Research shows that an estimated 14-20% of horses in the northern hemisphere suffer from severe equine asthma (SEA). Therefore, this condition is one many practitioners will see. Veterinarians agree that unmanaged or recurrent severe equine asthma is both an economic and healthcare burden. It leaves untreated horses with an extremely poor quality of life.
Considered under the umbrella term “equine asthma,” severe equine asthma falls at the extreme end of the spectrum. Affected horses suffer cough, nasal discharge, nasal flaring, increased respiratory effort at rest, and exercise intolerance. In chronic cases, a heaves line might be present.
If you read the textbooks and expertly authored articles such as the ACVIM Consensus Statement (available online through open access at https://onlinelibrary.wiley.com/doi/full/10.1111/jvim.13824), guidelines for approaching severe equine asthma are clear: diagnostics first, to rule out other conditions that could mimic SEA, then targeted treatment for managing asthma with a concerted effort to minimize the use of medications. But as we know, the academic world is not always applicable to the real world.
The goal of this article is to relay to practicing veterinarians “in the trenches” what a textbook picture of diagnosing and managing severe equine asthma currently looks like. We then address the challenges associated with achieving a textbook treatment. After, we will look at how front-line practitioners can encourage clients to move in the direction of expert recommendations. This requiers overcoming barriers to successful management.
The Gold Standard Approach to Severe Equine Asthma
“SEA is a non-septic inflammatory condition of the lower airways occurring in susceptible horses upon exposure to environmental allergens. SEA is characterized by bronchospasm, mucus accumulation in the airways and airway smooth-muscle remodeling, all of which are a consequence of inflammation,” said Laurent Couëtil, DVM, DACVIM, associate professor of large animal medicine and director of the Equine Sports Medicine Center at Purdue University’s College of Veterinary Medicine.
Diagnosis
In a world without financial, equipment/skill or geographic barriers (i.e., proximity of referral centers), Couëtil said, “I think that the minimum database for an initial evaluation of a horse suspected of SEA is a detailed history such as risk factors like hay feeding, a thorough physical exam, hematology and inflammatory biomarkers (serum amyloid A or fibrinogen), as well as bronchoalveolar lavage (BAL) cytology.” Only then, if findings are consistent with SEA, should the horse be treated and the horse’s response to treatment evaluated.
“If there is no response to treatment, then we need to do a more in-depth workup,” advised Couëtil. “This advanced workup would include endoscopy and pulmonary function tests, either with or without thoracic radiography. I rarely do the latter but routinely do pulmonary function tests.”
Treatment
The current agreed-upon goals of treatment are to decrease airway inflammation, relieve bronchospasm and reduce mucous secretion.
Environmental Control of Allergens
The main environmental sources of allergens are respirable dust particles in feed and bedding—such as mold spores, mites, endotoxins, antigenic materials and pollens. Of those, the number one cause of allergen exposure is hay dust and exposure from round bales.
Simões et al. (2020) therefore recommended the following for minimizing exposure of horses to common environmental allergens:
- Have at least two openings in a stall for fresh air circulation to address ventilation.
- Stable horses minimally.
- Ensure horses spend at least 12 hours per day outdoors.
- Remove horses from the stables while cleaning/mucking.
- Use dust-free wood shavings, cardboard or other low-dust options.
- Avoid dry hay and instead offer soaked hay or cubed/pelleted feeds.
When environmental control of allergens alone provides insufficient control of severe equine allergy, owners should consider pharmacologic options. Owners need to appreciate that there is no cure for this condition. It is recurrent in nature (hence its previous name, recurrent airway obstruction or RAO).
More on Environmental Control
In addition to those referenced in research by Simões et al., key environmental control strategies recommended by Couëtil include the following:
- Don’t feed round bales without a covered feeder.
- Don’t keep affected horses in the barn during cleaning.
- Don’t feed hay in a hay net or rack in the stall.
- Do turn out horses to grass pasture and supplement with omega-3 fatty acids when grass is not available.
- Do feed horses from the ground.
- Do reduce dust exposure in severe equine asthma horses by wetting hay down or using hay steamers.
Here are some handy tips when soaking/steaming hay:
- Both soaking and steaming hay significantly reduce respirable dust particles.
- Soaking hay also reduces water-soluble carbohydrates (WSC). This can be a benefit for metabolic horses that require diets with restricted non-structural carbohydrates. Kate Hodson, DVM, of Hodson Veterinary Services LLC in Hebron, Indiana, points out that a number of horses with severe equine asthma are also metabolic.
- Soaking for 15-30 minutes will reduce WSC without resulting in nutrient deficiencies.
- Soaking too long leaches nutrients from the hay. Extended soaking might mean owners should offer horses a ration balancer or vitamin and mineral supplement.
- Steaming hay reduces bacterial counts almost entirely, thereby benefiting horses with severe equine asthma. In contrast, soaking hay increases bacterial counts and is less hygienic than steaming hay unless fed directly after soaking.
Horses appear to prefer steamed hay over soaked hay in terms of palatability.
Corticosteroids
“If finances are a major limiting factor, I recommend systemic dexamethasone for three to five days. If the horse improves, then taper the dose down,” recommended Couëtil. But, he said, it is paramount to first address the cause (i.e., exposure to allergen/dust).
“If not, the horse will be back to square one with one to two weeks of stopping medication,” Couëtil warned.
He added, “Depending on the situation, I may recommend aerosol therapy with either ciclesonide or a nebulized corticosteroid. Specifically, my first choice for aerosol therapy would be ciclesonide. It is an all-in-one device (Aservo Equihaler) that would be cheaper than other alternatives. Budesonide would be my first choice for nebulization, but the owner will need to purchase a nebulizer. Hence, this is the more expensive option.”
Ciclesonide (Aservo EquiHaler), which has only been available in the United States since the spring of 2021, is a glucocorticoid product that is de-esterified in the horse’s lung to its active metabolite. That metabolite has up to a 120-fold increase in glucocorticoid receptor binding affinity than ciclesonide itself (Pirie et al. 2020).
According to Couëtil, “This product has a very good safety profile and is effective in horses with SEA.”
For example, Pirie et al. (2020) reported a success rate of 73.4% in 109 horses after 10 days of treatment (doses used in the study were eight actuations twice daily for five days, then 12 actuations twice daily for five days, then 12 actuations once daily for the next five days).
Although used in some veterinary practices, Couëtil does not recommend nebulized dexamethasone as it “does not work in SEA.” This was demonstrated in a study conducted by Wasseige et al. (2020) in six horses treated with 5 mg nebulized dexamethasone. Lung function tests showed no improvement following treatment (once daily for seven days). In that study, however, improvement in lung function was appreciated following orally administered dexamethasone (5 mg by mouth once daily for seven days).
That research further indicated that inhaled dexamethasone, but not ciclesonide or low-dose fluticasone, suppresses the hypothalamo-pituitary-adrenal (HPA) axis. This occurs when supraphysiologic doses of glucocorticoids suppress hypothalamic secretion of corticotrophin-releasing hormone and adrenocorticotropic hormone (ACTH) by the pituitary gland, resulting in secondary adrenal cortex atrophy.
With profound or prolonged ACTH deficiency, the adrenal glands might be temporarily unable to generate sufficient cortisol. The HPA axis plays a major role in the stress response and in host defense against infection. Suppression of the HPA axis results in an impaired stress response and an inadequate host defense against infection.
The goal of treatment is not only to improve inflammation but also to reverse smooth airway remodeling.
“Smooth muscles are arranged in a spiral formation around bronchi and bronchioles. Their contraction results in narrowing of the airway lumen,” explained Couëtil. “This occurs normally to protect the lung from inhalation of irritants (e.g., feed particles, dust, ammonia). Chronic airway inflammation occurring in asthma, however, results in thickening of the smooth muscle and subsequently enhanced narrowing of the airway lumen. This makes it harder for horses to inhale.”
In addition to decreasing inflammation in the airways, inhaled corticosteroids can also improve airway remodeling to some extent. In the study by Bullone et al. (2017), a combination of fluticasone and the long-acting Beta 2 agonist salmeterol or fluticasone monotherapy improved peripheral smooth muscle remodeling after 12 weeks. But that improvement was no better than when horses were turned out to pasture. This again highlights the importance of control in cases of severe equine asthma.
Other Therapies
Additional medications recommended for severe equine asthma include bronchodilators, mucolytic agents and omega-3 fatty acids as described in the ACVIM Consensus Statement (Couëtil et al. 2016).
In summary, Couëtil said that bronchodilators allow horses with severe equine asthma to improve clinically faster because they reduce the effort of breathing within minutes of administration. Mucolytic agents might help some horses in combination therapy but are not sufficient by themselves. Omega-3 polyunsaturated fatty acid supplementation can result in clinical improvement within a couple weeks when combined with reduced dust exposure (e.g., switching from hay to pelleted feed).
SEA in Daily Practice
With the seemingly “obvious” or pathognomonic clinical signs, diagnosing severe equine asthma might outwardly appear rather straightforward. Indeed, some owners pre-diagnose their horses before their veterinarians even see the animal. Further, some owners might have already decided that a short course of steroids will resolve those clinical signs, hopefully permanently.
“This is a condition in which we are practicing with one hand tied behind our backs,” said Jake Jensen, DVM, partner at Janssen Veterinary Clinic in Sheridan, Indiana. “Most clients want to try empiric treatment first. Often it is only in the face of treatment failure will owners consider a BAL or referral.”
Diagnosis
Kate Hodson, DVM, is in a good position, as she has both the equipment and the wherewithal to perform BALs.
“That said, I have many backyard situations in which the owner’s pocketbook is the major limiting factor, not equipment,” said Hodson.
Hodson can easily perform a BAL in a backyard setting under light sedation. However, the procedure requires two assistants—one to hold the horse and the other to serve as a “driver” for the endoscope. “This is a specialty procedure in my solo practice. If I am super busy, then I need to refer my client to Purdue because they can get in faster,” said Hodson.
Once Hodson collects the BAL fluid, she submits it to Antech, which offers interpretation and can also offer guidelines.
Additional important points made by Hodson are that: 1) owner buy-in will be improved by explaining to them that horses tolerate the procedure very well, remain standing during the procedure, and are awake in 30 to 40 minutes; and 2) a transtracheal wash is not going to provide needed diagnostic information.
Considering that severe equine asthma is relatively common and should be definitively diagnosed prior to initiating treatment, practitioners might wish to consider continuing education to become (more) adept at performing BALs.
Hodson encourages new practitioners to attend as many hands-on workshops as possible and to work with clinicians at referral centers.
“Phone a friend,” Hodson quipped. “I email Dr. Couëtil frequently for advice when managing difficult cases.”
Jensen added: “One thing I find is that if I use the term ‘equine asthma syndrome,’ then it helps owners understand that there will be long-term management. As a result, they may be inclined to be more aggressive up front with testing.”
Treatment
Environmental Control of Allergens
In their survey, Simões et al. found that owners have poor compliance when it comes to minimizing allergen exposure. In that study, 39 horses diagnosed with severe equine asthma were included.
One year after giving clear instructions to owners of severely asthmatic horses, the study authors conducted a follow-up telephone interview. They asked owners a series of questions regarding their current management routines. The goal of those questions was to determine whether the owners had followed the guidelines and whether the recommendations had a positive effect on the horses’ respiratory tracts. The survey found that the overall compliance to the environmental management guidelines was poor.
Specifically, the survey found:
- Compliance was “extremely low” in 51.3% of the cases.
- More than half of the owners/managers implemented two or fewer of the six suggestions for allergen avoidance.
- Compliance was “good” in only 15.4% of cases (n=6), with owners implementing five to six of the recommended parameters.
- Only three owners/managers were able to adopt all six management recommendations.
- Four owners failed to adopt any management changes.
Not surprisingly, where there were no management changes implemented, the horses failed to show any clinical improvement over the course of that year. They required corticosteroids and bronchodilators to control clinical signs of disease.
The two most difficult management strategies the study authors identified were hay soaking and keeping horses on pasture. At least 16 horses with severe equine asthma remained in stables with poor ventilation.
The easiest recommendation to adopt was a change in the type of bedding.
In Jensen’s experience, owners are often “gung-ho” to try new types of bedding—such as dust-free shavings or cardboard—if the horse is housed at home. However, owner dedication to environmental change is often short-lived, he admitted.
As recognized in other studies, hay was identified as a trigger in a substantial number of cases (64.1%). According to the research team, owners appeared reluctant to change their horses’ forage. Hay was not appropriately soaked but rather “sprinkled,” which was ineffective as the center of the hay remained dry. Admittedly, hay soaking is time-consuming, which likely explains why this management recommendation was poorly adopted.
“At boarding facilities, control of SEA is hard,” explained Katie Coleman, DVM, of Premiere Equine Ambulatory Veterinary Service in Lemont, Illinois. “Owners have little control over most of the management of their horse because they’re not there every day. Boarding facilities aren’t usually willing to soak hay, and they typically have round bales out in the field. Managing asthmatic horses is much easier in privately owned and manged horses.”
Jensen concurred, saying that owners capable of controlling their horses’ environments at boarding facilities are few and far between.
Simões et al. suggested, “It is possibly easier for owners to adopt a change in protocol which solely involves a new purchase (e.g., wood shavings) rather than one that requires a change in everyday habits (e.g., turning out the horses or taking them from the stables during periods of increased activity) or modifications of infrastructures (e.g., ventilation).”
That said, the basic management strategies described earlier do appear efficacious. The research noted that “increased compliance had an effet on the absence of respiratory effort at rest, absence of cough, decreased frequency of clinical signs, non-administration of pharmacological treatment, and clinical improvement of the horse.”
Backyard Horses
“The gold standard goes out the window sometimes with backyard horses,” Hodson acknowledged. As a result, primary practitioners treat many horses with a short course of dexamethasone.
“This can be either oral or injectable, just to get the flare under control,” said Jensen. After that initial treatment, Jensen attempts to have owners use inhalers. In his hands, the preloaded, one-time-use ciclesonide (Aservo EquiHaler) is useful short-term for flare-ups. Budesonide through a nebulizer works well for longer-term management of severe equine asthma.
Other Therapies
“For horses with SEA that are breathing hard at rest, I typically prescribed clenbuterol and antihistamines in addition to the short tapering dose of dexamethasone,” shared Hodson.
Jensen said he also uses clenbuterol, but only in short bursts because the horse’s response to that medication decreases over time. He also routinely recommends an omega-3 supplement for his asthmatic horses.
Hodson said that inhalers have been a game-changer. “Inhalers delivering bronchodilators such as albuterol directly to where they need to go have totally improved the quality of life of these horses.”
That said, both Hodson and Jensen said that the number of owners using these inhalers are few and far between.
“But the Aservo product has made it much easier for owners compared with purchasing a nebulizer,” Jensen said.
Pitfalls with Shortcuts
In the face of a “classic picture of asthma,” it might be tempting to go ahead and treat with systemically administered corticosteroids such as dexamethasone or prednisolone. Indeed, prednisolone 1.1-2.2 mg/kg by mouth once daily does benefit horses with severe equine asthma, improving their clinical signs.
As mentioned above, blindly reaching for systemic corticosteroids can have important side effects (e.g., HPA axis suppression, laminitis), might not address underlying comorbidities, and could delay application of other beneficial therapies.
“Giving dexamethasone without decreasing dust exposure is like giving phenylbutazone to a lame horse without addressing the cause of the lameness,” stated Couëtil.
Summary of Recommendations
Focusing on environmental management first is of utmost importance in effectively managing horses with severe equine asthma. As noted in the study by Simões, owner compliance is “pivotal.” Veterinarians have the challenge of effectively relaying to owners the “risks of lack of compliance,” according to that study.
Couëtil said, “Environmental control to reduce dust exposure is essential, and pharmacological agents will help speed up resolution. It will take two to three months for horses to recover with just environmental control; three to six weeks when supplemented with omega-3 polyunsaturated fatty acids; and one to two weeks when combined with an inhaled corticosteroid.”
Careful follow-up by veterinarians will identify an owner’s adherence to management recommendations. This could have life-altering effects on the horse’s long-term well-being.
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