Heaves (Recurrent Airway Obstruction) in Horses

Key takeaways

Heaves, or Recurrent Airway Obstruction (RAO), is a severe form of equine asthma. It is an inflammatory reaction in the lower respiratory tract to inhaled dust particles, endotoxins, or mold spores typically found in hay and straw.

  • The condition causes airway constriction that impairs breathing, even while at rest
  • Symptoms include rapid, labored breathing, flared nostrils, nasal discharge, coughing, and exercise intolerance
  • Diagnosis of RAO is based on physical examination and the presence of coughing or labored breathing
  • Horses with severe difficulty breathing require emergency veterinary attention
  • Medications including bronchodilators and corticosteroids address immediate symptoms
  • Environmental management which aims to reduce contact with organic dust, mold and endotoxins is critical
  • Most horses show reduced symptoms with appropriate management
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A closer look: Heaves (Recurrent Airway Obstruction) in Horses

RAO is a progressive disease, meaning it gets worse the longer it is left untreated. Severity depends on the state of progression of the disease and the quality of the air in which the horse spends its time. Horses showing early signs of RAO whose environment is adjusted to limit contact with environmental triggers often recover fully without need for ongoing treatment.

One variety of RAO is also known as Summer Pasture-Associated Obstructive Pulmonary disease (SPAOPD, aka summer heaves or pasture-associated heaves), which affects horses living on pasture during the late summer and early fall. In these cases, mold spores from the grass have an inflammatory effect on the lungs of the horse. Symptoms of this condition are similar to traditional RAO, however the symptoms wane during the winter and spring. In these cases, stabling the horse or reducing access to grass pasture during the late summer and early fall is typically sufficient for management.

RAO is related to, but not the same as, Inflammatory Airway Disease (IAD). IAD is an inflammatory condition of the lower respiratory tract with symptoms similar to RAO, but seen in younger horses. The most significant difference is that with IAD affected horses breathe normally at rest, with symptoms only appearing during exercise. IAD and RAO are syndromes within the Equine Asthma complex.

RAO is not curable, but managing the environmental triggers is most often sufficient for recovery to the point that there is a complete absence of symptoms. Without a reduction in the environmental triggers, RAO is a progressive disease that can cause serious discomfort and shorten life expectancy.

Risk factors

Horses living in barns, who are stabled in dusty environments, who eat hay, or who are bedded in straw are most susceptible to RAO. Onset of RAO typically happens after the age of seven.

Initial signs are hard to detect. By the time symptoms are noticeable, the condition is serious and requires prompt veterinary attention. Cases where the horse is having severe difficulty breathing require emergency veterinary care.

In some cases, secondary bacterial pneumonia develops. Symptoms of pneumonia include severe lethargy, yellow or cream-colored nasal discharge, and fever.

Risk factors of developing RAO include:

  • Being fed dry hay
  • Sleeping in dry straw
  • Not spending enough time outdoors
  • Being stabled in a dusty environment
  • Access to moldy hay or straw

Horses with difficulty breathing cannot perform well in sports or competitions. If detected early and managed with environmental changes, recovery is sufficient to continue competing in some cases.

Possible causes

RAO is caused by a sensitivity in the lungs to the dust, mold spores, and endotoxins commonly found in hay and straw. The underlying causes of the sensitivity are still under investigation, but evidence for a genetic component is emerging.

Main symptoms

In addition, horses suffering from heaves may have a “heave line.” The effort of the abdomen to assist with exhaling produces a visible line of muscle along the flank.

Testing and diagnosis

Horses with severe difficulty breathing require emergency care from a veterinarian.

Often, an assessment of the patient’s risk factors and the presence of labored breathing or coughing are sufficient for diagnosis. Other diagnostic tools include:

  • X-rays or ultrasound to rule out other causes
  • Upper airway endoscopy
  • Bronchoalveolar lavage
  • Lung function testing

Steps to Recovery

Veterinary interventions aim to treat severe breathing difficulties with bronchodilators to open the airways and corticosteroids to reduce inflammation. These medications produce short-lived improvement, and are typically only useful for severe, acute symptoms.

In cases where RAO is left untreated for some time, the physical changes in the lungs become permanent. Even where permanent changes have occurred, controlling the air quality ensures that many horses with RAO are healthy enough to continue pleasure or trail riding.

For horses with SPAOPD, allowing pasturing only in winter or early spring is usually sufficient for recovery.

For recovery of more serious cases, it is crucial to increase air quality and reduce contact with triggers that cause inflammation. Dry hay and straw are the primary sources of the triggers for RAO.


Prevention strategies, like management strategies, aim to increase air quality and limit contact with triggers such as mold spores, organic dust, and endotoxins from hay and straw. The general approach to improved management includes:

  • Increasing time outside including keeping the horse outdoors (for horses without SPAOPD)
  • Wetting or steaming hay to reduce dust
  • Avoiding round bales, which have higher levels of dust, spores, and endotoxins
  • Switching to complete pelleted food
  • Feeding from the floor to help clear the airways
  • Frequent cleaning of stalls with frequent complete bedding changes
  • Avoiding storing hay or straw near the horse, particularly overhead
  • Switching to a low-dust bedding such as cardboard, paper, wood chips, or wood shavings
  • Turning out horses for at least an hour during and after cleaning the barn
  • Providing good ventilation
  • Avoiding the use of leaf blowers, tractors, and other machines that move the air
  • Keeping arena footings moist
  • Sprinkling areas with water before raking
  • Supplementing the diet with omega-3 fatty acids

Is Heaves (Recurrent Airway Obstruction) in Horses common?

RAO is common in horses, particularly those over the age of seven.

Typical Treatment

  • Environmental management
  • Bronchodilators
  • Corticosteroids

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