The Newest Treatment Options for Equine Asthma

Back in 2016, the equine industry adopted the term “equine asthma” to describe horses with chronic airway inflammation. This included young horses with inflammatory airway disease as well as older horses with what veterinarians used to refer to as recurrent airway obstruction (RAO) or heaves.1 In the more severe forms of equine asthma, horses suffer from bronchoconstriction (a narrowing of the lower airways called bronchioles), mucus production, and bronchospasm (a temporary constriction of the airways into the lungs caused by muscle contraction). Those hard-hit by asthma usually have an increased respiratory rate, cough, nasal discharge, exercise intolerance, and respiratory difficulty. In some cases a heave line might form along the bottom edge of the ribs due to enlarged abdominal muscles to assist with breathing. Weight loss, anorexia, and exercise intolerance might also occur.

Veterinarians currently have a variety of medical treatments available to manage horses with asthma. Options include anything from over-the-counter breathing aids, natural/herbal/holistic products, and breathing implements to a variety of prescription-strength drugs. Many medications used to treat asthma can have adverse effects on the horse’s body, including laminitis—an intensely painful inflammation of the tissues in the horse’s foot.

Pick Your Weapons

According to the latest asthma treatment guidelines, equine veterinarians endorse a four-pronged approach to controlling airway inflammation. Of these, environmental management remains the most important management strategy.¹

Clearing the Air

While a small population of horses do suffer pasture-associated asthma, the bulk of horses with asthma develop signs of disease when housed in stables and fed hay.²,³ With the lack of airflow typical of many stables (compared to pasture), the more than 50 known allergens in hay and straw can build up quickly! Such allergens including mold spores, mites, and bacterial endotoxins. Inorganic dusts and irritating gases such as ammonia from urine can also aggravate the respiratory tract.4

Veterinarians and researchers consider equine asthma a “disease of domestication” because of the high concentration of airway irritants in stabled areas. An estimated 15% of horses in the Northern Hemisphere have asthma, where horses are stabled for at least part of the year due to inclement weather.4
One veterinary team wrote, “… environmental management should be the primary goal because clinical signs and lung function quickly improve in a low-dust environment, even without medication.”³

In an ideal world, asthmatic horses live in day-round turnout and eat fresh grass rather than hay, which can contain dust, mold, and other airway irritants. While environmental management presents as a simple concept (just keep the horses outside, right?), it can be a demanding process and one with which many owners find difficult to manage.

In a study of 39 horses with asthma, researchers provided owners with guidelines to control their horses’ asthma³: ‘

  • Keep your horse at pasture at least 12 hours a day. If/when stabled, ensure the stall has two openings for fresh air circulation.
  • Do not keep your horse indoors during grooming or stable maintenance.
  • Use dust-free wood shavings, cardboard, etc., for bedding, not sawdust or straw.
  • Do not feed hay unless it is completely soaked. Alternatively, offer cubed or pelleted feed.

One year after being given this list of environmental control strategies for their severely asthmatic horses, only three owners out of 39 adopted all the recommended strategies. In fact, about 50% of owners had “poor” compliance to the management guidelines. The most unpopular strategies were hay soaking and keeping the horse at pasture. Changing to low-dust bedding was the most popular method of improving air quality.

When owner compliance was good, their horses rarely had clinical signs of asthma, and pharmaceutical drugs were unnecessary to maintain respiratory function.

“Owner compliance is therefore pivotal, and persistence in maintaining the newly adopted environmental changes is essential for the horse’s long-term well-being,” concluded the study authors.

Tried and True Oral and Inhaled Options

The two medications most widely recommended for controlling equine asthma are corticosteroids (also commonly referred to as steroids or glucocorticoids) and bronchodilators (β2 agonists).1,5

While systemic anti-inflammatory drugs such as oral prednisolone and injectable dexamethasone can effectively reduce airway inflammation, veterinarians typically try to reserve their use for flare-ups (exacerbations). For daily control to prevent exacerbations, look to inhaled corticosteroids. These medications include fluticasone, budesonide, and beclomethasone that come as inhalants delivered via an aerosol chamber placed over one nostril through which the horse breathes. Inhaled steroids rapidly and effectively improve clinical signs associated with asthma. However, glucocorticoids should be used with caution in horses with a history of laminitis, or at a higher risk for laminitis.

The newest glucocorticoid on the market, Aservo® EquiHaler® (ciclesonide inhalation spray), gained FDA approval for treatment of severe equine asthma in April 2020 after rigorous scientific evaluation.5-7 This inhaled glucocorticoid boasts an improved safety profile compared to fluticasone and other traditional glucocorticoids. ASERVO EQUIHALER comes as an inactive form, only activating once it reaches the lungs.5 When administered to horses with experimentally induced severe asthma, ASERVO EQUIHALER proved itself an effective treatment without the untoward side effects observed with dexamethasone. ASERVO EQUIHALER has not been evaluated in pregnant or lactating mares. ASERVO EQUIHALER, according to the manufacturer’s guidelines, should be administered for 10 consecutive days—five days at eight puffs twice daily and another five days at 12 puffs once daily.6 In a large clinical field study, the most common adverse reactions reported were coughing, nasal discharge, sneezing, and nasal irritation/bleeding.

ASERVO EQUIHALER is not for use in humans and is licensed for intranasal inhalation in horses only. It may be used as part of the overall management of severe equine asthma in addition to other appropriate strategies to control clinical signs, such as environmental changes/modifications and bronchodilator therapy, as needed.6

β2s to the Rescue

Bronchodilators—also called β2 agonists due to the specific type of cellular receptor they bind to in the horse’s body—result in immediate relaxation of smooth muscles. Relaxing the lower airway muscles minimizes airway hyperresponsiveness (one of the causes of coughing in asthmatic horses) and bronchospasm. Examples of commonly prescribed bronchodilators include oral clenbuterol and inhaled albuterol. Less commonly, veterinarians prescribe other β2s such as terbutaline, ritodrine, fenoterol, salbutamol, and salometrol.8 Researchers recently found that both fenoterol and salbutamol more effectively relaxed precontracted bronchial smooth muscle than clenbuterol, opening new doors to this traditional class of medications.

Other Sighs of Relief 

β2 agonists and corticosteroids control asthma most effectively when used together, especially with environmental management and when adding omega-3 fatty acid nutritional supplementation (1.5 mg DHA by month once daily).¹

Clear Breathing Ahead—Novel Treatments in Development

No single solution for equine asthma exists. Lack of perfection, however, doesn’t stop progress, and many equine researchers continue to explore novel paths to make breathing second nature again.

Stem cell therapy. Because stem cells have potent anti-inflammatory and immunomodulatory properties, scientists are looking to harness those properties for diseases like asthma. To date, researchers have only conducted studies in mice, cats, and humans. This technique shows promise, but dosing, frequency of administration, and treatment timing need to be explored further.9

Inhaled microscopic immunomodulatory particles. This technology uses small inert, biodegradable particles to alter the immune system’s function to decrease airway inflammation. These particles work like “allergy shots” but don’t need to be custom-made for each patient. When researchers tested the particles in horses, they noted significant improvements in respiratory parameters that persisted for eight weeks.4   

Azithromycin, an immunomodulatory antibiotic. Azithromycin can do far more than just fight infections. This antibiotic also has immunomodulatory properties. When studied in asthmatic horses, levels of proinflammatory interleukins-1b and -8 (that attract white blood cells) decreased following treatment. This medication has not yet been studied in enough detail, however, to warrant use in actual patients.10

Breathing It All In

If this feels like a lot of information, it is. Take a deep breath, review your options, and focus on preventing asthma exacerbations via environmental management strategies. Importantly, always consult your veterinarian before initiating any form of treatment to ensure the diagnosis is correct. Several infectious diseases can present similarly. If entering a competition, always check with the appropriate governing body when using any of the above-described medications to ensure they are not banned substances.

Sources

  1. Couëtil LL, Cardwell JM, Gerber V., et al. Inflammatory airway disease of horses–revised consensus statement. J Vet Intern Med 2016;30(2):503-15.
  2. Managing Summer Pasture-Associated Obstructive Pulmonary Disease, an Asthma-like Disease of Horses. Available at: https://aaep.org/horsehealth/managing-summer-pasture-associated-obstructive-pulmonary-disease-asthma-disease-horses.
  3. Simões J, Sales Luís JP, Tilley P. Owner compliance to an environmental management protocol for severe equine asthma syndrome. J Equine Vet Sci 2020;87:102937.
  4. Klier J, Bart C, Geuder S, et al. Immunomodulatory asthma therapy in the equine animal model: A dose-response study and evaluation of a long-term effect. Immun Inflamm Dis 2019;7(3):130-149.
  5. Lavoie JP, Bullone M, Rodrigues N, et al. Effect of different doses of inhaled ciclesonide on lung function, clinical signs related to airflow limitation and serum cortisol levels in horses with experimentally induced mild to severe airway obstruction. Equine Vet J 2019;51(6):779-786.
  6. Freedom of Information Summary. 2020. Original New Animal Drug Application. Aservo® Equihaler® (ciclesonide inhalation spray).
  7. Beckstett A. Inhaled ciclesonide: potential help for asthmatic horses. Available at: https://thehorse.com/190483.
  8. Pozzoli C, Bertini S, Poli E, et al. Relaxing effects of clenbuterol, ritodrine, salbutamol and fenoterol on the contractions of horse isolated bronchi induced by different stimuli. Res Vet Sci 2020;128:43-48.
  9. MacDonald ES, Barrett JG. The potential of mesenchymal stem cells to treat systemic inflammation in horses. Front Vet Sci 2020:6;507.
  10. Mainguy-Seers S, Vargas A, Labrecque O, et al. Randomised study of the immunomodulatory effects of azithromycin in severely asthmatic horses. Vet Rec 2019;185(5):143.

ASERVO® and EQUIHALER® are registered trademarks of Boehringer Ingelheim Vetmedica GmbH, used under license. ©2021 Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All rights reserved. US-EQU-0303-2020