Heaves is a chronic respiratory condition in horses now formally called severe equine asthma. It involves inflammation and obstruction of the lower airways, causing labored breathing, persistent coughing, and exercise intolerance. An estimated 14 to 17% of the horse population shows signs consistent with severe equine asthma, and when milder forms are included, up to 80% of horses may be affected to some degree.
The condition has gone by many names over the years, including broken wind, chronic obstructive pulmonary disease, and recurrent airway obstruction (RAO). Veterinarians now group all of these under the umbrella of “equine asthma,” with heaves representing the severe end of the spectrum. A related form, summer pasture-associated recurrent airway obstruction, affects horses turned out on grass during warm months rather than those kept in barns.
What Happens Inside the Lungs
In a horse with heaves, inhaling dust and organic particles triggers an exaggerated immune response in the airways. White blood cells called neutrophils flood into the lungs, causing inflammation that thickens the airway walls and produces excess mucus. The smooth muscle surrounding the airways constricts, narrowing the passages and making it harder for air to move in and out. Over time, this cycle of inflammation and constriction leads to airway remodeling, where the structural changes become more permanent even between flare-ups.
The good news is that during periods of remission, lung function can return to normal despite those structural changes. Heaves is rarely fatal on its own unless complications like right-sided heart failure develop from the chronic strain on the cardiovascular system.
Recognizing the Signs
The hallmark of heaves is increased breathing effort at rest. Healthy horses breathe quietly and effortlessly, so visible rib movement or flared nostrils while standing in a stall are red flags. Other common signs include a frequent cough (especially when eating hay or during exercise), nasal discharge, and a noticeable drop in stamina or willingness to work.
The most distinctive physical sign is the “heave line,” a visible ridge of muscle along the bottom edge of the ribcage. It develops because the abdominal muscles have to work overtime to push air out of obstructed lungs. Over weeks and months, those muscles enlarge from the constant extra effort, creating a line you can see and feel. Severely affected horses may also lose weight, show decreased appetite, and experience genuine respiratory distress.
What separates heaves from milder equine asthma is that labored breathing at rest. Horses with mild or moderate asthma may cough occasionally and underperform during exercise, but they breathe normally when standing still. That distinction is often the first thing a veterinarian looks for.
What Triggers a Flare-Up
Barn dust is the primary culprit. It contains a cocktail of mold spores, endotoxins (bacterial cell wall fragments), and fine organic particles that work together to provoke an airway response. Research has shown that endotoxins and mold spores have a synergistic effect, meaning their combined impact is far worse than either trigger alone. Removing endotoxins from barn dust significantly reduced airway inflammation in susceptible horses, and adding them back re-established the full allergic response.
The most common sources of these particles are hay and straw bedding. Round bales and poorly cured hay are especially problematic. For horses with summer pasture-associated disease, the triggers are airborne pollen and mold spores from grasses during warm, humid months.
How It’s Diagnosed
A veterinarian can often make a preliminary diagnosis based on the history and a physical exam, particularly if a heave line is present and the horse shows labored breathing at rest. To confirm the diagnosis and rule out infection, the gold standard is bronchoalveolar lavage, a procedure where a small amount of fluid is flushed into the lungs and collected for analysis. In a healthy horse, neutrophils make up 6% or less of the cells recovered. A horse with severe asthma will have 20% or more neutrophils, while horses with milder forms fall in the 7 to 19% range.
Treatment: Medication and Environment
Treatment has two pillars that work best together: reducing airway inflammation with medication and removing the environmental triggers that cause flare-ups.
Anti-inflammatory steroids are the cornerstone of medical treatment. They can be given by injection, orally, or through inhalation using a specially designed mask. Intravenous or oral forms tend to produce faster and more dramatic improvement, with noticeable changes in lung function within two to three days and some relief appearing as early as two hours after an IV dose. Inhaled steroids offer the advantage of delivering medication directly to the lungs with fewer whole-body side effects, and high-dose inhaled formulations can match the effectiveness of injected steroids.
Bronchodilators, medications that relax the muscles around the airways, are often used alongside steroids to open up constricted passages quickly. Veterinarians typically prescribe both together because steroids help prevent the airways from becoming less responsive to the bronchodilator over time.
Medication alone, however, will not solve the problem if the horse returns to a dusty environment. This is the most common reason owners become frustrated with treatment and why heaves horses are more likely to be euthanized due to the expense and effort of ongoing management than from the disease itself.
Environmental Management
Reducing airborne dust in the horse’s living space is the single most effective long-term strategy. For barn-dust-triggered heaves, maximizing turnout time on pasture keeps horses away from the concentrated allergens inside a stable. When stalling is necessary, every detail of the environment matters.
Hay
Dry hay is one of the biggest sources of respirable particles. High-temperature steaming using a purpose-built hay steamer reduces airborne particles and microbial contamination by 99%. Improvised steaming methods (using a wheelie bin or kettle) are less effective, cutting particles by about 88%. Soaking hay in water does reduce dust, but it leaches minerals and water-soluble carbohydrates from the forage and increases bacterial counts by 1.5 to 5 times, making it a less desirable option. Replacing hay entirely with haylage or complete pelleted feeds is another approach that eliminates the dust problem at the source.
Bedding
Straw bedding produces two to three times more dust than many alternatives and carries higher levels of endotoxins. Peat mixed with shavings produces the lowest dust levels in studies, measuring about 1 mg per cubic meter compared to roughly 2.4 mg for straw. Wood shavings, straw pellets, and paper-based bedding are all lower-dust options worth considering. Crushed wood pellets, somewhat counterintuitively, tested highest for respirable dust in one study at over 4 mg per cubic meter, so not all “alternative” bedding is automatically better.
Ventilation
Good airflow through the barn dilutes airborne particles. Opening doors and windows, using stalls with open tops, and avoiding sweeping aisles while horses are inside all help. Storing hay and straw in a separate building from the horse keeps ambient dust levels lower.
Long-Term Outlook
Heaves is a chronic condition, not a curable one. With consistent environmental management, many horses achieve functional remission where their lung function returns to normal and they can work comfortably. Horses with summer pasture-associated disease face a particular challenge because they need to be kept off grass during exactly the months when turnout and riding are most practical.
The earlier the condition is managed aggressively, the better the outcome. Horses left in dusty environments through repeated flare-ups accumulate more airway remodeling, making each episode harder to reverse. For owners willing to commit to low-dust housing and ongoing veterinary care, most horses with heaves can maintain a reasonable quality of life for years.