrates, which is useful for horses prone to laminitis, but does not match steaming for reducing allergen load.

What bedding is best for a horse with RAO?

Rubber matting with minimal bedding on top offers the lowest dust exposure. Where bedding is required, dust-extracted shavings or paper bedding are preferable to straw. Cardboard can be used but may become compacted. The key is choosing products specifically labelled as dust-extracted. See our stable management terminology for further guidance.