Heaves is a chronic respiratory condition in horses that causes difficulty breathing, persistent coughing, and reduced exercise tolerance. It’s now officially classified as severe equine asthma, the most serious end of a spectrum that also includes milder forms of airway inflammation. The name “heaves” comes from the visible abdominal effort affected horses use to push air out of their lungs, a hallmark sign that distinguishes this condition from a simple cough or cold.
How Equine Asthma Is Classified
Veterinarians have used many names for this condition over the years, including broken wind, chronic obstructive pulmonary disease (COPD), and recurrent airway obstruction (RAO). The current terminology groups all of these under the umbrella of “equine asthma,” which covers three related conditions: inflammatory airway disease (IAD), recurrent airway obstruction, and summer pasture recurrent airway obstruction.
Mild to moderate asthma corresponds to IAD, which typically causes subtle performance issues and occasional coughing. Severe asthma corresponds to what horse owners have traditionally called heaves. The progression isn’t always linear. Some horses develop mild symptoms that stay mild for years, while others deteriorate quickly if their environment doesn’t change.
What Happens Inside the Airways
In a healthy horse, the resting respiratory rate sits around 8 to 12 breaths per minute, and exhaling is a passive, effortless process. In a horse with heaves, the small airways deep in the lungs become inflamed, constricted, and clogged with excess mucus. This makes it progressively harder to move air out.
Because the airways won’t empty on their own, the horse recruits abdominal muscles to actively force air out. Over time, one of the abdominal wall muscles thickens from constant overuse. This creates a visible ridge running diagonally along the horse’s barrel, known as a “heave line.” That line is a telltale sign of chronic, long-standing disease and indicates the horse has been compensating for restricted airflow for a significant period.
Common Signs to Recognize
Early symptoms can be easy to dismiss. A horse may cough a few times at the start of work or produce a small amount of clear nasal discharge. As the disease progresses, you’ll notice more obvious changes:
- Increased breathing effort at rest. The nostrils may flare, and the abdomen visibly contracts with each breath.
- Persistent cough. It may worsen in the barn or during feeding.
- Nasal discharge. Usually clear or whitish, not the thick yellow discharge associated with infection.
- Exercise intolerance. Horses tire faster, recover more slowly, and may resist work they previously handled easily.
- The heave line. A ridge of muscle along the lower belly wall that develops in chronic cases.
These signs often wax and wane depending on the horse’s environment. Many owners first notice the problem in winter, when horses spend more time stabled and exposed to dusty hay and bedding.
Environmental Triggers
Heaves is driven by an allergic-type response to inhaled particles, particularly organic dust from hay and straw. The risk of flare-ups increases during winter months when horses are confined indoors and breathing air loaded with endotoxins, mold spores, mites, and fine particulate matter.
The most common culprits are fungal species that thrive in stored hay. Aspergillus fumigatus, found in moldy hay, is a major offender. Penicillium, Fusarium, and various yeasts are also widespread in stables. These organisms and their byproducts become airborne every time hay is shaken out or bedding is disturbed. Research has shown that the concentration of Aspergillus fumigatus can be twice as high inside stables compared to outdoor air. Dry hay poses a greater respiratory risk than haylage or silage because it’s more likely to harbor mold contamination.
The sources of airborne irritants in a barn go beyond feed. Horse manure, bedding material, and even the horses themselves release microorganisms into the air. In a poorly ventilated barn, these particles accumulate quickly.
The Summer Pasture Variant
Not all horses with heaves get worse in the barn. A distinct form called summer pasture recurrent airway obstruction affects horses that live outdoors, particularly in warm, humid climates. Research from the American Veterinary Medical Association found that flare-ups of this variant are associated with rising temperatures, high humidity, elevated fungal spore counts, and grass pollen. In the southeastern United States, Bermuda grass sheds pollen from March through September and Johnson grass from June through November, both windows that overlap with the peak season for this condition. If your horse’s breathing worsens in summer despite living outside, this variant may be the explanation.
How Veterinarians Diagnose Heaves
A vet can often suspect heaves based on history, physical exam, and the pattern of symptoms (worse in the barn, better on pasture, or vice versa). For confirmation, they may perform a procedure called bronchoalveolar lavage, which involves flushing a small amount of fluid into the lungs and collecting it back for analysis. In horses with severe equine asthma, that fluid shows a significant increase in a type of white blood cell called neutrophils, typically above 10 to 25 percent of the total cell count. Healthy horses have far fewer neutrophils in their lung fluid.
Endoscopy, where a small camera is passed into the airways, can also reveal excess mucus accumulation and signs of inflammation. Combined with the clinical picture, these findings help distinguish heaves from infections, allergies, or other causes of chronic cough.
Managing the Environment
Environmental changes are the single most important part of managing heaves, often more effective than medication alone. The goal is to reduce the volume of respirable particles your horse inhales every day.
Soaking hay for 10 to 30 minutes reduces respirable dust particles by up to 90 percent for one to two hours after feeding. It also lowers the concentration of fine dust in the horse’s breathing zone. Steaming hay with a commercial hay steamer is another option that kills mold spores and bacteria without leaching nutrients the way soaking can. Some owners switch to haylage, hay cubes, or complete pelleted feeds to eliminate dry hay entirely.
Bedding matters just as much. Straw is a significant source of mold and dust. Low-dust alternatives like shavings, cardboard, or paper bedding reduce airborne irritants considerably. Ideally, mucking out happens while the horse is outside, giving dust time to settle before the horse returns.
Maximizing turnout is one of the simplest interventions. Horses that live outdoors full-time breathe far fewer irritants than those stabled even part of the day. When stabling is necessary, good ventilation is critical. Open barn designs with cross-ventilation outperform closed barns with little airflow, even in cold weather.
Medical Treatment
When environmental changes alone aren’t enough, veterinarians typically use two categories of medication. Anti-inflammatory drugs, usually corticosteroids, reduce the swelling and immune overreaction in the airways. These can be given systemically or delivered directly to the lungs through an inhaler designed for horses. Inhaled forms target the lungs while limiting effects on the rest of the body.
Bronchodilators relax the muscles surrounding the airways, opening them up so the horse can breathe more easily. These provide relatively quick relief during a flare-up but don’t address the underlying inflammation, so they’re typically used alongside anti-inflammatory treatment rather than on their own.
Medication can dramatically improve a horse’s comfort during an acute episode, but it’s not a cure. If the horse goes back into the same dusty environment, symptoms return. Long-term management almost always revolves around controlling what the horse breathes.
Long-Term Outlook
Heaves is a chronic condition. The airway changes that develop over time, particularly the thickening of airway walls and the loss of elasticity in lung tissue, are not fully reversible. However, horses whose environments are carefully managed can remain comfortable and functional for years. Some return to moderate work with consistent environmental control and appropriate medical support during flare-ups.
Horses that continue to live in dusty conditions without intervention tend to deteriorate. The heave line deepens, resting respiratory effort increases, and the horse’s capacity for exercise progressively declines. Early recognition and aggressive environmental changes give the best chance of preserving lung function and quality of life over the long term.