Lower airway inflammation: effects and what vets can do

The horse is physiologically adapted to be a superior athlete. Compared to other species of similar size, they have a massive aerobic capacity, large stores of energy in their muscles and the ability to increase the oxygen-carrying capacity of their blood through splenic contraction at the onset of exercise.

Exercise physiologists agree that the respiratory system of the horse is the major limiting factor to athletic performance. As we know from our physiology lectures, oxygen diffuses from the air in the alveoli into the tiny capillary blood vessels across the alveolar membrane.

Inflammation in the lower airway impedes ventilation by physically obstructing ventilation (mucous plugging) or narrowing the airways (bronchoconstriction). Infiltration of inflammatory cells further inhibit diffusion and gas exchange. This directly impacts the amount of oxygen that enters the blood.

Less oxygen means more anaerobic metabolism, increased fatigue, lower speed, less power and prolonged recovery.

Non-septic lower airway inflammation is the most common disease affecting the horse’s respiratory system and is currently classed under the umbrella of equine asthma. The early stages often go unnoticed – particularly in horses that are not working at elite level.

A multitude of causes exist, which include high levels of particulates in the environment, viral disease, air pollution, genetic predisposition and bacterial infection, to name a few. It is essentially either a disease of domestication (stable dust, straw, hay, bedding, endotoxin, ammonia and so on) or pollution (pollen, dust, smog).

Diagnosis

Clinical signs of early or mild lower airway inflammation are easy to miss. Astute owners may notice a decrease in performance, respiratory dysfunction at exercise or prolonged recovery times. Signs are often exacerbated in hot or humid weather. Sport horse owners may notice a decrease in quality or ability during collection, while racehorses may fade as fatigue sets in. These signs can be difficult to distinguish from other causes of poor performance or a lack of ability.

The most useful field test for lower airway inflammation is a broncho-alveolar lavage (BAL). Neutrophilia (more than 5%), eosinophilia (more than 0.5%), mastocytosis (2%) or combinations of these are indicative of lower airway inflammation.

Several studies have shown that tracheal aspirates are not necessarily representative of the lower airways and, therefore, BALs are preferred if lower airway disease is suspected. Endoscopy allows for evaluation of airway mucous, which has been associated with coughing.

Performance horses with lower airway inflammation have impaired physiological responses to exercise compared to healthy horses. Objectively, this is measured as lower speeds for a blood lactate of 4mmol/L and heart rate of 160bpm or 200bpm.

The author is a big advocate of the use of exercise testing to record objective physiological measures of performance that can be repeated and monitored over time. Her equine athletes are often contending with a multitude of factors that negatively affect their performance, including musculoskeletal discomfort, gastric ulceration and airway inflammation, to name a few.

It isn’t always straightforward to determine which of these factors are most significant at any given time and serial objective measures remove the guesswork.

Blood lactate – a measure of anaerobic metabolism – is particularly useful to monitor the effect of subclinical lower airway inflammation on performance.

The author uses standardised exercise tests designed around the horse’s job (racing, endurance, eventing or sport) and takes serial timed lactate measurements at specific points during each test. Together with heart rate and GPS data, this allows a better understanding of each horse’s physiology/pathophysiology at exercise.

Treatment

Treatment goals include managing the inciting causes, relieving bronchospasm, decreasing inflammation and mucous, and preventing future exacerbations.

The most important (and most difficult to achieve) treatment for airway inflammation is appropriate management of the horse’s environment. This is where most treatments fail.

No amount of pharmaceutical intervention will resolve airway inflammation in a horse living in an adventitious environment. Hay, straw, stable dust (cobwebs) and ammonia are the biggest culprits, as well as seasonal pollen or other environmental allergens.

Each environment will have to be evaluated to determine the best steps to improve it. All risk factors should be identified and addressed.

In general, dust-extracted wood shavings and hay pellets will be superior to straw and hay. If fed, hay should be fed from the ground and not from a hay net, which causes a lot of particles in the horse’s breathing zone. Hay should be steamed, soaked well or ensiled, and feed should be wet down.

Horses should be removed from the stall or barn while stables are cleaned. Any activity that increases the number of particles in the air (such as sweeping or using a blower) should not be undertaken while horses are nearby. Other sources of dust, such as sand schools and dirt roads, should be addressed by wetting it down or removing horses from the area during dusty times. Cobwebs should be routinely removed from stables.

Horses that suffer from outdoor allergens (for example, pollens and pesticides) may benefit from being stabled in clean, well-ventilated stables prior to their expected seasonal exacerbation. Keeping a detailed diary will assist in identifying each horse’s triggers, which is imperative for successful long-term management.

For some horses, simply steaming their hay and feeding it from the floor may be all that is needed, but others may require a complete change in management (for example, to live out permanently) or intensive environmental management to get their lower airway inflammation under control.

It is also worth noting that some factors exist that are inherent to a particular environment that no amount of management or medication will change.

The author has had several patients that simply could not thrive in a specific environment, regardless of the owners’ efforts, that did very well when moved to a different environment.

Targeted pharmaceutical intervention can be very effective, but should be seen as an adjunct to environmental management or last resort. When environmental management fails to control lower airway inflammation, inhaled or systemic corticosteroids and bronchodilators can be considered.

It is important to adhere to appropriate withdrawal times in performance horses, as all these drugs are performance-enhancing and prohibited during competition.

A tendency exists for some practitioners to routinely prescribe long courses of bronchodilators without adequately addressing either the environment or the airway inflammation. It appeases the owner, because the horses feel better initially, which may give owners the false impression that they have addressed the problem. In truth, they have neither addressed the problem (airway inflammation) nor the cause (the environment) – they have just temporarily relieved a symptom. This mistaken belief may also make owners less motivated to take the necessary environmental management steps in the horse’s interest.

With prolonged use, horses also experience tachyphylaxis, with one study showing a measurable increase in airway reactivity within 21 days of clenbuterol treatment. For this reason, the author only uses bronchodilators for short-term exacerbations and tries not to use them as first-line treatments.

In horses that have no clinical signs at rest, the author would typically ask owners to make the appropriate environmental changes and then reassess.

Nebulised dexamethasone in saline is her treatment of choice for managing the inflammatory cell infiltration in cases where environmental management alone is insufficient, where a quick resolution is needed or to prevent an exacerbation in response to a known but unavoidable trigger. Omega-3 polyunsaturated fatty acids improve the clinical signs of airway inflammation and the BAL neutrophilia.

Often-overlooked contributing factors to the decreased performance associated with airway inflammation are the role of fitness and bodyweight. A fit, lean horse in training will be much better adapted to tolerate a small amount of lower airway inflammation compared to a fat, unfit horse. A weight management and fitness plan can go a long way towards improving a horse’s performance, irrespective of airway inflammation.

Conclusion

In summary, lower airway inflammation is extremely common and significantly impacts equine performance. A veterinarian should provide an honest assessment of the horse’s environment and risk factors to enable owners to make the most appropriate interventions, and devise pharmaceutical interventions around environmental triggers and the competition schedule.