A Fresh Look at Heaves
Researchers definition of this respiratory issue continues to evolve; here’s what we currently know
Is it common to hear your horse’s cough echoing from his dusty stall? If so, don’t dismiss it as you would your own throat tickle in the same setting—as something that will resolve on its own. Your horse might be confined in this setting, unable to avoid the dust, and he could have developed a chronic breathing problem called heaves. These horses suffer impaired respiratory function (and, hence, athletic performance) due to airway congestion and constriction. The condition generally results from breathing such dust, along with the mold particles and pollen innate to the environments in which we keep our horses.
In this article we will describe the mechanisms behind heaves and what researchers are discovering about it.
What is Heaves?
Historically, many people believed horses with heaves suffered from chronic obstructive pulmonary disease (COPD)—a common lung disease in humans that causes difficulty breathing, says Rose Nolen-Walston, DVM, Dipl. ACVIM, assistant professor of large animal internal medicine at the University of Pennsylvania’s New Bolton Center.
“It is a chronic disease, and it does cause obstruction … and it is a lung disease,” she says. “But COPD in humans is very different from heaves in horses.”
The leading cause of COPD in humans is cigarette smoking, which causes airway inflammation and damage and produces airway-blocking mucus. Long-term exposure to other irritants can cause the same. Over the past 15 years researchers have learned that heaves is more akin to human asthma, in which the airways tighten and make respiration difficult. “Whereas COPD involves permanent scarring (and can be the end stage of heaves after many years of breathing problems, seen as damaged lungs), the most common case of heaves that we treat is simply an airway constriction,” says Bruce Connally, DVM, an equine sports medicine practitioner in Longmont, Colorado. “If you can get the airways open again, the horse does fine.”
But asthma isn’t a perfect comparison either. While asthma sufferers have allergic airway disease, horses with heaves experience something that allergy testing won’t diagnose, says Nolen-Walston.
“Heaves is generally not an allergic response, even if the trigger is hay dust or hay mold,” she explains. “So we try to think in terms of triggers rather than allergens.”
Dorothy Marie Ainsworth, DVM, PhD, Dipl. ACVIM, professor of large animal medicine at Cornell University’s College of Veterinary Medicine, likens heaves to occupational asthma—a condition in people who work in farming, constantly breathing organic dust particulates. “We still need to find out what the underlying immunological mechanisms are and how all the cells in the interstitial lung tissue, the airways, and the lining (epithelium) work together to cause this inflammatory reaction,” she adds.
Researchers know that certain equine family lines seem more likely to develop heaves when placed in environmental conditions that favor it. And within these lines, horses that have one parent with heaves are more likely to develop the condition, and horses with both parents affected are significantly more likely to have heaves. “What we don’t know is if there are ways we can prevent the development of clinical heaves in horses that have the genetic propensity for heaves,” Nolen-Walston adds. Identifying the exact genes involved will help scientists determine this.
Signs That Something’s Wrong
Heaves is part of a chronic inflammatory process involving the lung’s mucosal lining. “The inflammatory component is characterized by neutrophils (a type of white blood cell) coming into the lungs, mucus production that plugs the airways, and constriction of the bronchial tubes,” Ainsworth says. Small airways might also spasm, and their walls might thicken.
Well-known obvious clinical signs of heaves include the constant coughing described, labored breathing as the horse exhales, and wheezing, which might be especially noticeable when a horse eats hay or when he exerts effort under saddle. “Milder symptoms include a slowdown in performance,” says Connally. “For instance, a barrel horse may run the barrels a half-second or a second slower,” due to reduced lung capacity.
Why the emphasis on the exhale? While drawing air into the lungs through constricted airways is difficult, expelling it is even more so because it requires an extra push from the abdominal muscles. The effort is such that chronic heaves sufferers develop a characteristic “heave line” along the lower abdomen—a ridge of muscle that becomes thicker from the extra breathing exertion.
“Chronic, long-standing cases get to where the horses are breathing so hard that they lose weight,” Connally says. “They can’t hold body condition because they are putting so much effort into breathing.” They also spend more time and effort trying to breathe than eating.
“This is similar to a person with end-stage lung disease from smoking,” he adds. “These people are often very thin because it takes so much effort to breathe. I occasionally see horses at this stage. You can’t ‘fix’ those, and often you can’t even maintain them; they are near the end of the line.”
Prevention and Treatment
The good news is you can examine your horse’s risk factors and potentially stave off heaves altogether. The typical horse with heaves develops signs when you put him in a dusty barn and feed hay. “The best thing for those horses is to stay outside all the time,” says Ainsworth.
Nolen-Walston shares a story about a researcher planning a heaves study: “She got a group of horses donated that all had heaves,” she recalls. “As she accumulated the horses she put them out in a big pasture on grass. When she had enough to start her study she brought them all in and tested their lungs … and they were all essentially normal. These were horses that had heaves so severe that people had given them up, but after just turning them out on pasture they became symptom-free.”
Your best prevention might simply be housing your horse in a field (not a dusty paddock) with a run-in shed for shelter and eliminating access to dusty hay.
If full-time outdoor housing is not a reality, you can soak—not just spritz, says Nolen-Walston—the horse’s hay for about five minutes before feeding to eliminate some of the dust. Other alternatives include purchasing dust-free hay, less-dusty pelleted feed or hay cubes, or hay steamers, which she says are quite effective.
Also turn horses out while cleaning and bedding stalls. “Research has shown that for at least two hours after the stall is cleaned, the level of dust is highest,” says Nolen-Walston.
And, if possible, do not bed these horses on straw. “Straw has an enormous amount of dust—more fine particles than hay dust,” she says. “One of the big offenders is endotoxin, which is just a part of bacterial cell walls found commonly in soil and manure. In straw dust, the level of inhalable endotoxin is often 10 times the acceptable level for a human to inhale during an eight-hour shift. If a horse is standing in a stall for 20 hours, he is breathing very toxic air. The level is much lower on wood chips or wood shavings.”
Horses that are symptomatic, or horses for which living in a dusty environment is unavoidable, can be medicated with steroids, Ainsworth adds. Systemic steroids can be given orally or they can be administered into the lungs (topically) via an inhaler or nebulizer.
Nolen-Walston warns, however, that steroid use can adversely impact the stomach, kidneys, or feet. “If a horse has a tendency toward laminitis or has equine metabolic syndrome or uncontrolled Cushing’s disease, we try to avoid systemic steroids,” she says.
These horses might be better off with bronchodilator therapy, using agents such as clenbuterol (trade name Ventipulmin, the only FDA-approved equine bronchodilator) to reduce the lungs’ smooth muscle contractions and help the airways stay open. Some bronchodilators also have anti-inflammatory effects.
“This would reduce the inflammatory mediators generated in the airways,” Ainsworth says. “The bronchodilator therapy by itself is not nearly as effective as steroid therapy, however.”
Nolen-Walston cautions that she and colleagues at New Bolton Center discovered that if they administer either clenbuterol or another beta-2 agonist, such as albuterol, for more than 14 days by itself, the treatment stops working. “If you keep using it longer than that, the horse will have more bronchospasm and more trouble breathing,” she says. “It is only a band-aid. You can use it when the horse is in trouble—having a heaves attack—to immediately give him relief for a few days. But it does not solve the problem.”
In other words, clenbuterol helps the horse breathe better but only temporarily. “The most common mistake we see is horses being treated with just clenbuterol for months, and it quits working,” Nolen-Walston says.
Ainsworth says that sometimes she and fellow clinicians use clenbuterol and corticosteroids together in an acute case in which a horse is having trouble breathing.
Horse owners should think of heaves as “horse asthma,” a performance- and quality-of-life-limiting condition. While veterinarians can turn to treatments such as bronchodilator therapy to relieve clinical signs when necessary, it’s even more important to try to avoid the inhaled triggers—such as barn/hay dust and molds—in the first place.
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