Recurrent airway obstruction (RAO)

Many horses across Canada are affected by recurrent airway obstruction (RAO), a respiratory disease that’s also known as heaves or broken wind. RAO is similar to asthma in people and horses affected by the disease can have trouble breathing — especially when exercising. 

The key to breathing easy is proper management and early diagnosis and many affected horses can remain just as active as ever. Being able to recognize the signs of an RAO episode and acting quickly will make a big difference to your horse.

What is RAO?

Recurrent airway obstruction is a chronic condition that waxes and wanes. Affected horses experience episodes of respiratory distress followed by periods of remission. These episodes are usually triggered by exposure to dust, mould, pollen or allergens in the environment. When the horse inhales these particles, they cause an inflammatory reaction in the small airways of its lungs. 

When an inflammatory reaction occurs in the horse's lungs, the airways narrow and there’s an excessive production of mucus secretions — both of which contribute to airway obstruction and difficulty in breathing. Episodes can vary in severity, but for some horses, these episodes can be a major emergency.

Not all horses react the same to dust and mould: horses with RAO tend to over-react to these allergens while others in the same environment may not be bothered. For this reason, researchers suspect that there may be a genetic factor in the development of RAO.

RAO is not an infectious disease and it isn’t contagious to other horses. However, bacteria in the lungs may perpetuate the condition and contribute to damage of the airways.

What are the clinical signs of RAO?

RAO typically affects middle-aged to older horses and is more common in horses housed indoors. Affected horses often have more frequent episodes in the winter if they spend less time outside and if their barns have poor ventilation.

In Western Canada, RAO is often seen in horses fed from round bales. Horses tend to tunnel their muzzle into the bale, and as a result, they inhale more dust and mould. Horse owners can prevent this scenario by forking out hay from the bale when feeding or by using a hay net. However, the best approach is to use square bales or other types of feed. These choices will reduce the amount of dust and airborne particles that can trigger an inflammatory response in a horse’s lungs.

The key sign of this condition is recurring episodes of difficulty in breathing and coughing. The condition progresses with age, and the episodes become more severe and frequent. Proper treatment and management is necessary to prevent the progression of the disease and permanent damage to the lungs.

Coughing is the most frequent clinical sign of RAO. Coughing is a normal behaviour in horses, but if it becomes regular, it may be time to talk to your veterinarian. Initially, affected horses typically cough at the beginning of exercise, and as they warm up, the coughing subsides. A thick, white nasal discharge may also be present in more advanced cases of RAO.

Affected horses also have an increased respiratory effort. A horse’s nostrils may flare and its owner may notice more movement in the animal’s abdomen as it tries to breathe. More severely affected horses often develop a heave line — a muscle ridge along the belly that develops as the muscles for exhalation become over-developed.

How is RAO diagnosed?

As part of your horse's physical exam, your veterinarian will listen to your horse's lungs using a stethoscope and a re-breathing bag. Some of the abnormal sounds that may be present in the lungs of a horse with RAO include the following:

  • Harshness. Lung sounds can be louder and harsher when there is an abnormality in the lungs.
  • Wheezes. A high-pitched squealing noise may be present as a result of air being forced through narrowed airways.
  • Crackles. A popping noise indicates that there is some inflammatory fluid in the lungs.
  • Tracheal rattle. A rattling noise heard over the trachea is a result of mucous in the trachea.

To accentuate these abnormal lung sounds, your veterinarian may place a plastic bag (re-breathing bag) over the horse’s muzzle and allow it to breathe in to the bag for a minute or two. Re-breathing the expired air encourages the horse to breathe deeper, and the practice is similar to when your doctor asks you to take a deep breath.

In some cases, the history of the horse's problem and physical exam can be enough for your veterinarian to start treating your horse for RAO. However, to make a definitive diagnosis, it’s necessary to perform a bronchoalveolar lavage (BAL).

To perform a BAL, your veterinarian will pass a long tube up the horse's nostrils and down into the lungs until it wedges in one of the small airways. Next, sterile saline is infused through the tube into the horse's lungs and then retrieved. Finally, laboratory professionals examine the fluid under a microscope for evidence of lung inflammation. 

The BAL procedure, which is very safe for the horse, allows your veterinarian to determine the severity of the inflammation in the lungs. It also provides baseline information that can help clinicians determine if your horse is improving with treatment.

It may also be necessary to perform a tracheal wash to rule out pneumonia, an infection in the lungs. A tracheal wash is similar to the BAL and can be performed in two different ways:

• The veterinarian inserts an endoscope (a small camera on the end of a flexible rod) into the horse’s nostrils and guides it into the trachea to visualize the animal’s airways. A small tube can be passed through the endoscope into the trachea to infuse and retrieve sterile saline. Veterinarians evaluate the sample for evidence of inflammation and bacterial infection in the lungs.

• The veterinarian makes a small incision through the muscles along the bottom of the neck and into the trachea. Through this site, a tube is passed into the trachea to collect the sample.

It is important to diagnose RAO early on in the disease process. If the disease is identified before any permanent damage is done to the lungs, your horse can continue to breathe easy with proper treatment and management. However, once changes occur in the lung, there is irreversible damage.

 How is RAO treated?

To prevent damage to the lungs, it’s critical to treat RAO-affected horses whenever they are having an episode so they can achieve remission and be managed with environmental changes.

Medical therapy involves a combination of steroids and bronchodilators. The steroid treatment reduces the inflammation in the small airways of the horse's lungs while the bronchodilators help the animal to breathe by opening up these airways.

Veterinarians traditionally give these medications orally, but they can also use the equine version of an inhaler to administer the drugs. The inhaler fits over the horse’s nostril and delivers puffs of medication directly into its lungs. Many horses tolerate this method even better than oral medications. One advantage is that the inhaler delivers the drug directly to the site of inflammation. Since less of the medication is distributed throughout the body, it also helps to reduce the risk of side effects.

Steroid medications can have some side effects at higher doses. Steroids reduce the inflammation present in the lungs, but on the negative side, they reduce the animal’s immune response and can increase the risk of infection. There is also a link between high doses of steroids and laminitis (inflammation in the hoof wall). However, this connection has not been well established by researchers.

If you know your horse will be exposed to a “trigger” of RAO, veterinarians or horse owners can begin medical therapy before the clinical signs become apparent in the animal. For example, many horses in Western Canada experience episodes during harvest season when there is more dust in the air so some owners begin treating their horses at this time of year to prevent an episode. Consult with your veterinarian to determine the situations that may trigger an RAO episode in your horse.

How can RAO be managed?

Managing a horse with RAO is the most important part of the treatment plan. The goal is to reduce your horse's exposure to dust and mould in his environment. Here’s a list of changes that you can make that may benefit your RAO-affected horse.

Housing:

  • Keep your horse outside as much as possible
  • Ensure that your barn has good ventilation and air quality
  • Keep your horse's stall clean to prevent the build-up of ammonia, an airway irritant
  • Avoid cleaning your barn when the horse is in its stall
  • Wet down the arena before riding to reduce dust

Feeding:

  • Inspect your hay and do not feed any hay that is mouldy or dusty
  • Avoid feeding from round bales. Instead, feed from a square bale, fork out hay or use a hay net
  • Feed from a hay net to reduce the amount of time your horse spends rummaging in hay and dirt
  • Soak hay to reduce the amount of dust and airborne particles
  • Consider switching to hay cubes or complete feed for more severely-affected horses

Always be on the lookout for an increase in clinical signs of RAO in your horse. An episode of RAO requires rapid treatment to prevent permanent damage to the lung. Strict management to reduce your horse's exposure to airway irritants will help to maintain your horse in remission and keep it breathing easy.

Written by Dr. Fernando J. Marqués, DVM, Dipl. (ACVIM) with Hayley Kosolofski. Reprinted with permission from Horse Canada (horse-canada.com)

Dr. Fernando J. Marqués is an associate professor at the Western College of Veterinary Medicine (WCVM) and a board-certified specialist of large animal internal medicine in the WCVM’s Veterinary Medical Centre.  

Download the RAO fact sheet PDF