Equine asthma

What it is

Equine asthma is a chronic inflammatory disease of the lower airways that develops when the horse inhales particles from its environment: organic dust from hay and straw, mould spores, endotoxins, β-glucans and forage mites. The bronchi and bronchioles develop bronchospasm, excess mucus and inflammation that, if it persists, leads to remodelling of the bronchial wall (smooth muscle hypertrophy and fibrosis). It is not an infection and is not contagious.

Two forms are recognised, now grouped under the term equine asthma:

• Mild-moderate asthma (formerly inflammatory airway disease, IAD): occasional cough and poor performance, without breathing difficulty at rest.
• Severe asthma (formerly recurrent airway obstruction, RAO, or heaves): episodes of airflow obstruction with increased breathing effort at rest, which resolve when the cause is removed and return with new exposures. There is a summer pasture-associated variant in hot, humid climates. There is a hereditary component.

Who it affects

• Mild-moderate asthma: horses of any age, often young sport horses or racehorses kept in stables.
• Severe asthma: mainly adult and older horses, stabled and fed dry hay, worsening in winter when they spend more time indoors.
• Risk factors include stabling, poor-quality hay, straw bedding, poor ventilation and age.

Anatomy involved

• Lungs: the horse lung shows hardly any external lobation (cranial and caudal lobes on each side and an accessory lobe on the right) and its lobules are poorly separated, so it has good collateral ventilation. The lesion lies in the bronchioles: smooth muscle contraction, mucus plugs and neutrophils in the lumen. Obstruction of the small airways traps air and forces active expiration.
• Trachea: mucus coming up from the bronchi pools on the ventral floor of the trachea, especially at the thoracic inlet, where it is seen and scored by endoscopy. The tracheal wash is collected through it, and the endoscope or tube for bronchoalveolar lavage is passed along it.
• External abdominal oblique muscle: in severe asthma expiration is no longer passive and the abdominal muscles push at the end of each breath. Over time the external oblique hypertrophies and forms a ridge along the costal arch, the heave line, a sign of chronic disease.

Clinical signs

• Mild-moderate asthma: intermittent cough, especially at the start of exercise or when eating, poor performance, slow recovery after exercise and sometimes scant nasal discharge. Resting respiratory rate and effort are normal and there is no fever.
• Severe asthma: increased breathing effort at rest, with flared nostrils, a biphasic expiration and a heave line; frequent cough, mucoid nasal discharge, exercise intolerance and, in chronic cases, weight loss. Wheezes and crackles are heard on auscultation. There is no fever either, which helps distinguish it from an infection.

Diagnosis

• History and examination: link between signs and stabling or hay, and absence of fever and systemic signs.
• Airway endoscopy: amount of mucus in the trachea.
• Bronchoalveolar lavage cytology, the key test. In mild-moderate asthma more than 10% neutrophils, or more than 5% mast cells or eosinophils, is considered abnormal; in severe asthma neutrophils usually exceed 25%.
• Lung function tests and response to a bronchodilator, at specialist centres.
• Thoracic radiography and ultrasonography mainly serve to rule out other diseases.

Treatment and prevention

• Environmental control, the basis of treatment: more time at pasture, replacing dry hay with soaked or steamed hay, haylage or pelleted forage, shavings or paper bedding instead of straw, good ventilation, and not mucking out or handling hay with the horse inside.
• Corticosteroids: systemic (dexamethasone, prednisolone) to control exacerbations, and inhaled (fluticasone, beclomethasone, ciclesonide) with an inhaler or a mask designed for horses. Laminitis is a common concern with systemic corticosteroids, although the evidence for that risk is limited.
• Bronchodilators: β2-agonists such as oral clenbuterol or inhaled salbutamol (albuterol) to relieve obstruction, and intravenous N-butylscopolamine bromide in a crisis. They do not treat inflammation, so they should not be used alone.
• Asthma is not cured: the aim is sustained remission while the environment is controlled.
• Drug use in horses is subject to each country's regulations and to withdrawal periods. In the EU the horse is a food-producing species, and its identification document (passport) states whether it is excluded from slaughter for human consumption, which determines which drugs may be given. In competition, anti-doping rules also apply.

Key studies

• ACVIM consensus (Couëtil et al., 2016): revises the definition of the mild-moderate form (then IAD): cough, poor performance, excess tracheal mucus, inflammatory bronchoalveolar lavage cytology or pulmonary dysfunction, without fever, systemic signs or increased resting respiratory effort. It is expert opinion based on the literature, not a study.
• Hotchkiss et al. (2007), survey of owners randomly selected through veterinary practices in Great Britain, using a validated questionnaire. The estimated true prevalence of severe asthma was 14.0% (95% CI 10.7–17.4%). Age, an urbanised environment, hay exposure and respiratory infections early in life were associated with the disease. Diagnosis relied on what owners described, not on examination.
• Couëtil et al. (2005), randomised controlled trial in 28 horses with severe asthma kept outdoors on pelleted feed: inhaled fluticasone, oral prednisone or placebo for 4 weeks. All horses improved, and at 4 weeks those treated with corticosteroids did not differ from controls; in severe cases fluticasone sped up the improvement in lung function at 2 weeks. The authors conclude that environmental control is the most important factor. The sample is small.
• Pirie et al. (2021), multicentre, randomised, double-blind, placebo-controlled trial in 224 client-owned horses with severe asthma. After 10 days, 73.4% of horses treated with inhaled ciclesonide (80 of 109) improved by at least 30% in clinical score, versus 43.2% with placebo (48 of 111), with few adverse effects. There was a marked placebo effect, and several authors were employees of the drug's manufacturer.
• Pistocchini et al. (2025), meta-analysis of 4 trials with 252 asthmatic horses comparing inhaled corticosteroids with placebo. About 3 horses need to be treated for one to improve clearly (NNT 3.2; 95% CI 2.3–4.7), and the probability of improvement was 1.73 times that with placebo; GRADE rated the quality of evidence as high. The trials are few and short.
• Kinnison et al. (2022), systematic scoping review of the definition of mild-moderate asthma: 44 studies with 6,092 horses, mostly racing Thoroughbreds. They were mostly observational, with a median of 74 horses per study and a high risk of bias for statistical power and masking in almost half. Heterogeneity precluded meta-analysis. The link between cough and tracheal mucus was more consistent than that between lavage cytology and lung function: the definition of the mild-moderate form rests on weak evidence.