Bit & Bridle

Equine Asthma (Heaves)

Other Names

Heaves, recurrent airway obstruction (RAO), chronic obstructive pulmonary disease (COPD) — now largely replaced by the term equine asthma, inflammatory airway disease (IAD), small airway inflammatory disease (SAID). The umbrella term “equine asthma syndrome” is now preferred in veterinary literature.

What is Equine Asthma?

Equine asthma is a non-infectious, inflammatory condition of the lower respiratory tract characterised by airway inflammation, bronchoconstriction (narrowing of the airways), excessive mucus production, and in severe cases, structural changes to lung tissue. It is one of the most common causes of poor performance and respiratory distress in horses in the UK and is particularly prevalent in stabled horses exposed to dust and mould spores from hay and bedding.

The condition exists on a spectrum of severity. At the mild end, inflammatory airway disease (IAD) causes subtle signs — slightly reduced performance, occasional cough — without overt breathing difficulty at rest. At the severe end, the condition previously known as heaves or RAO causes significant breathing difficulty even at rest, exercise intolerance, and progressive structural lung damage.

The prevalence of equine asthma increases with age, and older horses (above 7–8 years) with prolonged stable-based management are most frequently affected. However, horses of any age can develop the condition, particularly following heavy exposure to dusty hay, fungal spores, or other airway irritants.

What Causes Equine Asthma?

The primary trigger in the UK and temperate climates is inhaled organic dust — particularly fungal spores (Aspergillus, Alternaria, Fusarium), mould, endotoxins (bacterial cell wall components), and forage mites from poor-quality hay and straw bedding. Indoor stabling concentrates these irritants to levels horses did not encounter in their evolutionary environment as open-grassland animals.

Other contributing factors include:

  • Ammonia fumes from poorly managed stable flooring and bedding
  • Viral upper respiratory tract infections that trigger ongoing airway inflammation
  • Environmental pollutants (diesel exhaust, crop dusts in agricultural areas)
  • Pasture-associated severe equine asthma — seen in late summer and autumn in some horses, triggered by outdoor fungal spores from cut hay or mould in the field environment
  • Genetic predisposition — some horses appear inherently more susceptible

The condition is not caused by a single allergen but by a complex inflammatory response to multiple organic particles. Mast cells, neutrophils, and eosinophils all play roles in the airway inflammation, and the condition shares similarities with human asthma and chronic obstructive pulmonary disease (COPD).

Symptoms and Signs

Signs range from mild to severe depending on the degree of airway inflammation and obstruction:

Mild (IAD):

  • Intermittent cough — often worse at the start of exercise or in dusty environments
  • Subtle reduction in exercise tolerance or performance
  • Occasional nasal discharge (usually bilateral and mucoid)
  • No respiratory distress at rest

Moderate to severe (classic heaves/RAO):

  • Persistent cough — productive, often with mucoid to mucopurulent nasal discharge
  • Laboured breathing at rest — increased respiratory rate and effort
  • Flared nostrils during exertion and at rest in severe cases
  • “Heave line” — a characteristic hypertrophied line of muscle along the flank, visible as a double expiratory effort as the horse uses abdominal muscles to push air out of stiffened airways
  • Reluctance to eat — coughing interrupts feeding
  • Weight loss in chronic, severe cases
  • Exercise intolerance — horses may stop during work due to breathing difficulty

The heave line is pathognomonic (uniquely characteristic) of chronic severe equine asthma and indicates longstanding, significant disease. Its presence suggests that structural changes in the lung parenchyma have occurred and that complete resolution may not be achievable.

Diagnosis

Diagnosis is based on clinical signs, history of management and environment, and diagnostic testing to confirm airway inflammation and rule out infectious causes.

Clinical examination: Lung auscultation — abnormal lung sounds including wheezes, crackles, and prolonged expiration. Percussion may reveal hyperinflation (increased resonance) in severe cases.

Endoscopy (bronchoscopy): A flexible endoscope is passed into the trachea and lower airways, allowing direct visualisation of mucus accumulation and airway inflammation. Excessive mucus in the trachea is a hallmark finding.

Bronchoalveolar lavage (BAL): Fluid is instilled into a lung segment via the endoscope and retrieved. The cellular composition of the lavage fluid confirms and characterises the inflammation — elevated neutrophils in severe asthma (heaves), elevated mast cells or eosinophils in mild-moderate forms.

Tracheal wash: Samples tracheal mucus for cytology and bacterial culture to exclude infectious tracheobronchitis.

Spirometry and lung function testing: Available at referral centres; quantifies airway obstruction objectively.

See our entry on Rhodococcus equi infection for an important differential diagnosis in young horses with respiratory signs.

Treatment

Treatment has two pillars: environmental management (reducing exposure to triggers) and medical management (controlling inflammation). Environmental change is essential — medication without reducing dust exposure provides only partial and temporary benefit.

Environmental management:

  • Maximise turnout on pasture — full-time turnout is the single most effective intervention for stable-associated equine asthma
  • If stabling is unavoidable, ensure maximum ventilation — open the top door, remove partitions, use a well-ventilated barn design
  • Replace dusty hay with haylage or vacuum-packed forage, or soak hay thoroughly (minimum 30 minutes, up to 60 minutes for severely affected horses)
  • Replace straw bedding with dust-extracted shavings, paper, or rubber matting
  • Avoid mucking out in the horse’s presence — dust peaks during yard work
  • Position feed from ground level to facilitate mucus drainage from the respiratory tract

Medical management:

Bronchodilators: Relax bronchospasm and provide rapid short-term relief. Clenbuterol (oral) is widely used in UK equine practice. Inhaled salbutamol (beta-2 agonist) can be used via equine spacer devices for acute relief. Long-term bronchodilator use without addressing the underlying inflammation is not appropriate.

Corticosteroids: Reduce airway inflammation. Systemic dexamethasone provides rapid, significant relief in severe cases. Inhaled fluticasone or beclomethasone via equine spacer devices are preferred for long-term management as they deliver the drug directly to the airway with minimal systemic absorption, reducing the risk of laminitis and other steroid-related side effects.

Mucolytics: Dembrexine helps thin and clear mucus from the airways and is used as an adjunct to primary treatment.

Prevention

  • Maximise turnout — horses evolved to spend most of their time outdoors
  • Feed top-quality, low-dust forage — haylage or tested hay from reputable sources
  • Choose low-dust or dust-free bedding materials (shavings, paper, cardboard, rubber matting)
  • Ensure excellent stable ventilation — a horse breathes up to 60,000 litres of air per day in its stable
  • Keep horses away from dusty areas during hay handling and mucking out
  • Monitor horses regularly for early signs of airway disease — a persistent cough is never normal

When to Call the Vet

  • Breathing difficulty at rest or laboured breathing after mild exertion
  • Persistent cough lasting more than 2 weeks despite environmental improvements
  • Nasal discharge that is thick, discoloured, or foul-smelling (may indicate secondary infection)
  • Development of a heave line — indicates significant, chronic disease requiring assessment
  • Sudden deterioration in a known asthmatic horse — may indicate acute exacerbation or secondary infection
  • Horse refuses to work or is clearly in respiratory distress

Prognosis

For mild IAD, the prognosis with appropriate environmental management is excellent — many horses return to full performance. For moderate-to-severe equine asthma (heaves), the condition is manageable but rarely fully curable. Horses with an established heave line have irreversible structural lung changes; they can be maintained in reasonable comfort and limited work with ongoing management, but high-level performance may be unrealistic. Prognosis is significantly better when the diagnosis is made early and environmental management is implemented promptly and consistently.

Frequently Asked Questions

Is equine asthma the same as COPD in horses?

COPD (chronic obstructive pulmonary disease) was the older term used for what is now called severe equine asthma or recurrent airway obstruction (RAO). The current preferred umbrella term is equine asthma syndrome, which encompasses conditions from mild inflammatory airway disease through to severe RAO/heaves. The renaming reflects improved understanding that the condition is primarily inflammatory and, unlike human COPD, is not caused by smoking but by inhaled organic dust.

Can I ride a horse with equine asthma?

It depends on severity. Horses with well-managed mild IAD can often remain in work with appropriate environmental management. Horses in an acute exacerbation of heaves — showing laboured breathing at rest — should not be exercised until clinically improved. For horses with moderate disease, work intensity should be reduced during exacerbations. Always discuss exercise management with your vet.

Does soaking hay really make a difference?

Yes, significantly. Soaking hay for 30–60 minutes reduces respirable dust, mould spores, and forage mites by up to 95% compared to dry hay. However, soaking reduces water-soluble carbohydrates (beneficial for weight-managing horses) and also leaches out some nutritional value. Haylage generally has even lower dust content than well-soaked hay and is an excellent alternative for asthmatic horses.

My horse’s asthma is better in summer — why?

This is the classic pattern of stable-associated equine asthma — symptoms improve dramatically when horses are turned out full-time in summer, because they are no longer exposed to the hay and straw dust that triggers airway inflammation in the stable. This pattern is itself diagnostically useful. If your horse’s asthma is worse outdoors in late summer, consider pasture-associated equine asthma triggered by outdoor fungal spores from mould on grass or cut forage.