Guelph, ON July 18, 2023–There’s nothing like hearing a horse cough to set people scurrying around the barn to identify the culprit. After all, that cough could mean choke, or a respiratory virus has found its way into the barn. It could also indicate equine asthma. Yes, even those “everyday coughs” that we sometimes dismiss as “summer cough” or “hay cough” are a wake-up call to the potential for severe equine asthma.
Formerly known as heaves, broken wind, emphysema, chronic obstructive pulmonary disease (COPD), or recurrent airway obstruction (RAO), this respiratory condition is now called severe equine asthma (sEA). These names reflect how our scientific and medical understanding of this debilitating disease has changed over the years. We now consider heaves to be most comparable to severe asthma in people.
But what if your horse only coughs during or after exercise? This type of cough can mean that they have upper airway irritation (think throat and windpipe) or lower airway inflammation (think lungs) meaning inflammatory airway disease (IAD) – which is now known as mild-to-moderate equine asthma (mEA). This airway disease is similar to childhood asthma, including that it can go away on its own. However, it is still very important to call your veterinarian out to diagnose mEA. This disease causes reduced athletic performance and there are different subtypes of mEA that benefit from specific medical therapies. In some cases, mEA progresses to sEA.
What has Equine Asthma got to do with Air Quality?
A lot, it turns out. Poor air quality, or air pollution, includes the barn dusts – the allergens and molds in hay and the ground up bacteria in manure – as well as arena dusts and ammonia from urine. Also, very importantly for both people and horses, air pollution can be from gas and diesel-powered equipment. This includes equipment being driven through the barn, the truck left idling by a stall window, or the smog from even a small city that drifts nearly invisibly over the surrounding farmland. Recently, forest-fire smoke is another serious contributor to air pollution.
Smog causes the lung inflammation associated with mEA. Therefore, it is also likely that air pollution from engines and from forest fires will also trigger asthma attacks in horses with sEA. Smog and smoke contain many harmful particulates and gasses, but very importantly they also contain fine particulate matter, known as PM2.5. The 2.5 refers to the diameter of the particle being 2.5 microns. That’s roughly 30 times smaller than the diameter of a human hair. Because it is so small, this fine particulate is inhaled deeply into the lungs where it crosses over into the blood stream. So, not only does PM2.5 cause lung disease, but it also causes inflammation elsewhere in the body including the heart. Worldwide, even short-term exposure is associated with an increased risk of premature death from heart disease, stroke, and lung cancer. This PM2.5 stuff is not trivial!
In horses, we know that PM2.5 causes mEA, so it’s logical that smog and forest-fire smoke exposure should cause exacerbation of asthma in horses, but we don’t know about heart disease or risk of premature death.
Symptoms, Diagnostic Tests, and Treatments
Equine asthma manifests with a spectrum of symptoms that vary in severity and degree of debilitation they cause. Just like in people with asthma, the airways of horses with mEA and sEA are “hyper-reactive”. This means that the asthmatic horse’s airways are extra sensitive to barn dusts that another horse’s lungs would just “ignore”. The asthmatic horse’s airways constrict, or become narrower, in response to these dusts. This narrowing means it’s harder to get air in and out of the lungs. Think about drinking through a straw. You can drink faster with a wider straw than a skinnier one. It’s the same with air and the airways. In horses with mEA the narrowing is mild. In horses with sEA the constriction is extreme and is the reason why they develop the “heaves line” – they have to use their abdominal muscles to help squeeze their lungs to force the air back out of their narrow airways. They also develop flaring of their nostrils at rest to make their upper airway wider to get more air in. Horses with mEA do not develop a heaves line, but the airway narrowing and inflammation do cause reduced athletic ability.
The major signs of mEA are coughing during or just after exercise that has been going on for at least a month, and decreased athletic performance. In some cases, there may also be white or watery nasal discharge particularly after exercise. Often, the signs of mEA are subtle and require a very astute owner, trainer, groom, or rider to recognize them.
Another very obvious feature of horses with sEA is their persistent hacking cough, which worsens in dusty conditions. Hello dusty hay, arena, and track! The cough develops because of airway hyper-reactivity and because of inflammation and excess mucus in the airways. Mucus is the normal response of the lung to the presence of inhaled tiny particles or other irritants. Mucus traps these noxious substances so they can be coughed out, which protects the lung. But if an asthma-prone horse is constantly exposed to a dusty environment it leads to chronic inflammation and mucus accumulation, and the development or worsening of asthma – along with that characteristic cough.
Veterinarians use a combination of the information you tell them, their observation of the horse and the barn, and a careful physical and respiratory examination that often involves “rebreathing”. This is a technique where a bag is briefly placed over the horse’s nose causing them to breathe more frequently and more deeply to make their lung sounds louder. This helps your veterinarian hear subtle changes in air movement through the lungs and amplifies the wheezes and crackles that characterize a horse experiencing a severe asthma attack. Wheezes indicate air “whistling” through constricted airways, and crackles mean airway fluid buildup. The fluid accumulation is caused by airway inflammation and contributes to the challenge of getting air into the lung.
Other tests your veterinarian might use are endoscopy, bronchoalveolar lavage, and in the specialist setting, pulmonary function testing. They will also perform a complete blood count and biochemical profile assay to help rule out the presence of an infectious disease.
Endoscopy allows your veterinarian to see the mucus in the trachea and large airways of the lung. It also lets them see whether there are physical changes to the shape of the airways, which can be seen in horses with sEA.
Bronchoalveolar lavage, or “lung wash” is how your veterinarian assesses whether there is an accumulation of mucus and inflammatory cells in the smallest airways that are too deep in the lung to be seen using the endoscope. Examining lung wash fluid is a very important way to differentiate between the different types of mEA, between sEA in remission and an active asthma attack, and conditions like pneumonia or a viral lung infection.
Finally, if your veterinarian is from a specialty practice or a veterinary teaching hospital, they might also perform pulmonary function testing. This allows your veterinarian to determine if your horse’s lungs have hyperreactive airways (the hallmark of asthma), lung stiffening, and a reduced ability to breathe properly.
Results from these tests are crucial to understanding the severity and prognosis of the condition. As noted earlier, mEA can go away on its own but medical intervention may speed healing and return to athletic performance. With sEA, remission from an asthmatic flare is the best we can achieve. As the disease gets worse over time, eventually the affected horse may need to be euthanized.
Management, Treatment, and Most Importantly – Prevention
Successful treatment of mEA and sEA flares, as well as long-term management, requires a multi-pronged approach and strict adherence to your veterinarian’s recommendations.
Rest is important because forcing your horse to exercise when they are in an asthma attack further damages the lung and impedes healing. To help avoid lung damage when smog or forest-fire smoke is high, a very useful tool is your local, online, air quality index (just search on the name of your closest city or town and “AQI”). Available worldwide, the AQI gives advice on how much activity is appropriate for people with lung and heart conditions, which are easily applied to your horse. For example, if your horse has sEA and if the AQI guidelines say that asthmatic people should limit their activity, then do the same for your horse. If the AQI says that the air quality is bad enough that even healthy people should avoid physical activity, then do the same for you AND your horse. During times of poor air quality (Image 2), it is recommended to monitor the AQI forecast and plan to bring horses into the barn when the AQI is high and to turn them out once the AQI has improved.
Prevent dusty air. Think of running your finger along your tack box – whatever comes away on your finger is what your horse is breathing in. Reducing dust is critical to preventing the development of mEA and sEA, and for managing the horse in an asthmatic flare. Logical daily practices to help reduce dust exposure include:
- turning out all horses before stall cleaning
- wetting down the aisle prior to sweeping
- never sweeping debris into your horse’s stall
- using low-dust bedding like wood shavings or dust-extracted straw products –which should also be dampened down with water
- reducing arena, paddock, and track dust with watering and maintenance
- when selecting footing substrate, consider low dust materials.
- steaming (per the machine’s instructions) or soaking hay (15-30 minutes and then draining, but never storing steamed or soaked hay!)
- feeding hay from the ground or using hay feeders that sit on the ground
- feeding other low-dust feeds
- avoiding hay feeding systems that allow the horse to put their nose into the middle of dry hay – this creates a “nosebag” of dust
Other critical factors include ensuring that the temperature, humidity and ventilation of your barn are seasonally optimized. Horses prefer a temperature between 10-24 ºC (50-75 ºF), ideal barn humidity is between 60-70%. Optimal air exchange in summer is 142 L/s (300 cubic feet/minute). For those regions that experience winter, air exchange of 12-19 L/s (25-40 cubic feet/minute) is ideal. In winter, needing to strip down to a single layer to do chores implies that your barn is not adequately ventilated for your horse’s optimal health. Comfortable for people is often too hot and too musty for your horse!
Medical interventions for controlling asthma are numerous. If your veterinarian chooses to perform a lung wash, they will tailor the drug therapy of your asthmatic horse to the results of the wash fluid examination. Most veterinarians will prescribe bronchodilators to alleviate airway constriction. They will also recommend aerosolized, nebulized, or systemic drugs (usually a corticosteroid, an immunomodulatory drug like interferon-α, or a mast cell stabilizer like cromolyn sodium) to manage the underlying inflammation. They may also suggest nebulizing with sterile saline to help loosen airway mucus and may suggest feed additives like omega 3 fatty acids, which may have beneficial effects on airway inflammation.
New Research and Future Directions
Ongoing research is paramount to expanding our knowledge of what causes equine asthma and exploring innovative medical solutions. Scientists are actively investigating the effects of smog and barn dusts on the lungs of horses. They are also working to identify new targeted therapies, immunotherapies, and other treatment modalities to improve outcomes for affected horses.
Both mild and severe equine asthma are caused and triggered by the same air pollutants, highlighting the need for careful barn management. The alarming rise in air pollution levels poses an additional threat to equine respiratory health. Recognizing everyday coughs as potential warning signs and implementing proper diagnostic tests, day-to-day management practices and medical therapies are crucial in combating equine asthma. By prioritizing the protection of our horse’s respiratory health and staying informed about the latest research, we can ensure the well-being of our equine companions for years to come.
Dr. Janet Beeler-Marfisi works at the University of Guelph’s Ontario Veterinary College. She is a specialist in veterinary clinical pathology, dedicated teacher, and an asthmatic who is devoted to researching why horses get asthma and what role smog plays in damaging horses’ lungs. She is particularly grateful to her research horses for the lessons they have shared, to her horse trainer husband for his astute observations and for horse transportation, and to Equine Guelph for their generous financial support of her research, and dissemination of her research results and insights.
From Equine Guelph