Recurrent Airway Obstruction (RAO)

Cold winter weather means that our horses may be spending more time inside. It is tempting to close up the barn to keep warm, but doing so can compromise the quality of the air. Poor ventilation can result in high levels of allergens from molds and endotoxins in the air. These substances can affect a horse’s respiratory system and cause Recurrent Airway Obstruction (RAO), also known as heaves. Heaves is a chronic, non-infectious airway condition of horses.

RAO is the most common chronic respiratory disease plaguing housed mature horses. It occurs most frequently in the winter when horses are stabled more frequently. Conversely, summer pasture–associated obstructive pulmonary disease, which is clinically indistinguishable from RAO, is mostly found in southern regions, where horses are kept on pasture throughout the year.

Who is affected?

Horses older than 5 or 6 years of age are most frequently affected, and the prevalence increases with age. There does not seem to be a predisposition for gender, but breed and heredity seem to be involved. A study using a control group found an important effect of the sire on the prevalence of RAO in a population of horses. The typical profile of a horse for a diagnosis of heaves is usually a mature horse kept indoors most of the time with limited environmental control of allergens.

What causes RAO?

Hay contains microorganisms such as bacteria and fungi as well as tiny particles of feed grains, plants, feces, dander, and pollen. These tiny particles become aerosolized in hay dust and elicit an allergic response when they are inhaled by horses. While it is believed that the hypersensitivity reaction seen in horses is in response to many different allergens, the primary microorganisms involved in the etiology of heaves are Aspergillus fumigatus, Thermoactinomyces vulgaris, and Faenia rectivirgula. Aspergillus fumigatus is a mold that grows on dead and decaying matter such as poorly cured hay. It is thermophilic (“heat-loving”) and can thrive in the high temperatures achieved in decomposing vegetation. A. fumigatus forms spores which become airborne and can be inhaled. These spores are antigenic (they are recognized as “foreign” by the immune system and provoke an immune response) and allergenic. Both Thermoactinomyces vulgaris and Faenia rectivirgula are bacteria which produce spores that become airborne and can be inhaled. All three of these species of microorganisms are numerous in moldy hay.

Challenge studies with A. fumigatus and T. vulgaris have been conducted that show a change in respiratory function, although not as marked as that seen in clinical cases. Airborne endotoxins are also implicated as an inciting factor. The complete etiology of heaves in horses is still not known.

Pathophysiology of RAO

When these allergens are inhaled they cause several events to occur in the lungs. Inflammation and thickening of the tissues of the lining of the bronchioles (airways), constriction of the smooth muscles that surround the bronchioles (bronchoconstriction) which also includes coughing, and accumulation of mucus in the airways. These are also the normal natural defense mechanisms used by the lungs to eliminate inhaled particles. The difference in horses with heaves is that the reaction is hyper-reactive in them.

It takes only 4 to 6 hours for inflammation to develop in the lungs after allergens are inhaled. When this occurs, neutrophils, specialized white blood cells that kill bacteria, are recruited to the airways in massive numbers. Some of the substances that they release to kill the bacteria can also harm the lining of the airways. Each time a horse with heaves is exposed to allergens this reaction takes place. Over time, the repeated episodes result in edema and thickening of the airways that obstruct normal airflow.

Clinical Signs of RAO

Clinical signs of heaves often start out very subtly. It may just be a slight nostril flare or a very slightly faster respiratory rate. These early signs can easily be missed. Most owners first notice a soft cough. This can be during rest or at the start of exercise. It may be accompanied by exercise intolerance. These signs will progressively worsen if the horse is continuously exposed to allergens without treatment. Due to the obstruction of the small airways, a horse with heaves works harder to pull air into and expel air from the lungs than a healthy horse. This increased respiratory work forces the horse to use its abdominal muscles during the late phase of exhalation. Over time, the additional workload results in the visible enlargement of the abdominal muscles and the formation of a heave line. The extra effort of breathing also results in anal pumping, as the horse uses all of its abdominal muscles to expel air. With progression of the disease it becomes increasingly difficult for the affected horse to expel the air from the lungs at the end of exhalation and the lungs may remain over-inflated, which is called emphysema. If left untreated, non-reversible damage to the lung tissue may occur resulting in the permanent loss of lung function.

Diagnosis of RAO

Diagnosis of RAO starts with a complete physical examination. Most horses with RAO will have some degree of nostril flare. In later stages anal pumping and a heave line will be noticeable. Most RAO horses will not have a fever, unless they have a secondary bacterial or viral pneumonia. The lungs are auscultated and may reveal an expanded lung field, increased bronchovesicular sounds (although some low airflow areas may have decreased sounds), expiratory wheezes throughout the auscultation area, and crackles at the periphery of the lung. These abnormalities may be associated with the trapping of air in the lower airway and alveoli secondary to the bronchoconstriction. The higher intrapleural pressure necessary to move the air through the constricted airway increases the velocity of air and the turbulence at the origin of audible noises. When large amounts of tracheal secretions are present, tracheal wheezes may be heard throughout the whole lung auscultation field and even at a distance from the nostrils of the horse.

Blood work, such as a complete blood count and blood biochemistry analysis, and chest x-rays are often of little value in confirming the diagnosis of heaves. Yet, these tests may be beneficial in ruling out other causes of respiratory disease, such as pneumonia, pleuritis and neoplasms of the chest cavity. Ultrasound can be useful to show pleural roughening and to rule out pneumonia, pleuritis, and neoplasia, as well. Endoscopy can be used to show mucus in the trachea and bronchioles. A transtracheal wash can be performed to evaluate the tracheal secretions for evidence of infection (if pneumonia or secondary pneumonia is suspected) and to look for neutrophils and mucus plugs, called Curschmann’s spirals that are indicative of RAO.

The most reliable way to achieve a diagnosis of RAO is with a bronchoalveolar lavage. Bronchoalveolar lavage is a process whereby a tube is passed through one nostril of the horse into the peripheral airways and then sterile saline is quickly injected and withdrawn from the air passages through the tube. This sample is then analyzed microscopically for both the total number of cells present and the number and percentage of each cell type present (i.e. macrophages, lymphocytes, neutrophils, eosinophils, and mast cells). In normal horses, the predominant cells are macrophages and lymphocytes with neutrophils comprising less than five percent of all the cells present. In horses with severe COPD, the percentage of neutrophils in bronchoalveolar lavage (BAL) fluid may be 50-70% (or more) of the total cell count. However, horses with greater than 20% neutrophils will likely have impaired lung function and may have COPD.

Treatment of RAO

The best and most effective long term treatment for a horse with RAO is to eliminate exposure to allergens. For many horses, just getting them out of the barn and onto full time turnout will put them into clinical remission. It a horse must be stabled, then it is critical to eliminate as many allergens as possible. This includes eliminating straw and shavings bedding, dry hay, and many grains. Even though the dust levels in the barn may seem insignificant, research has shown that the dust levels in the breathing zone (i.e. around the nose) of a horse eating hay can be as much as thirty to forty times higher than in the rest of the stall. Alternative bedding such as peat moss or newspaper can be used. Haylage or soaked hay can reduce the number of mold spores in the air. Low dust grains are also available. Ideally, horses in adjacent stalls would also be treated the same way to prevent the spread of allergens from their stalls. The overall ventilation of the barn should be evaluated as well. Keep good airflow at all times. Horses don’t need the barn as warm as we would like it in the winter, so don’t be afraid to keep some windows open.

When management changes are not enough, it may be necessary to administer anti-inflammatory drugs and bronchodilators. Corticosteroids are the drugs of choice for relieving inflammation of the airways. Corticosteroids can be administered by mouth, by injection, or by inhalation. When they are administered by mouth or by injection, therapy usually begins with a high dose and, as the horse improves, the dose is reduced to a maintenance level. Inhaled steroids offer the advantage of a high dose within the airways and minimal systemic side effects but a special mask is necessary for administration.

Bronchodilators relax airway smooth muscle and relieve airway obstruction. In mildly affected horses, they may be the first line of therapy. They can also be safely combined with anti-inflammatory drugs for treatment of more severely affected horses. This combination is beneficial because anti-inflammatory drugs can reduce airway wall thickening but have no direct effect on the smooth muscle regulating the diameter of the airways. Bronchodilator drugs can be given orally, by injection, or by inhalation. Oral administration is the most convenient method but inhalation therapy is the most effective treatment for relief of airway obstruction. As with anti-inflammatory therapy, administration of bronchodilators by inhalation requires the use of a special mask.

If you suspect that your horse has any signs of RAO, make an appointment with your veterinarian. Early intervention and treatment can help decrease or eliminate clinical signs and allow your horse to return to his normal level of activity. As always, if you horse is competing, be sure to ask your veterinarian about drug withdrawal times.

Registered 2014 by Equestrian Collections
Author: Sallie S. Hyman, VMD, DACVIM, CVA

Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian, such as TEVA, for their horses' medical care.

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