With three disease severities and varying clinical signs, however, “do we treat them the same?” said Renaud Léguillette, DVM, MSc, PhD, Dipl. ACVIM, ACVSMR. “And what is the evidence for treatments that work in severe asthma versus mild-moderate asthma? The conclusion, in a nutshell, for now is we treat them with the same medications, but with subtle differences.”

Léguillette, a professor of equine internal medicine and Calgary Chair in Equine Sports Medicine at the University of Calgary Faculty of Veterinary Medicine, in Alberta, Canada, reviewed current equine asthma treatment options and discussed potential new approaches at the 2021 American Association of Equine Practitioners Convention, held Dec. 4-8 in Nashville, Tennessee.

“Equine asthma is a triad,” he said. “There are three important pathologies involved. One is inflammation, one is bronchoconstriction, and one is accumulation of mucus.”

Veterinarians can treat each separately, but a more practical—and effective—approach is to treat existing, then prevent future inflammation. This, of course, means owners must embrace significant management changes.

Léguillette said practitioners should discuss with them the horses’ environments and how to manage dust and allergen exposure that can trigger asthma clinical signs. Then make a plan to decrease and control lung inflammation.

“When I talk to clients, I make it very clear to them that there is no cure,” he said. “That’s it. He’s stuck with asthma for, if he’s a young horse, years. If he’s an older horse, probably the rest of his life. And all those horses are going to be, unfortunately for the owners, high-maintenance horses.”

Treatment Challenges

A previous convention presentation covered causes of asthma in-depth, but Léguillette added a few key points.

“Dust is a well-known trigger, and dust is complicated,” he said. “It’s not just the dust you can see up in the air. It’s a complex mixture where you can have beta glucan (β-glucan, a component of fungal and plant cell walls), you can have mold, you can have bacteria and endotoxins, you can have pollen.”

And it’s not just the dust in barns, stalls, and arenas, he said. Some of these triggers are present in pastures, as well.

“It makes sense, and it’s actually the key, to prevent exposure to those triggers,” he said. “The problem is that it’s very difficult to do because of the modern management of horses.

“We need them to be shipped in a trailer. We need them to be fed hay in the winter. We need them to be in stalls,” he continued.  These factors make it very challenging to realistically manage a performance horse in aa dust-free environment. .

This was well-represented in a recently published study revealing poor owner compliance when implementing equine asthma management methods, he added; 20 of 49 participating owners had “very bad” compliance with veterinarians’ recommendations, he said.

“It’s no wonder that owner compliance is really poor when you tell them, ‘Change the hay, the bedding, this and that,’” he said—you’re asking them to shift well-ingrained management habits.

However, he added, the six owners that complied well with management recommendations were able to reduce dust in their asthmatic horses’ environment with very good results. “None of these horses had labored breathing effort at rest, and none of these horses required more treatment,” he said. Something else that can complicate asthma management is any pause or break in dust management protocols can cause the horse to regress to labored breathing and other signs of severe asthma.

“Any mistake is not forgiven with asthma,” Léguillette said. “Doing a good job 80% of the time, unfortunately, most of the time is not enough.”

Practical Dust Management

Practically speaking, what are the best ways to manage equine asthma? Start by evaluating and managing the horse’s environment, Léguillette said. He offered tips to remember:

• Dust particle size matters. The big, visible dust particles might look bad, but they’re not typically damaging in terms of causing equine asthma for most horses, he said. “The worst is the tiny, invisible dust, 10 to 2.5 micron in diameter,” he said.
• Consider breathing zone. The most important spot to evaluate is your horse’s breathing zone—what’s in the area around his nose that he can inhale into the lungs. Veterinarians can measure these very accurately using aspirating devices, filters, and other technology, but Léguillette noted these expensive machines probably aren’t practical for the average practitioner. Rather, he encouraged a “dust investigation” in the horse’s barn and his outdoor environments.“Have a walk with the owners and look at what’s going on,” he said. “Look for dust, large particulates versus smaller ones. Where is the dust coming from? When? It’s been very well-shown that the amount of dust (fluctuates) over 24 hours in a barn. Look for allergens and pollen; when you see flowers and trees everywhere, sometimes that could also be a sign of what’s going on there. Look for anything unusual. It’s a full assessment of the environment and it takes time, but that’s how you get to the bottom of it.”
• Make hay changes. Hay has the biggest impact on the horse’s breathing zone, so focus on that first. “Horses spend hours with their nose in the hay,” Léguillette said. Round bales are particularly problematic for horses with asthma, he noted. Square bales are typically less problematic than round bales, but pelleted or cubed hay is most appropriate for horses with equine asthma. “Even if (hay cubes) ‘look dusty,’ it’s a heavy dust that tends to stay in the upper airways,” he said. Complete feeds designed to be fed sans hay can also help affected horses, and, if long-stem hay is the only option, soaking or steaming it can help reduce dust levels. Haylage is also much less dusty than hay, but it’s less common in the United States than in other countries (and might require precautions to prevent botulism).
• Choose low-dust bedding. Some beddings seem more problematic than others. Wood shavings have been shown to contain nearly 10 times less dust than conventional straw, he said.
• Remove affected horses from stalls during cleaning and aisle sweeping. Sprinkling water in aisles can also help reduce dust levels.
• Know your limits. Understand there will always be things you won’t be able to control, such as smoke from wildfires or dust from dirt roads that aren’t on your property, for example. That’s where medical treatment can help.

Medical Treatment Options

When used in conjunction with environmental modifications, medical treatments can help alleviate clinical signs of equine asthma. Léguillette again emphasized that the condition has no cure, and medications are just designed to reduce inflammation in the airway to improve the horse’s breathing ability.

Veterinarians use systemic (administered intravenously or orally) or inhaled corticosteroids to reduce airway inflammation.

Systemic Corticosteroids Léguillette said dexamethasone seems to be the most effective corticosteroid (and is often used as a reference treatment in research on equine asthma). Other options include isoflupredone and triamcinolone. Prednisone isn’t well-absorbed in the horse’s body, so he recommended avoiding it.

“I use as low of a dose as possible but (continue treatment) as long as necessary,” he said. This could be a short duration of 10 to 15 days or as long as a few weeks, he said.

Of course, systemic corticosteroids aren’t without risks, especially in horses with equine metabolic syndrome or pituitary pars intermedia dysfunction (formerly known as equine Cushing’s disease) or ponies in general. These drugs could induce laminitis in such equids, so Léguillette said he avoids using them in those cases.

Inhaled Corticosteroids The other steroid-based option is to treat the lungs locally with an inhaled product. Léguillette said this allows microscopic droplets of varying sizes containing corticosteroids to essentially coat and penetrate the horse’s respiratory tract to reduce inflammation. He listed three options for administering inhaled corticosteroids:

• Mechanical nebulizers These typically have a mask with a powered device to convert a solution into droplets that will get into the horse’s airway. In one study researchers found that a low dose of dexamethasone administered via nebulization didn’t improve lung function in horses with severe asthma, he said, and more work is needed to determine the optimal dose for such cases.
• Metered dose inhalers (MDI) Léguillette said these have good efficacy, but the key is to use the device with a chamber—don’t puff the canister containing the solution directly into the nostril—to ensure all the droplets get where they need to be. He said the Aerohippus mask is his go-to when he prescribes an MDI. Fluticasone administered via an MDI can help relieve clinical signs of severe asthma but doesn’t work quite as well as dexamethasone, he said.
• A soft mist This new option on the market sends a metered dose of solution through an engineered nozzle, producing two streams of liquid that collide, form small droplets, and enter the airway. It uses ciclesonide, Léguillette said, a prodrug (molecules with little or no pharmacological activity) that doesn’t have much anti-inflammatory (or other) effect; it essentially runs through the body. However, when it reaches the lungs’ epithelium, he said, it is metabolized into des-ciclesonide, which has a potency more than 100 times greater than ciclesonide. “That’s why it’s brilliant,” he said. “The principle is basically to get a prodrug that has no side effects if you swallow it, but when it reaches the lungs it turns into a very effective drug.” Studies have confirmed the delivery device and the ciclesonide at the proper dose are effective in alleviating clinical signs in horses with severe asthma. Another benefit is ciclesonide can be used in horses with PPID and EMS, as well as in ponies.

While researchers have studied each type of distribution device, Léguillette said, “it’s difficult to compare studies, between the (different) delivery methods, equipment, (drug) formulations, doses, ages and types of horses, types of asthma, and environmental conditions.”

Finally, Léguillette touched on bronchodilators.

“I see too many people go to bronchodilators first,” he said. “The bronchodilator should be used after you’ve done environmental changes and after steroids. For me, most of the bronchodilators are for severe asthma, when you have a need for immediate short-term rescue.” They can also be used immediately before nebulizing corticosteroids, to improve their distribution throughout the lungs, but this adds a treatment step.

Take-Home Message

“In summary,” Léguillette said, “we have a high prevalence of lung inflammation and asthma; it’s very common and there is no cure. The key is prevention.”

Reducing exposure to irritants with management changes is the first step, with medical management coming second, he said, noting again that a new, safer treatment option now available on the market.