24 Jun 2025
Catriona Mackenzie
Job Title
Image: Halfpoint / Adobe Stock
Equine asthma (EA) is a condition of chronic lower airway inflammation, occurring as a result of susceptible horses being exposed to aerosolised allergens. It is a global disease and can be a major cause of poor performance and reduced quality of life. In the UK, where it is common for horses to be stabled for at least part of the year, the estimated prevalence of EA is 14% (Hotchkiss et al, 2007).
While the exact pathogenesis remains incompletely defined, a multifactorial process, including genetic susceptibility, environmental exposures and the immune response, is suspected (Couëtil et al, 2020). Disease occurs when predisposed individuals are exposed to respirable allergens in the environment.
The airway hyper-reactivity that occurs results in bronchospasm and mucous accumulation, leading to airway narrowing and reduced airflow. Clinical signs vary depending on the severity of airway inflammation and remodelling, but can include coughing, nasal discharge, increased respiratory effort at rest, exercise intolerance and poor performance. Within the stable environment, multiple agents can act as respirable aero-allergens and contribute towards the aetiology of disease. Feed and bedding materials are the main sources of respirable dust particles, which include mould spores, mites, endotoxins, antigenic material and pollen (Robinson et al, 2006; Pirie, 2014).
As a result, exposure to these environmental triggers increases during winter months when horses tend to spend more time stabled. An increase in EA is also recognised throughout mid to late spring, with an increase in environmental allergens such as tree or grass pollens.
To successfully manage EA, a holistic approach must be taken. The two main therapeutic strategies that are required are environmental management and pharmacological treatment. Pharmacological therapy is aimed at decreasing bronchospasm and reducing lower airway inflammation and mucous production. However, it is now well established that environmental management aimed at reducing exposure to inciting aero-allergens is the key component in the control of airway inflammation and dysfunction in horses with asthma.
Ongoing monitoring and regular follow ups to assess treatment efficacy and maintain compliance will also help to ensure optimal clinical outcomes.
Effective environmental management is the primary goal of long-term management and is fundamental to maintaining remission once control has been achieved. Studies have shown that clinical signs can rapidly improve in a low-dust environment, even without the use of medication (Holcombe et al, 2001; Couëtil and Ward, 2003). Therefore, effective management of the horse’s environment is fundamental in reducing clinical signs and improving outcomes. To achieve successful environmental control, all aspects of management must be evaluated. Housing facilities, bedding materials, stable management and feeding should all be modified with the aim of reducing exposure to airborne triggers.
It has been demonstrated that feed, in particular forage, has the most significant effect on respirable dust concentrations in the horse’s breathing zone (Clements and Pirie, 2007). Hay, which is the most common forage source in stabled horses and is also often used to supplement the feeding of horses on pasture, tends to have high concentrations of dust particles, moulds and endotoxins, and has been associated with airway inflammation in both healthy and asthmatic horses (Di Pietro et al, 2022).
In most cases, the optimal solution would be the provision of permanent pasture with no requirement for supplementary forage (Jackson et al, 2000). However, this is rarely an option – particularly over the winter months, when grass availability is poor. Other strategies must, therefore, be implemented aiming to reduce the impact of forage on horses with EA. The type of forage being fed, the forage quality, how it is stored and methods of feeding can all affect hygienic quality and the concentration of respirable allergens.
Hay quality and methods of storage can both have a profound impact on EA; for example, hay that has been baled and stored with a higher moisture content has been recognised as a course of organic particles and allergens that can act as triggers in susceptible horses (McGorum et al, 1993). Hay should be preserved with a dry matter content of greater than 85% in an attempt to reduce fungal and microbial growth.
The most commonly used method of reducing the respiratory challenge associated with forage is hay soaking. Full immersion for 60 minutes has been shown to reduce respirable dust exposure by approximately 60% (Blackman and Moore-Colyer, 1998; Clements and Pirie, 2007). Soaking should be timed to avoid the hay drying out prior to or during feeding, which would result in the release of respirable particles.
Another alternative is steaming, which has been demonstrated to reduce respirable particles by up to 90% using a commercial hay steamer (Moore-Colyer et al, 2016). It is important to consider that soaking, and to a lesser extent steaming, can reduce the nutritional value of hay, with leaching of nutrients, minerals and vitamins; therefore, dietary supplementation may be required.
An alternative to soaking or steaming hay is the use of haylage. Good quality haylage has been shown to have lower concentrations of allergens and other respirable particles (Séguin et al, 2012; Siegers et al, 2018). Haylage has the additional benefit of being less labour intensive than soaking or steaming, but its greater nutritional value and ability to induce a higher postprandial insulinaemic response makes it unsuitable for horses with obesity or insulin dysregulation (Diez de Castro and Fernandez-Molina, 2024). A further option is the use of dust-free, complete hay replacement fibre pellets. This can be very effective in terms of reducing respirable particles and has been used effectively to control EA (Woods et al, 1993). Cost and palatability tend to prohibit this as a practical long-term solution, but they can be useful as a supplementary feed for horses kept predominantly at pasture (Leclere et al, 2012).
Methods of feeding have also been shown to have some impact on the respiratory challenge. Feeding from a haynet can result in a four-fold increase in exposure to respirable dust compared to feeding from the ground (Ivester et al, 2014). Interestingly, a more recent study has shown that using hay nets hung outside open-fronted stables reduced challenge to a level similar to feeding from the floor (Ivester et al, 2018).
Choice of bedding can significantly impact air quality within the stable and is another key factor in the management of EA. Changing bedding from straw to low-dust cardboard material has been shown to reduce respirable dust levels by around 50% (Kirschvink et al, 2002).
Other low-dust options include dust-extracted shavings, paper and wood pellets (Clements and Pirie, 2007; Beeler-Marfisi et al, 2010). In addition to dust concentrations, other potential triggers, such as fungi and ammonia, should be taken into account; for example, studies have shown that wood shavings and paper bedding result in lower fungal concentrations compared to straw (Diez de Castro and Fernandez-Molina, 2024). Deep litter beds should be avoided, and stables should be mucked out and cleaned regularly to help prevent the build up of respirable particles such as ammonia.
It is important to consider not only the stable of the horse with EA, but also neighbouring stables and the surrounding environment. Factors such as barn activity and traffic, storage of feed or bedding materials, barn design and ventilation can all affect dust exposure. Dust exposure is increased during times of peak activity levels; for example, at times of feeding and mucking out (Millerick-May et al, 2011). Removing horses with EA from the barn during these times (for example, coinciding with periods of turnout) can help to reduce exposure to dust. Similarly, grooming should be performed in well-ventilated areas to prevent the accumulation of dust within a confined space.
Adequate ventilation is essential for maintaining air quality, as it reduces dust particles and concentrations of harmful gases such as ammonia. A more open stable design and opening of barn windows and doors can both help to improve ventilation and decrease exposure to respirable particles (Ivester et al, 2012; Rosenthal et al, 2006; Millerick-May et al, 2013).
Maximising turnout time can also help to improve respiratory health. Respirable, total particulate and endotoxin levels measured in the breathing zone at pasture have been shown to be significantly less than when a horse is stabled in a low-dust environment (McGorum et al, 1998; Berndt et al, 2010). However, it is important to consider seasonal allergens such as pollens, which can also act as aero-allergens in susceptible horses.
The success of environmental management is heavily dependent on owner compliance; therefore, educating owners about the importance of these measures, and providing clear, practical guidelines, will lead to improved outcomes in cases of EA. Simões et al (2020) demonstrated poor owner compliance in the management of severe EA and emphasised that clinical signs and the need for pharmacological management were related to the successful implementation of environmental strategies.
Alongside environmental management, pharmacological therapy is commonly required for the effective control of EA. Treatment is based on the use of bronchodilators to control bronchoconstriction, and reducing airway inflammation through the use of corticosteroids.
Medical management is frequently used to provide rapid symptomatic relief following initial diagnosis or acute exacerbations of EA. However, where environmental changes are not sufficient in controlling disease and respiratory triggers cannot be fully eliminated, longer-term reliance on medical management may be required.
Horses with EA can experience significant airway narrowing as a consequence of airway inflammation, bronchospasm and mucous accumulation, as well as the potential for airway smooth muscle remodelling in the longer term. Bronchodilators are, therefore, an essential part of rescue therapy, reducing bronchoconstriction and improving clinical signs in acute exacerbations.
Further potential benefits of bronchodilation include enhanced clearance of mucus and allowing better delivery of aerosolised drugs to the lower airways. Although effective in alleviating the acute clinical signs and improving lung mechanics, bronchodilators will not reduce the underlying inflammation; therefore, it is essential they be used in conjunction with corticosteroids and environmental management.
Both sympathomimetic and parasympathomimetic drugs can be used to achieve bronchodilation. However, as airway smooth muscle is generally under parasympathetic control, the latter are more effective.
Due to similar potency, but reduced risk of systemic side effects compared to atropine, N-butylscopolamine (buscopan) is now considered the preferred drug for rescue therapy. N-butylscopolamine has been shown to have a rapid onset of action, improving clinical and pulmonary function testing within 10 minutes (Couëtil et al, 2012). Its short duration of action (30 to 60 minutes) make it more suitable as a rescue therapy and less appropriate for ongoing treatment. For short-term bronchodilator therapy early in the course of disease, beta-2 agonists (for example, inhaled salbutamol or oral clenbuterol) are considered to be more suitable options (Calzetta et al, 2020).
In addition to bronchodilator therapy, anti-inflammatory drugs are indicated to reduce airway inflammation. Due to their potent anti-inflammatory effects, corticosteroids are still considered to be the most-effective treatment for EA and can be administered either systemically or by inhalation (Couëtil et al, 2020).
Systemic corticosteroids, most commonly dexamethasone or prednisolone, have been shown to rapidly and effectively reduce inflammation, improving clinical signs and lung function (Couëtil et al, 2016). They are typically used for short-term management – particularly during acute exacerbations or in refractory cases. While very effective, the potential for serious side effects, such as immunosuppression, suppression of the hypothalamic-pituitary-adrenal (HPA) axis and laminitis, limits the long-term use of systemic corticosteroids (Rush et al, 1998).
The administration of inhaled corticosteroids allows direct drug delivery to the site of inflammation while reducing the risk of systemic side effects; therefore, it is preferable for longer-term use (Mainguy-Seers and Lavoie, 2021).
Options for inhaled corticosteroid therapy include fluticasone propionate or beclomethasone via a metered dose inhaler, budesonide via nebulisation or ciclesonide via a soft-mist inhaler. Nebulised dexamethasone was previously recommended; however, studies have shown a lack of efficacy with this treatment (Mainguy-Seers et al, 2019; de Wasseige et al, 2021). Similarly, beclomethasone is also now used less frequently due to reduced potency in comparison to fluticasone (Léguillette et al, 2017).
Where more severe inflammation is present, drug delivery via the inhaled route may be compromised due to bronchospasm and mucus accumulation; therefore, systemic treatment tends to be more effective. It is also worth noting that nebulised dexamethasone has been shown to cause HPA suppression (indicating systemic absorption) in horses with severe EA, but not in healthy horses, suggesting breakdown in normal pulmonary epithelial barriers (Mainguy-Seers and Lavoie, 2021). This has only been evaluated for dexamethasone, so does not necessarily apply to other corticosteroids.
While bronchodilators and corticosteroids are the mainstay of pharmacological management of EA, other adjunctive therapies can be beneficial in some cases. Supplementation with omega-3 fatty acids may help to modulate inflammatory responses and has been demonstrated to improve clinical signs and lung function in horses with EA (Nogradi et al, 2015; Diez de Castro and Fernandez-Molina, 2024).
Inhaled mast cell stabilisers, such as sodium cromoglycate, have been used in the treatment of airway inflammation and have been shown to improve clinical signs in horses with high mast cell counts on bronchoalveolar lavage fluid cytology (Hare et al, 1994). Mucolytics (for example, N-acetylcysteine) can be used to reduce mucus viscosity and facilitate clearance in certain cases. However, evidence for their use is lacking.
In summary, EA is a multifactorial condition that requires a holistic approach to achieve successful management. This is particularly important during winter months, when more prolonged periods of stabling can result in increased exposure to environmental triggers.
Environmental strategies must be comprehensive and encompass all areas of management, and educating owners on the importance of these measures is key to enhancing compliance and improving outcomes.
By addressing environmental factors, in addition to implementing appropriate therapeutic interventions, and providing ongoing care and monitoring, we can significantly improve respiratory health and overall outcomes in horses with EA.
