Equine asthma does not discriminate. It can affect horses young and old, male and female, large and small, and of all breeds. It appears in all climates and geographic regions. And it doesn’t care if horses are high-octane elite athletes or plump pasture puffs. Equine asthma can cause respiratory distress, reduced performance, and other problems anywhere, anytime.
Because many treatment and management options are based on how it develops and impacts a horse, veterinarians need a good understanding of the mechanisms behind equine asthma. Laurent Couëtil, DVM, PhD, Dipl. ACVIM, reviewed these topics at the 2021 American Association of Equine Practitioners’ Convention, held Dec. 4-8, in Nashville, Tennessee.
“The prevalence of asthma in horses is quite high,” said Couëtil, a professor and section head of Large Animal Internal Medicine, and director of both the Equine Sports Medicine Center and Equine Research Programs at the Purdue University College of Veterinary Medicine, in West Lafayette, Indiana.
Studies suggest that based on evidence of excess mucus in the trachea, mild-moderate equine asthma (MEA, formerly known as inflammatory airway disease and other terms) can be found in about 13% to 22% of racehorses, 31% of sport horses, and 20% of pleasure horses. Based on bronchoalveolar lavage fluid (BALF) cytology, however, researchers estimate the prevalence of MEA is as high as 80% to 95% in Thoroughbred and Standardbred horses racing in Europe and the United States.
Severe equine asthma (SEA, previously known as heaves, recurrent airway obstruction, or chronic obstructive pulmonary disease, among other terms) is the most common chronic respiratory disease in adult and senior horses, affecting an estimated 14% in horses living in cool climates in the Northern Hemisphere.
Couëtil noted that he and others recommended using equine asthma—the newest term used to describe these issues—to help people understand the condition. He started using the term asthma with his clients and realized immediately that they better comprehended what he was describing, and quickly. “It helped the discussions tremendously,” he said. “So we decided to introduce that new terminology (in a consensus statement); we felt that it was actually a simplification to use asthma syndrome in horses and accurate based on the current understanding of the disease in both horses and humans.”
While MEA and SEA are part of the same disease process, they generally present differently. Horses with MEA are typically younger to middle age and experience decreased performance and sometimes cough. When the inciting factors are removed, MEA typically improves and has a low recurrence rate. On the other hand, SEA commonly affects older horses (often 7 years and older, Couëtil said) and presents as frequent coughing, exercise intolerance, and difficulty breathing at rest.
“Young horses with mild asthma, most of them actually grow out of it, a bit like the kids with atopic asthma,” Couëtil said. “The serious asthmatic horses are affected for life. You can manage them by controlling their environment. But, for the most part, if you expose them (to irritants) again, they’ll have recurrence of the disease.”
Horses with MEA and SEA both have excess mucus in the trachea when viewed endoscopically, but a BALF analysis can help differentiate between the two conditions.
“Severe asthmatic horses tend to have a severe neutrophilic inflammation—neutrophils typically over 25%,” he said. “Mild asthmatic horses can have a mild neutrophilia (somewhere between 5% and 20%), eosinophilia (more than 1%), or mastocytosis (more than 2% mast cells) or any combination thereof.”
Couëtil also noted that researchers in the U.K. and Australia have linked tracheal inflammation to high bacterial counts in tracheal washes of racehorses. “The relationship between that bacterial tracheitis and mild equine asthma is not quite clear at this point,” he noted. “We do know, certainly, that in horses with severe equine asthma it’s quite common to isolate bacteria in their tracheal mucus, as well.”
What’s Behind Asthma?
It’s a simple question with a seemingly simple answer: “The main driving force for asthma is exposure to dust and mold,” Couëtil said.
Dust, mold, and other irritants lurk in many places horses frequent and food they consume, such as hay or pasture, he said.
The etiology can vary between horses with MEA and SEA, however.
In horses with MEA, Couëtil said, “many causes have been considered, such as an infectious component, either bacterial or viral. Certainly, there could be some environmental factor at play—exposure to dust or whatever endotoxin content may be there. And then, of course, the horse itself will react differently depending on those factors, based on their genetic background, immune status, age, etc.”
He reviewed some key study findings on the topic:
- Researchers have identified a strong association between tracheal inflammation and the presence of bacteria (especially over 1,000 colony-forming units [CFU] per milliliter of tracheal wash); in that study, Streptococcus equi subspecies zooepidemicus, pneumoniae, and Mycoplasma spp were the three main bacteria identified, he said.
- In a study of young Thoroughbreds, horses that coughed during training in the morning were more likely to have a tracheal bacterial culture containing at least 1,000 CFU/ml; horses that didn’t cough during morning training either had no or small numbers of bacteria isolated from the wash, he said.
- In another study, researchers found no relationship between post-race presence of neutrophils and other cells on tracheal wash cytology and performance. “What was associated with performance was tracheal mucus accumulation after the race,” Couëtil said. If the horse had no excess mucus or just a few blobs here or there, there was no impact on performance; any more mucus than that was associated with a poor showing.
- Researchers are still exploring the potential role of less common respiratory viruses (such as equine rhinitis or equine herpesvirus- 2 or -5) in MEA, because studies have shown they’re common in nasal swab samples from sick horses and healthy control horses. There have also been conflicting study results about the association between MEA, BAL cytology, clinical signs, performance, and respiratory viruses, he added; some found links between MEA and respiratory viruses while others did not .
- Scientists have linked irritant exposure to decreased performance. “Dust exposure is really the driver of airway inflammation, especially neutrophilia, in racehorses at the track,” Couëtil said. For instance, “one study in Thoroughbreds at a racetrack showed an association between the tracheal mucus score and exposure to small dust particles that we call PM 10—particulate matter less than 10 microns in diameter.” Studies in Thoroughbred yearlings and racehorses have shown that as respirable dust (less than 4 microns in diameter) exposure increases, so does the percent of eosinophils and neutrophils, respectively, in BAL. Researchers have shown an association between β-glucan—a component of fungal and plant cell walls—in dust and mast cell accumulation in BAL. More importantly, he said, all the horses with dust-related airway inflammation also experienced decreased performance.
“The bottom line is dust is driving inflammation, and inflammation is bad for performing, even with a small number of inflammatory cells,” he said.
In horses with severe asthma, he said, “we’ve obviously known for quite some time that this is really an allergic disease caused by mold content in hay. This has been described in horses in the wintertime, stuck in the barn and fed hay.”
But it’s not just poor-quality hay that causes issues. Frequently, in his university practice, “when horses come in as a referral, I ask the owner about what they feed to their horses,” he said. “They say they’re feeding hay, and I tell them we know it’s triggering this condition. They say, ‘Oh no, it’s not possible. This is really good hay.’”
When the owner brings a hay sample, it often looks pretty good, he said. But in a soon-to-be-published study, Couëtil tested whether what appears to be high-quality hay could cause issues in seven severely asthmatic horses. When challenged with moldy hay, he said, six horses with SEA developed clinical signs of respiratory distress. When they consumed the good-looking hay, still four of them developed signs of respiratory distress, he said.
Horses with SEA also developed neutrophilia evident on BAL following hay exposure, he said. Additionally, he said, the team identified up to 33 fungal species in both the moldy and good-looking hay, indicating even hay that looks fantastic can harbor molds that can irritate a horse with asthma.
“This study tells me that even good hay is not good for horses if they have severe asthma,” Couëtil said.
In addition, researchers have shown that endotoxins in dust have a synergistic effect on neutrophilic inflammation: “If you have the (dust) particles and the endotoxin together, the effect is greater. You have more neutrophilic inflammation compared to any single component,” he said.
Couëtil touched on the fact that airway inflammation, especially if it’s chronic, can result in remodeling that negatively impacts lung function and gas exchange.
“Pulmonary gas exchanges are the limiting factor to performance in fit horses exercising strenuously,” he said; their maximum aerobic capacity during maximal intensity exercise is known as the VO2max. “In this context, a relatively mild degree of MEA can significantly impair gas exchanges and result in decreased performance.”
Horses with MEA exercising at less than 50% of VO2max likely won’t experience exercise intolerance. However, researchers have shown that Standardbreds with MEA performing an exercise test below VO2max did have higher pulmonary artery pressures than did healthy controls. Couëtil said this could be due to increased vascular resistance.
Horses with SEA, on the other hand, have more obvious issues, including pulmonary hypertension and hypoxemia.
Ventilation-perfusion mismatching—one or more areas of the lung receiving oxygen but no blood flow or vice versa—is largely responsible for gas exchange abnormalities during asthmatic episodes, and affected horses have a harder time breathing.
Additionally, scientists know that horses with SEA have more smooth muscle in their airways than do healthy controls, which likely explains why the airways shut down and clinical signs develop.
Couëtil closed by reemphasizing his key points for veterinarians to remember.
“I hope I convinced you that mild-moderate asthma is associated more with the exposure to small dust particles, and there’s just a minor role for infectious agents,” he said. “For serious asthma, this is classically associated with exposure to different molds, and these molds can be present in good-quality hay.
“And finally, there are a lot of changes in those horses’ lungs due to the inflammation and the remodeling of these airways from chronic inflammation,” he said. “Airway obstruction will lead to impaired gas exchanges.”