If you are picturing an older, swaybacked, retired horse knee-deep in pasture struggling for breath, you could indeed be correct. However, it is important to appreciate that younger, athletic horses genetically susceptible to equine asthma can suffer bouts that are just as intense as a geriatric horse’s attack. In one recent study of adult horses with equine asthma, researchers found that patients were, on average, younger than 10, and those horses were actively participating in reining, barrel racing, trail riding, and dressage. About 50% of those asthmatic athletes were “competitive” per the owner rather than pleasure mounts (Leclere et al., 2018).
Severe equine asthma affects an estimated one in seven horses (about 14%) in the Northern Hemisphere.
Severe equine asthma, or SEA, primarily occurs in adult horses, usually those 7 years or older. Considering horses are living longer than ever, 7 years is a young horse—too young to be left suffering such a debilitating condition. Severe equine asthma affects an estimated one in seven horses (about 14%) in the Northern Hemisphere. Knowing how to recognize asthma promptly and institute appropriate treatment and management strategies will help keep affected horses breathing easy and physically active for years to come.
Which Horses are Susceptible to SEA?
Not all horses over the age of 7 years are at risk for SEA. “A genetic or heritable basis for SEA has been reported in several breeds,” says Vinzenz Gerber, PhD, of the University of Bern’s Swiss Institute of Equine Medicine. “For example, Lipizzans and Warmblood horses with one affected parent had an increased risk of developing SEA as an adult. That risk increased markedly when both parents were affected. To date, no genes that ‘cause’ asthma have been identified, although several genes that might contribute to SEA have been found.” Based on recent research, Gerber says, it is unlikely that a single gene will be found that is responsible for SEA. “Instead, a combination of genes that each contribute a small amount to the development of SEA in combination with environmental risk factors will help identify at-risk horses,” he says.
For those of us unlucky enough to have suffered an asthma attack ourselves or seen our horses struggle for breath during a flare-up, the definition of severe equine asthma doesn’t do the condition justice.
Asthma describes a chronic noninfectious, inflammatory obstructive lower airway disorder.
“In a nutshell, this means that small-diameter airways deep in the lungs become chronically inflamed, which obstructs the flow of air in and out of the lungs,” says Laurent L. Couëtil, DVM, PhD, professor of Large Animal Internal Medicine and director of Equine Research Programs and the Equine Sports Medicine Center at Purdue University’s College of Veterinary Medicine, in West Lafayette, Indiana.
In normal lungs the respiratory tract looks like a tree. The trunk of the tree is the main airway, called the trachea or “windpipe.” The trunk branches out over and over until the airways become very narrow, like the tiny branches and twigs at the ends of each bough. The airway measures about 5 centimeters or so in diameter near the throat and becomes increasingly smaller, measuring less than 2 millimeters deep in the lung. Those narrow airways, called bronchioles, terminate in a sort of pouch-like structure referred to as the alveolus—like a bud at the tip of a tree branch. It is here in the alveoli that gas exchange takes place. Specifically, oxygen diffuses from the air inside the alveoli’s lumen, across that structure’s narrow wall, and into the bloodstream. The heart then pumps that oxygenated blood to all the cells, organs, and tissues throughout the body.
Laurent L. Couëtil, DVM, PhD
Professor of Large Animal Internal Medicine and director of Equine Research Programs and the Equine Sports Medicine Center at Purdue University’s College of Veterinary Medicine.
“In horses with asthma, the walls of the airways become inflamed, which causes the lumen of the airways to become narrower, making it more difficult to draw air into alveoli,” says Jean-Pierre Lavoie, DMV, Dipl. ACVIM, a professor and the director of the Equine Asthma Research Laboratory at the University of Montreal, in Canada. “Think about trying to breathe underwater through a straw (like a snorkel) … the thinner the straw, the harder it is.”
In addition to the inflamed airway walls, the layer of smooth muscle that wraps around the lower airways and bronchioles constricts in asthmatic horses. This muscle contraction—or spasm, really—contributes to airway narrowing and the increased effort it takes to move air to the alveoli for gas exchange.
Further, mucous and other secretions in the airways contribute to airflow obstruction—like speed bumps in the passages.
Over time, airway remodeling occurs.
“In these horses the smooth muscles actually undergo excessive growth (hyperplasia and hypertrophy), not just spasm,” says Lavoie. “There is also fibrosis (scarring) of the airway walls. Together, those changes cause continued obstruction of the airways, even when horses are otherwise symptom-free.”
Many asthmatic horses will develop “heave lines” on their abdomens, a tell-tale sign of labored breathing caused by overuse of the muscles to draw air in and out of the lungs.
Jean-Pierre Lavoie, DMV, Dipl. ACVIM
Professor and director of the Equine Asthma Research Laboratory at the University of Montreal, in Canada.
Each inhale a horse takes can include up to 4.6 million particles present in hay and bedding. You may not be able to see these particles, but you can hear your horse’s cough, which could mean your horse is suffering from severe equine asthma.
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Owners residing in temperate climates might take comfort knowing their horses are nestled in their warm stalls happily munching hay on cold, blustery days. However, this is the last place horses genetically susceptible to asthma should be.
Barns are full of inhalable aeroallergens—airborne particles as small as 2.5 microns. To comprehend how small this is, know that a single strand of human hair measures 100 microns in diameter. Those particles—sometimes observed dancing in the sunlight that streams through the barn windows—contain mold spores, mites, pollen, endotoxins (from bacteria), and irritants such as ammonia from urine.
When asthmatic horses inhale those particles—coming primarily from hay—the airways become inflamed, the muscles spasm, and the horse develops the telltale signs of asthma: coughing, nasal discharge, and exercise intolerance.
Additional Asthma Triggers
Severe equine asthma most commonly occurs in susceptible horses maintained indoors for a substantial portion of the day with dusty bedding and fed hay. Other types of asthma do occur, including the following:
Heat- and pollen-induced; and
Other potential triggers of equine asthma are exposure to traffic-related air pollution such as ozone, nitrogen dioxide, and fine particles. However, Couëtil says the evidence supporting these particles as aeroallergens is currently only circumstantial.
Researchers recently confirmed that latex—typically from surfaces on which horses work—is a major environmental trigger and could be associated with severe equine asthma. While further research in horses is needed, practitioners should consider latex exposure a potential danger to horses’ respiratory health.
A complete history, full physical exam, and routine bloodwork can often point toward SEA. While it might seem reasonable to skip diagnostic testing and jump straight into treatment, veterinarians caution owners against this approach.
“Specialist consensus recommendations clearly emphasize the use of bronchoalveolar cytology for the diagnosis of equine asthma,” says Vinzenz Gerber, PhD, of the University of Bern’s Swiss Institute of Equine Medicine.
Bronchoalveolar lavage (BAL) allows veterinarians to collect samples of small airway contents. Through this process they use a sterile tube to deliver a small amount of sterile saline into the bronchioles and alveoli. Then they suction the fluid, referred to as BALF, out of the small airways and send it to the laboratory for analysis.
“When analyzing those samples, we are primarily looking for the proportion of different cells present in the airways,” Couëtil explains.
Healthy airways maintain cell proportions in relatively constant ranges composed primarily of macrophages and lymphocytes (~50/50), which are types of white blood cells. Normal BALF only contains a few inflammatory cells: <5% neutrophils, <2% mast cells, and <1% eosinophils.
Changes in these cell populations, especially an increase in the proportion of neutrophils, more definitively supports an SEA diagnosis than case history alone.
“In addition, BAL provides a measure of inflammation severity, which helps in selection of therapy,” Couëtil says.
This means veterinarians with BAL results will have a better idea of whether they should prescribe inhaled or systemic medications and at what dose and duration.
Finally, veterinarians can only truly assess treatment response by repeating the BAL.
“Clinical signs of SEA usually improve quickly with therapy, but lung inflammation always lags behind,” says Couëtil. “If you stop therapy too soon and lung inflammation is still present, clinical signs will recur.”
Horses with SEA need a clean 2-foot breathing zone—the region encircling their muzzle from where they draw their breath—free of pollen, ammonia from urine and manure, hay particles, and dust.
Because hay is the No. 1 source of aeroallergens, horses with SEA should never be fed dry hay. Ever.
A group of researchers from the University of Lisbon in Portugal recently noted that “environmental management should be the primary goal because clinical signs and lung function quickly improve in a low-dust environment, even without medication.” (Simões 2020).
The research team, led by doctoral student Joana Simões, explained that horses living in an ideal world would be maintained exclusively on pasture and fed low-dust feeds such as fresh grass or a pelleted/cubed diet.
When owners cannot attain those high standards, they should strive for the following:
- If they house horses, the stalls should have good ventilation with at least two openings for fresh air circulation;
- Turn horses out for at least 12 hours a day;
- Groom horses outdoors, and clean stables when horses are turned out; and
- Use commercial dust-free wood shavings, cardboard, or other low-dust options when horses are housed. Avoid sawdust and straw.
PHOTOS: iStock / iStock / iStock / The Horse-Stephanie L. Church
“Most importantly, do not feed hay,” says Lavoie.
Instead, use a cubed or pelleted diet. If you do provide hay, soak it before feeding it to your horse. Hay soaking involves completely submerging the ration for 30 minutes, not just watering it down with a hose.
“Other hay alternatives include oiled hay such as the Nutri-Foin system, haylage, and pasteurized hay,” Lavoie says.
Steaming hay can be performed in place of soaking hay. Couëtil says molds will persist if the hay is not fully steamed to the inside of the bale. Further, the steamers need to be cleaned properly to ensure no molds get left behind. Commercial steamers take about 1 hour per bale, and some owners might find the cost prohibitive.
“Haylage, a well-known feedstuff widely used in Europe, can maintain SEA horses in remission while in the barn,” says Couëtil. “One drawback is that haylage is associated with the risk of botulism, but this is not typically a concern for properly harvested and stored haylage. Also, vaccination helps protect horses against botulism.”
While the above-mentioned feeding recommendations are clear and widely advocated for by veterinarians around the globe, putting them into practice has proven to be challenging for many horse owners.
In the study by Simões et al. involving 39 client-owned horses, researchers saw good compliance to allergy avoidance strategies in only six cases. This means about 85% of owners were unable to adopt many of the outlined management recommendations. Not surprisingly, many horses in the study did not show substantial improvements in clinical signs of their disease. In addition, many horses required medications to help control their asthma during the one-year study period.
The easiest management changes to adopt appeared to involve those that did not mandate a change in the owner’s behavior or time budget. They happily bought new bedding to replace straw but were less eager to turn horses out or soak hay.
In a 2018 study Mathilde Leclere, DVM, PhD, Dipl. ACVIM, and colleagues from the University of Montreal’s Faculty of Veterinary Medicine found more positive results but still saw room for improvement. Specifically, Leclere wrote, “adherence was surprisingly high, as was overall satisfaction. All owners indicated that they had implemented at least one of the recommendations to decrease exposure to hay dust or barn dust.”
More than half of the 33 owners reported hay soaking rather than feeding dry hay, and another 12 owners completely replaced hay with a pelleted diet. Further, 26 reported turning their horses out more frequently than before the asthma diagnosis.
Leclere said reasons owners might not be able to follow recommended management strategies include an incomplete understanding of why they’re important or a conscious decision that the “burden of the treatment outweighs the burden of the disease.”
In the long run, though, it appears owners can suffer “asthma fatigue” and revert back to their old habits. For Leclere’s 33 owners, only 30% reported they had made a “rigorous and permanent change in management” after their horses were diagnosed with asthma. Indeed, 44% of owners began offering dry hay again, and 26% of owners reported turning their horses out less over time than they originally did at the time of diagnosis.
Feeding Asthmatic Horses
Left to their own devices, horses graze for about 16-18 hours a day. This not only satisfies many horse’s energy (caloric) demands but also plays a role in protecting their stomachs from equine gastric ulcer syndrome (EGUS). Further, horses with inadequate grazing opportunities might be at a higher risk of developing stereotypic behaviors (i.e., weaving, cribbing).
Nutritionists recommend offering 1.5-2% of the horse’s body weight per day in forage, dry weight, to all horses. This does not mean the hay is fed dry, just measured while it is dry.
Even asthmatic horses on hay-alternative diets require 1.5-2% of their body weight in forage each day.
Time spent foraging can be extended by feeding soaked hay or haylage in slow feeders, such as smaller-holed haynets.
“If owners feed cubes or pellets, it may be advantageous to spread out the feedings into smaller, more frequent meals, about four or five per day,” says Kathleen Crandell, PhD, a nutritionist with Kentucky Equine Research in Versailles.
She notes that some of the challenges owners have with compliance could include:
Soaking hay is time-consuming and messy, and soaked forage could go “off” fairly quickly in hotter climates;
Horses tend to waste more hay when it is soaked rather than when fed dry;
Discarding wastewater after soaking can be problematic in some countries (e.g., in Europe);
Hay alternatives like haylage could be more expensive, difficult to procure, and the horse may not eat them; and
Cubes and pellets are expensive, and availability could be an issue in some areas.
In boarding situations where the staff is doing the feeding, having to prepare special meals for one horse can be demanding.
Minimizing the need to medicate asthmatic horses can save owners money (clean air is free!). That said, Lavoie says some horses must be medicated.
“It may take weeks to months before the airway obstruction resolves with only improving the environment, which again means avoiding dry hay, mostly,” he says. “Corticosteroids improved the obstruction rapidly, within days.”
Corticosteroids, such as ciclesonide, decrease inflammation in the airways. Veterinarians can use corticosteroids with bronchodilators, the other main type of medication administered to equine asthma patients. Bronchodilators increase airway diameter to reduce obstruction by relaxing the smooth muscles. Examples of these medications include fluticasone and salmeterol.
In one of his many studies on equine asthma, Lavoie reported that airway remodeling decreased significantly within 12 weeks of starting treatment with fluticasone/salmeterol (Scientific Reports, 2017).
“We also found that the amount of smooth muscle surrounding the airways remained twice as high as in healthy horses despite treatment,” he says, adding, “In a previous study, they had reported a similar finding after a three-month treatment with fluticasone or after one year of a strict control of the environment (Leclere M, et al. Am J Respir Cell Mol Biol 2011;45(1):181-187). This shows that treatment can’t reverse all airway changes, making this disease incurable but manageable.
“A surprising finding from this study was that the airway inflammation was controlled only in horses treated with corticosteroids and the bronchodilator, but not with either drugs alone, suggesting a synergetic effect,” Lavoie continues.
This cooperative effect might be at least partly due to the medications’ enhanced penetration within the bronchial tree when used together rather than individually. Further, Lavoie warns against using bronchodilators as a horse’s sole therapy, as they do not control the inflammatory process and could exacerbate the condition in the long run. Lavoie’s research also showed that BALF neutrophilia decreased following fluticasone/salmeterol treatment to the same degree as allergen avoidance within eight weeks of starting treatment. He cautions that neither corticosteroids nor bronchodilators alone control the neutrophilic inflammation and that these anti-inflammatory effects need confirmation in follow-up studies.
“This again shows that whenever possible, minimizing a horse’s exposure to a barrage of aeroallergens should be the first step to treating SEA rather than promptly reaching for corticosteroids and bronchodilators,” Lavoie says.
Equine experts and co-authors of the ACVIM consensus statement on asthma agree that veterinarians and owners can also use omega-3 fatty acids to help asthmatic horses (Couëtil, 2016).
According to those guidelines, offering 1.5 grams of DHA, a specific type of omega-3 fatty acid found in fish oil, by mouth once daily can curb inflammation in horses with SEA. This effect is more pronounced when supplemented horses also relocate to a low-dust environment.
“If horses are turned out on pasture, omega-3s are probably not indicated because grass pasture is usually a good source of omega-3s,” Couëtil says. “Further, haylage is a good source of omega-3s, so owners may not need to supplement if feeding this roughage either.”
Take the Guesswork Out of SEA
Although many veterinarians and owners may believe a diagnosis of SEA is straightforward, this is not always the case.
Infectious causes (e.g., bacterial or viral) need to be ruled out, as well as upper airway diseases that can cause coughing and poor performance.
“I have seen several cases presenting for chronic coughing and increased breathing efforts that had laryngitis caused by inflammation of the arytenoid cartilages or epiglottis,” says Laurent L. Couëtil, DVM, PhD, professor of Large Animal Internal Medicine and director of Equine Research Programs and the Equine Sports Medicine Center at Purdue University’s College of Veterinary Medicine, in West Lafayette, Indiana.
For cases that do not respond well to therapy directed toward SEA, thoracic (chest) radiographs might help rule out the following:
Equine multinodular pulmonary fibrosis (EMPF);
Idiopathic pulmonary fibrosis (IPF);
For a through work-up of coughing and poor performance, veterinarians can perform bronchoscopy—passing a scope into the bronchi to visualize the airways.
“Bronchoscopy is also useful to gauge airway inflammation, edema, and mucous grade. I have also diagnosed several horses with lung neoplasia that presented for chronic coughing, and we were able to visualize masses within airways during bronchoscopy,” says Couëtil.
Owners and veterinarians can effectively manage horses susceptible to SEA primarily by controlling the cleanliness of their breathing zone. This means avoiding dry hay at all costs and turning horses out (without a round bale—that also counts as dry hay) as often as possible. Vets can prescribe medical therapy involving a corticosteroid and bronchodilator, such as fluticasone and salmeterol, as needed as a second-line treatment to keep horses breathing easy. Some of the airway remodeling can be reversed with treatment, but the smooth muscle thickness will always be increased, contributing to persistent airflow obstruction.
Stacey Oke, MS, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.
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