Heaves is a chronic respiratory condition in horses, now formally called severe equine asthma. It causes recurring episodes of labored breathing, persistent coughing, and exercise intolerance, driven by inflammation and obstruction deep in the airways. The condition affects roughly 14% of horses, based on a large survey in Great Britain, making it one of the most common respiratory problems in the species.
If you’ve heard the older terms “broken wind” or equine COPD, those refer to the same disease. The modern umbrella term “equine asthma” covers a spectrum: mild to moderate cases (previously called inflammatory airway disease) and severe cases, which are what most people mean when they say “heaves.”
How Heaves Looks in a Horse
The hallmark of heaves is a horse that struggles to breathe out. During a flare-up, the horse stands in its stall with nostrils flaring and sides visibly heaving as it tries to push air from its lungs. Breathing out takes noticeably longer than normal, and you’ll often hear wheezing on the exhale. Frequent, deep coughing is common, sometimes producing nasal discharge. These episodes can be dramatic enough that everyone in the barn recognizes them.
Over time, the effort of repeatedly forcing air out causes a visible physical change. The external abdominal muscles thicken from overuse, creating a distinct line along the lower belly called a “heave line.” Think of it like a bodybuilder developing oversized muscles from repetitive exercise, except in this case the muscle growth reflects chronic respiratory distress. Horses with longstanding or poorly managed heaves also tend to lose body condition and show poor performance under saddle.
Milder forms of equine asthma look quite different. A horse with mild to moderate disease typically won’t struggle to breathe at rest. Instead, riders notice subtle signs: the horse coughs at the start of a ride, takes longer than expected to recover its breathing rate after hard work, or just doesn’t perform as well as it should. Many people dismiss a cough at the beginning of exercise as normal. It’s common, but it signals that something is off in the airways.
What Happens Inside the Lungs
Heaves is fundamentally an allergic-type reaction in the airways. When a susceptible horse inhales dust, mold spores, and bacterial toxins (called endotoxins), the immune system overreacts. The airways flood with white blood cells called neutrophils. In a healthy horse, fewer than 5% of the cells in the lower airways are neutrophils. In a horse with severe asthma, that number jumps to anywhere between 25% and 90%.
This inflammation triggers three things that make breathing difficult. First, the smooth muscle lining the airways constricts, narrowing the passages. Second, the airways produce excessive mucus, further blocking airflow. Third, and most damaging over the long term, the airway walls physically remodel. The tissue lining the airways thickens, the layer beneath it becomes denser with scar-like material, and the smooth muscle itself develops fibrosis. These structural changes explain why horses with long-standing heaves can have permanently reduced lung function even between flare-ups.
Research has found that even horses with mild to moderate asthma already show early signs of remodeling, including thickened airway linings and increased smooth muscle fibrosis compared to healthy horses. This suggests the damage begins well before the disease becomes severe.
What Triggers a Flare-Up
Dusty, moldy hay is the single biggest culprit. Hay and straw release a mixture of fungal spores, organic dust particles, and endotoxins that together provoke a strong inflammatory response in susceptible horses. Research has shown that endotoxins and mold spores act synergistically, meaning their combined effect is far greater than either would cause alone. Interestingly, two different batches of hay with similar overall dust levels can trigger very different reactions in the same horse, likely because the specific mix of mold species and toxins varies from batch to batch.
Stabling is a major risk factor. Horses kept indoors breathe higher concentrations of these particles, especially in barns with poor ventilation. Bedding made from straw adds to the dust load. Flare-ups often follow seasonal patterns, worsening in winter when horses spend more time inside eating stored hay, or in summer for horses sensitive to pasture-associated allergens (a variant called summer pasture-associated asthma).
How Vets Confirm the Diagnosis
A vet can often suspect heaves from the clinical picture alone: the labored breathing at rest, the audible wheezing, the heave line. But to confirm the diagnosis and rule out infection or other problems, they typically collect a sample of fluid from deep in the lungs using a procedure called bronchoalveolar lavage. The horse is sedated, a long tube is passed through the nose into the airways, and a small amount of sterile fluid is flushed in and suctioned back out. Examining the cells in that fluid tells the story. A neutrophil percentage above 25% points to severe asthma, while levels between 10% and 25% suggest the mild to moderate form.
Managing Heaves Day to Day
The single most effective long-term strategy is reducing what the horse breathes in. This means maximizing turnout time on pasture, improving barn ventilation, switching from straw bedding to low-dust alternatives like shavings or paper, and addressing hay quality. Steaming hay significantly reduces the number of respirable particles and mold spores compared to feeding it dry, and it’s more palatable than soaking (which leaches nutrients). Some owners switch to complete pelleted feeds to eliminate hay dust entirely.
The challenge is that true allergen avoidance is extremely difficult. Horses need forage, barns aren’t always well-ventilated, and environmental conditions change with the seasons. Because of this, affected horses tend to experience recurring flare-ups throughout their lives.
When environmental changes aren’t enough, medication becomes necessary. Anti-inflammatory steroids are the cornerstone of treatment. Inhaled versions delivered through a mask and spacer device designed for horses allow the drug to reach the lungs directly with minimal effects on the rest of the body. These inhalers can be combined with airway-opening medications (bronchodilators) that relax the constricted smooth muscle. Studies show that this combination can normalize lung function within about a week and fully resolve residual airway spasm after about 12 weeks of treatment. Steroids alone, without a bronchodilator, work more slowly, often taking a month or more to produce significant improvement.
Long-Term Outlook
Heaves is a manageable condition but not a curable one. Even during periods of remission, when a horse looks and breathes normally, low-grade inflammation and airway hyperreactivity persist beneath the surface. Exposure to the wrong environment can reignite symptoms at any time.
With consistent environmental management and appropriate medication during flare-ups, many horses live comfortably and continue to work. The key variable is how much structural damage has already occurred. Horses caught early and managed aggressively tend to retain good lung function between episodes. Those with years of poorly controlled disease may develop irreversible changes, including permanent scarring of the lung tissue and widened, damaged airways (bronchiectasis), which limit recovery even when inflammation is brought under control.