Recurrent Airway Obstruction (RAO) in Horses

Causes, Treatment, and Prevention

Horses in stall


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Horses have delicate lungs and are very sensitive to inhaled dust, pollen, and mold spores. Exposure to these irritants over time can lead to a persistent cough, difficulty breathing, and nasal discharge. This allergic response is called recurrent airway obstruction (RAO). Older terms still often used for this condition include chronic obstructive pulmonary disease (COPD), "heaves" or "broken wind."

RAO usually first appears when the horse is between nine and 12 years of age. Approximately 12 percent of mature horses have some degree of this respiratory affliction.

Although RAO is a chronic condition, many horses afflicted with this respiratory problem enjoy long and relatively normal lives. However, management of these horses does require vigilance to ensure the horse isn’t overworked or exposed to allergens that trigger breathing difficulties, such as dusty hay. While there’s no cure, most horses with heaves can be comfortable and can be lightly worked. Here’s how recurrent airway obstruction develops and what you can do to help your horse.

What Is Recurrent Airway Obstruction?

Horses' lower respiratory systems, as well as the respiratory systems of other mammals, consist of a large trachea (windpipe) that branches off into two main bronchi—these are something like the stems of the lungs—which then branch further into small airways called bronchioles. Recurrent airway obstruction is an allergic response that causes inflammation in the bronchioles. Over time, this causes them to thicken and leak mucus, leading to the characteristic cough, difficulty breathing, and other symptoms of this chronic condition. RAO is considered to be a type of equine asthma.

While the mechanisms of RAO are not well understood, it is ultimately believed to be caused by the horse's lung's hypersensitivity to allergens, especially hay, straw, mold, and dust. Horses that live in stables and eat hay are the likeliest to develop the condition, but it can strike pastured horses as well. Typically, stabled horses with RAO have the worst symptoms in the winter and spring, while pastured horses suffer the worst symptoms in the summer or early fall.

Symptoms of RAO in Horses

In the early stages of RAO, or in horses only mildly afflicted with the disorder, the symptoms are generally just a slightly runny nose and dry cough. As the disease progresses, however, symptoms generally become far more severe.


  • Chronic cough, which may bring up secretions
  • Nasal discharge/runny nose
  • Labored breathing
  • Exercise intolerance
  • "Heave line" of overdeveloped chest muscles at the bottom of the rib cage due to the strain of breathing
  • Flared nostrils
  • Tachypnea (breathing more rapidly than normal)
  • Wheezing or rattling sounds when breathing
  • Weight loss
  • Lack of appetite

Initially, horses with RAO tend to experience symptoms only when exercising. However, horses with more severe disease display symptoms even when at rest. These horses have a persistent cough and often "heave" their sides in and out with the effort of breathing. You'll notice that your horse's nostrils are flared, which is an attempt to draw in air more easily, and the animal is breathing rapidly. Often, there's a yellowish nasal discharge, as well. Your horse's ability to exercise or work will greatly diminish as the disease progresses.

Horses with severe RAO may lose weight due to a lack of interest in eating. However, a fever is not a normal symptom of this disorder. Development of fever usually indicates a secondary bacterial infection, often pneumonia.

Causes of Recurrent Airway Obstruction

Recurrent airway obstruction in horses is similar to asthma in humans and is believed to be an allergic response to inhaled antigens, primarily dust, mold spores, hay, and straw. When the horse inhales these common allergens, its airways respond by clamping shut (bronchospasm), narrowing (bronchoconstriction), and secreting mucus.

There also may be a hereditary component that contributes to a horse's likelihood of developing RAO, as horses with afflicted parents are far likelier to develop the disorder than horses with healthy parents.

Diagnosing RAO in Horses

Generally, an equine veterinarian can diagnose recurrent airway obstruction based on the horse's characteristic symptoms and history. At times, however, your vet might want to do further testing to establish the diagnosis or rule out other causes of a chronic cough. One such test is a bronchoalveolar lavage, which involves passing a tube into the horse's lungs, "washing" the lungs with a saline solution, and then suctioning the saline back out again. This provides a sample of cells from the horse's lungs, as well as the secretions within the airways. Testing this sample can reveal the presence of infection and/or inflammatory changes in the cells.

Occasionally, your vet might order x-rays of the horse's chest, particularly if there is a concern that the horse has pneumonia or a secondary infection as well as RAO.


Unfortunately, recurrent airway obstruction is a chronic disease that can be managed, but not cured. The mainstay of treatment for RAO is to remove the horse as much as is feasible from the allergy triggers. That means letting a stabled horse outside into a pasture whenever possible, storing hay away from the afflicted horse's stall, and switching from straw bedding to low-dust bedding such as shredded paper or cardboard.

Feeding the horse water-soaked hay is an option if the RAO is mild. However, horses with more severe disease will need a switch to a complete pellet feed instead of hay. The barn or stable should be kept as dust-free as possible.

Some horses will also require medications to control their condition, particularly during flares of symptoms. The most commonly prescribed medications include systemic or inhaled bronchodilators, which help to open the constricted airways, and systemic or inhaled corticosteroids, which help to relieve inflammation. These medications might be used only when the horse is in respiratory distress due to a flare of symptoms, or long-term to help manage symptoms in horses with severe disease that cannot easily be kept away from allergy-triggering antigens.

Prognosis for Horses With Recurrent Airway Obstruction

With proper management and treatment, horses with mild to moderate cases of RAO can lead fairly normal lives. However, they will need to be closely monitored for flare-ups of symptoms, and may never be able to work or exercise as hard as healthy horses.

More severely affected horses are not able to work or exercise, and are likelier to develop secondary pneumonia or other respiratory infections. These horses have a poorer prognosis.

How to Prevent Recurrent Airway Obstruction

It may be difficult to prevent RAO entirely, as it's hard to predict if your horse will develop the disease or not. But this chronic lung condition builds gradually. If recognized early, good feed and stable management may slow or prevent its progression, and the horse may be able to take on an almost normal workload. If the horse is continually exposed to the irritants, however, the disease may progress to the point where it is unable to thrive.

Make sure that any fodder and bedding are mold and dust-free. A well-ventilated stable is essential for keeping your horse’s lungs healthy. Outdoor turnout is the best way to provide lots of fresh air as well as fresh grass—rather than dusty hay—for feeding.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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  1. Recurrent Airway Obstruction in Horses. Merck Veterinary Manual.

  2. Recurrent Airway Obstruction (RAO) in the Horse. American Association of Equine Practitioners.