Reducing respirable particles in a horse’s indoor environment is a must, as Richard Hepburn FRCVS explains in the first of this two-part series
Most horses spend more time indoors in winter, both at rest in the stable and during exercise in an indoor school.
Compared to being at pasture, this represents a major change in a horse’s lifestyle – in terms of the bedding underfoot, the surface he works on, the forage he eats and the air he breathes when confined to a smaller space. These factors can affect the airway health of all horses, but most significantly those with airway inflammation.
Equine asthma (EA) is the term used to describe horses with chronic, reversible airway inflammation. This can range from mild exercise limitation (mild EA, previously called inflammatory airway disease), to horses who cough, have nasal discharge and a raised breathing rate at rest (severe EA, previously called recurrent airway obstruction).
Airway inflammation develops as a response to the quality of a horse’s breathing environment, in particular the levels of different “respirable particles” within his breathing zone – the area at nostril level. Organic dusts, mould spores, bacterial components (in particular endotoxins) and chemical irritants such as ammonia are the key particles small enough to enter a horse’s airways and cause inflammation. Cold air may slow down the ability of the lungs to remove these irritants.
Horses with severe EA will have obvious signs after exposure to high respirable particle environments. Because mild EA is subclinical (not observable) at rest, a horse may appear “normal” even though his airways are inflamed; with ongoing exposure, however, poor performance will develop.
Recent studies have shown that an increased number of mast (allergic inflammatory) cells in the airways of racehorses makes them 1.5 times less likely to win a race. So while it is vital to understand how to improve the environment for a severe EA horse, it is just as useful in milder EA cases.
There is also an appreciation that high respirable particle levels can increase mucus production in the lungs without causing inflammation, yet leaving a sport horse less willing to perform. All horses could therefore benefit from an improved breathing environment, although those with EA need more specific changes.
A horse on a straw bed in a conventional stable, eating dry hay, could be breathing up to 15 times more respirable particles than he would if he was grass-fed at pasture. Since an outdoor life may not be an option in winter, it makes sense to clean up his indoor environment in these key areas…
Currently, our understanding is there is greater potential to reduce the concentration of respirable particles with forage type than with bedding. Feeding haylage, as opposed to good-quality dry hay, can cut breathable dust by up to 95% and mould spores by up to 90%, as well as significantly reducing endotoxin levels. Fewer airway inflammatory cells will then be produced, meaning that mild to moderate EA may even resolve without the need for drug therapy.
Steaming dry hay with a Haygain steamer reduces respirable particles by over 90% and fungal elements by 100%, without significant nutrient loss. Steamed hay can also bring about a significant reduction in airway inflammation in horses with severe EA, but to a lesser degree than haylage. It is important to note studies have used a commercial Haygain steamer; similar results have not been shown with home-made steamers.
Soaking hay for 10 minutes can reduce respirable particles by over 90%. There is no significant advantage with more prolonged soaking, of 30 minutes or more. However, soaking hay does not reduce fungal element levels, and while haylage and steamed hay have been shown to improve clinical signs in horses with EA, the same has not been seen with soaked hay.
If feeding dry hay, the means of feeding the forage will have further consequences. When a horse pushes his nose into a haynet, particles are released directly into his breathing zone. Similarly, as he tunnels into dry forage placed in a manger, he will create a small, poorly ventilated breathing area.
Feeding dry hay from the ground – or from a floor-level forage feeder, if he is a messy eater – is a better option as it will increase drainage of respiratory secretions from the windpipe, helping limit the inflammatory response that respirable particles can cause. This is most useful in the winter, as cold weather can slow the mucociliary escalator (the fine layer of beating hairs which line the airways), so reducing airway clearance.
● Key message: for the EA sufferer, feeding haylage or steamed hay provides demonstrable benefits to airway health. If you just want to improve a stabled horse’s breathing environment, soaking dry hay for 10 minutes will really help. In all cases, feeding from the ground is advantageous.
Horses inhale respirable particles from their hard feed. Rolled, uncleaned grains can contain up to six times more respirable particles than commercially ready-prepared mixes and pellets, and up to three times more than whole grains. Adding oil, water or molasses to whole grains will help to reduce particle levels.
● Key message: if you have a horse with EA and are feeding whole or rolled grains, it is worth wetting the feed or switching to a commercially produced pellet.
A natural filter
The respiratory system prevents harmful substances entering a horse’s lungs…
- Hairs inside the nostrils obstruct larger particles.
- Humidification in the nasal sinuses and pharynx moistens particles, making them heavier and stickier.
- The conchal sinuses generate a swirling airflow that drives medium-sized particles into the mucus-covered sinus linings.
- In the pharynx, which acts as a holding chamber where air pauses between each breath, particles sink or spread to the sides and become trapped by mucus.
- The trachea (windpipe) speeds up airflow. This sweeps remaining particles on to its mucus-covered sides, where multiple cilia (tiny hairs) transport them back up to the pharynx to be swallowed or snorted away. If a horse is placed in an environment that overwhelms this natural filtering, airway inflammation and mucus accumulation can result.
Once you’ve modified feeding, the next step is to think about bedding choices.
Low-dust cardboard bedding produces the least respirable particles and in severe EA cases is associated with reduced levels of airway inflammation. Wheat straw produces the most particles – around twice that of low-dust wood shavings and straw pellets. A sealed, padded stable floor will mean that you can use less bedding, for the cleanest possible air.
Mucking out, in particular sweeping, can multiply particle levels within the stable by 19 times and within a barn arrangement will also compromise air quality in adjacent stables.
Be aware, too, of very high levels of ammonia from stagnant urine, which can irritate airway lining tissues.
● Key message: if a horse with severe EA must be stabled, use low-dust cardboard bedding, ideally on a padded floor. Move him elsewhere while he is mucked out (or his neighbours are, if in a barn setting) and the corridor is swept. In all cases, low dust bedding will improve air quality.
Ref Horse & Hound; 15 October 2020