As winter approaches, many horses spend more time stabled, and forage such as hay, makes up a larger proportion of their diet. These changes can increase their exposure to airborne dust, mould spores, bacteria and other irritants, particularly when ventilation is poor. Horses breathe a substantial volume of air, even at rest, so repeated exposure to airborne particles can affect respiratory health over time.
The horse’s respiratory system
A horse’s respiratory system supplies the body with oxygen and removes carbon dioxide. Air enters through the nostrils, where it is warmed, moistened and filtered, before travelling through the throat and windpipe (trachea). The trachea divides into two main airways, called bronchi, which branch into progressively smaller tubes known as bronchioles. At the ends of these are tiny air sacs called alveoli, where oxygen passes into the bloodstream and carbon dioxide is removed to be breathed out. Horses are obligate nasal breathers, meaning they can only breathe through their nostrils rather than their mouths.
Natural protective defences
The respiratory tract has several natural defences to protect the lungs from dust, pathogens and other airborne particles. The airway lining produces mucus, which traps inhaled particles and microorganisms. Tiny hair-like structures called cilia then move this mucus upwards towards the throat, where it can be swallowed or expelled. This process is known as mucociliary clearance and forms an important part of the respiratory system’s first line of defence.
Coughing and sneezing can also help remove unwanted material from the airways. However, the absence of coughing does not necessarily mean a horse’s airways are healthy; some forms of airway inflammation can develop without an obvious cough.
If particles or pathogens pass these physical defences, the immune system provides another layer of protection. Immune cells, including macrophages, help engulf and break down foreign particles and microorganisms. When a potential threat is detected, the immune system can trigger inflammation and recruit additional immune cells to help control it. Although this response is essential for protecting the lungs, excessive or prolonged inflammation can damage airway tissues and interfere with normal breathing.
Importantly, airborne irritants can trigger inflammation even when no infection is present. Repeated exposure to dust and mould spores can therefore contribute to ongoing respiratory problems.
Dust and mould in forage
When horses breathe in airborne particles, many are trapped in the mucus lining the respiratory tract and subsequently cleared. However, very small particles can travel deep into the lungs, reaching the alveoli, where they may trigger an immune response. Particles smaller than approximately 5 microns (μm) are of particular concern because they can penetrate deep into the respiratory system.
Dust can irritate the airways and stimulate coughing or sneezing, helping to remove unwanted material. If exposure is brief and the irritant is removed, inflammation may settle over the following days. However, repeated or prolonged exposure can contribute to persistent airway inflammation and respiratory disease.
Equine asthma is an important example. It is an umbrella term covering inflammatory airway conditions of varying severity. Older terms include inflammatory airway disease (IAD) and recurrent airway obstruction (RAO), the latter previously being known as chronic obstructive pulmonary disease (COPD). These historical terms describe different clinical presentations and are not always interchangeable.
Signs of respiratory irritation or disease in horses can include increased mucus production, a recurring cough, increased effort when breathing, nasal discharge and reduced exercise performance. Some horses may show only subtle signs, such as taking longer to recover after exercise.
Choosing forage to support respiratory health
Hay is a common winter forage, but its quality can vary considerably. Dust, mould and other contaminants are not always easy to identify by sight or smell, so apparently good-quality hay is not necessarily free from respiratory irritants. Where forage supplies are limited, choosing the most suitable option can be challenging.
For horses with respiratory sensitivities, the aim is to provide forage that is hygienic, palatable and as low in airborne particles as possible, while meeting the horse’s nutritional needs.
Haylage
Haylage has a higher moisture content than hay and undergoes a fermentation process during production. When made and stored correctly, it generally produces less airborne dust than dry hay and can be a useful option for horses with respiratory sensitivities.
However, haylage must be of good quality and handled correctly once opened. Spoiled or mouldy forage should not be fed, and the suitability of haylage will depend on the individual horse’s nutritional requirements and tolerance.
Soaked hay
Soaking hay is commonly used to reduce airborne dust. The effectiveness depends on the hay itself and how it is soaked. Even relatively short soaking periods may reduce some airborne particles, but a brief soak cannot be relied upon to remove all respiratory irritants.
Soaking can also reduce water-soluble carbohydrates, depending on the duration and method used, which may be useful for some horses with specific metabolic requirements. However, it also removes some water-soluble nutrients, including minerals, and the extent of these losses varies.
Once soaked, hay should be fed promptly while still wet. Leaving it for extended periods, particularly in warm conditions, can encourage bacterial growth. Any uneaten soaked hay should be removed rather than left to deteriorate.
Steamed hay
Correctly steaming hay can reduce airborne dust and substantially decrease viable mould and yeast levels. Unlike soaking, steaming generally retains more water-soluble nutrients, making it an attractive option when preserving nutritional value is important.
The effectiveness of steaming depends on the equipment, temperature and duration of treatment. Follow the manufacturer's instructions to ensure the hay is treated adequately.
Steaming can also make hay more palatable for some horses. However, it does not make poor-quality or visibly spoiled forage suitable to feed, and it cannot compensate for inadequate stable ventilation or other sources of dust.
Management beyond forage
Reducing respiratory irritation is usually most effective when several management changes are combined. A 2024 review of equine asthma management highlights forage quality, bedding and ventilation as important factors in reducing exposure to airborne irritants (read the review).
Practical steps include:
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Feed forage at a suitable height. Feeding from the ground or a low feeder allows a more natural head and neck position. It may support normal mucus clearance, although evidence that feeding height alone improves respiratory health is limited. Choose a clean feeding surface to avoid contamination with soil or droppings.
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Choose low-dust bedding. Options include dust-extracted wood shavings, shredded paper or cardboard, and suitable hemp or flax bedding. Product quality matters, and no bedding is guaranteed to be completely dust-free.
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Reduce dust during stable chores. Move horses away while mucking out, sweeping or distributing bedding, and allow airborne dust to settle before bringing them back inside.
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Improve ventilation. Ensure a continuous supply of fresh air and maximise turnout where practical. Consider the horse’s individual sensitivities, as pollen and other outdoor allergens can also trigger signs in some horses.
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Keep horses away from other sources of dust. Avoid stabling susceptible horses near hay stores, dusty indoor arenas or busy grooming areas where airborne particles may accumulate.
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Monitor respiratory health. Watch for recurring coughs, nasal discharge, increased breathing effort or changes in performance. Persistent signs should be investigated by a vet, as environmental management may need to be combined with prescribed treatment.
Summary
Respiratory health is an important part of a horse’s overall wellbeing, but it is easy to overlook until signs such as coughing, nasal discharge or reduced performance appear. Repeated exposure to dust and other airborne irritants can contribute to airway inflammation, even when a horse does not show obvious symptoms.
Reducing exposure through appropriate forage, low-dust bedding, good ventilation and thoughtful stable management can help support respiratory health in both susceptible horses and those with diagnosed respiratory disease.
Forage choice is an important part of this approach. Steamed hay and good-quality haylage are often useful options for horses with respiratory sensitivities, while soaking may be appropriate in certain circumstances. The best choice depends on the horse’s respiratory and nutritional needs, including any concurrent metabolic conditions.
Environmental management can make a substantial difference, but it does not replace veterinary assessment or treatment when needed. If a horse is struggling to breathe at rest, shows markedly increased breathing effort or develops rapidly worsening symptoms, contact a vet urgently.
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