The trachea then branches into two bronchi, one passing into the left lung and the other into the right lung. Within each lung these bronchi repeatedly branch forming smaller and smaller airways known as bronchioles.
Eventually the bronchioles end as alveolar sacs where oxygen is drawn into the blood and waste gases are expelled.
Built into the respiratory system is a series of defence mechanisms that are designed to prevent infection and remove any airborne particles or dust that a horse inhales.
The majority of harmful material is filtered out by the nasal passages. Smaller particles, including dust, bacteria and viruses may pass further into the lungs, even as far as the alveolar sacs, and be deposited there.
The lining of the airways provides protection against the potential harmful effects of these small particles in a number of ways:
- It secretes mucus and other substances that form a liquid barrier.
- Much of the airway lining has millions of microscopic finger-like projections that sweep dust and other substances upwards away from the lower airways and into the back of the throat.
- The horse naturally grazes from the ground, this head-down posture assists with clearing the airways.
- The respiratory system also has a very active immune system that is able to react to, fight and remove material ranging from bacteria to tiny particles of dust.
It is the quality of the air in the zone directly around a horse's nostrils that is critical to its respiratory health -- this is termed the breathing zone, as this is the air that a horse will draw into its lungs.
Even if a horse is in a well-ventilated stable, if it has its nose buried deep in mouldy and dusty hay, or is rooting in its bed, it will be inhaling thousands of airborne particles from the immediate surrounding breathing zone.
A stable is an unnatural environment for the horse, potentially exposing its respiratory system to a multitude of airborne challenges including dust, fungi, toxins and ammonia.
In fact, research has shown that some stables exceed the dust levels that are deemed safe for people working in factories.
When stabled, a horse's defence mechanism is constantly challenged and has to work overtime to remove harmful substances from the lungs. One component of the defence mechanism is the immune system, which may be particularly active and result in inflammation of the respiratory tract.
When stabled, even healthy horses have been shown to have inflamed respiratory tracts. For most horses, the respiratory tract is at its healthiest when the horse is at pasture 24 hours per day.
Respiratory disease associated with stabling has been recorded since the time of the ancient Greeks and since then the condition has gone under many names including hay or dust allergy, Chronic Obstructive Pulmonary Disease (COPD), Small Airway Disease (SAD) and broken wind.
However, most vets now refer to this disease as either Recurrent Airway Obstruction (RAO) or 'heaves'.
RAO is a respiratory disease associated with horses generally older than seven years of age.
Signs of RAO include coughing, nasal discharge and, in more severe cases, increased respiratory effort with involvement of the abdominal muscles, hence the name 'heaves'.
Even though apparently healthy horses may not show obvious clinical signs, such as coughing or nasal discharge, these horses may not be able to perform at their full potential and may have respiratory problems in later life.
The disease is linked to stabling (and is therefore often seen in winter). Horses with RAO are believed to have developed an allergy to one or more of the airborne particles (often called allergens) associated with housing.
Most evidence points to fungal spores, present in both hay and straw, as being the principle agents that cause RAO.
Effect of RAO
When a susceptible horse breathes in an allergen, the immune system of the lungs overreacts and the bronchioles go into spasm, reducing their diameter.
The airways also become inflamed and, as a result, clog with increased quantities of mucus.
All of these changes lead to obstruction of the airways, making it more difficult for the horse to breathe air in and out of its lungs.
To compensate, horses have to increase the effort associated with breathing (heaves).
The respiratory inflammation and excess mucus that occur in RAO result in the clinical signs of coughing and nasal discharge.
A horse with RAO has a less efficient respiratory system and often cannot perform athletically to their full potential.
Once this process has been set in motion, an affected horse's lungs react not only to allergens but also to other irritants, such as dust and ammonia from urine.
Horses with RAO have become hypersensitive and hyper-reactive to allergens associated with stabling. Medication is sometimes used to control the disease. However, in the long term, control of the environment is essential.
1. Turn your horse out as much as possible. The natural environment for a horse is out in the open air, away from all sources of dust present in a stable.
2. Only use best quality bedding materials to ensure dust-free bedding. Shredded wood fibre, wood shavings (dust extracted), paper, cardboard or hemp consistently contain minimal dust and fungal spore content. Good quality straw that has been well stored may have low dust content, however it is hard to guarantee that this is consistent for every bale.
Deep litter systems may encourage the growth of fungi and can be associated with other agents that inflame the respiratory system, for example ammonia and endotoxins.
3. Feed the best quality hay available and consider soaking it overnight with water. Fungal spores develop in hay because our climate results in it usually being cured in damp, humid conditions. Again, it is pointless having a perfect environment if a horse has its muzzle, and therefore nostrils, buried in hay containing fungal spores. Soaking hay does prevent the majority of fungal spores becoming airborne, as long as it does not dry out.
Prolonged soaking may reduce the nutrient and vitamin content of the hay. A compromise would be to complete immersion for around 20 minutes.
Alternatives such as haylage can be considered as the baling and wrapping process creates an environment that prevents the formation of fungal spores.
4. Make sure the stable ventilation is adequate. Install the right ventilation points for your type of stable to increase airflow.
5. Make sure the stable is very clean, particularly the walls and food containers. Ideally a horse should not be present in its stable during mucking out and bedding down, as this can often generate lots of airborne particles.
If possible, allow dust to settle after putting down a new bed before returning your horse to its stable.
Muck heaps should be kept at a reasonable distance away from a horse's stable as they are a source of dust and generate fungal spores.
6. Remove any unwanted food from the bowl to avoid mould growths.
Also, straights and concentrates in bucket feeds can produce dust, which can be inhaled when the horse has its nose buried in the bucket so it can be beneficial to damp down hard feed with a small amount of water.
7. Feed your horse at floor level, as lowering his head will help to clear secretions from the lungs.
8. Ensure adequate stable drainage to prevent urine and other liquids from collecting in the stable. Urine contains ammonia, a noxious gas, which can be irritating to the respiratory tract.
9. Avoid exercising or working your horse on dusty riding surfaces.