Recurrent Airway Obstruction in Horses

By Deidre M. Carson, BVSc, MRCVS & Sidney W. Ricketts, LVO, BSc, BVSc, DESM, DipECEIM, FRCPath, FRCVS.

RAO (previously called chronic obstructive pulmonary disease or COPD) is a relatively common cause of coughing and nasal discharge in stabled horses. In long-standing cases the horse may have difficulty in breathing and its chest and abdomen can be easily seen to move, hence the even older name 'heaves'.

What is RAO?

recurrent_airway_obstruction_rao-1RAO is a disease of the smaller airways (bronchioles) of the lungs and is caused by an allergy to small dust particles and spores that are inhaled by the horse when it breathes. Fungal spores and/or pollen are most important in terms of allergic 'trigger' factors. The allergic reaction in the airways results in the production of fluid and thickening of the walls of the small airways of the lungs, causing their obstruction. This means that the horse has to make an increased effort to breathe and usually develops a cough to clear trapped mucus. The pathology that occurs in the horse's lungs is reversible in the early stages, but if management changes are not made the damage may become permanent and the condition progressive. RAO cannot be 'cured' but the progress of the disease can be halted and the horse can be helped to accommodate to it, particularly when the problem is due to stable dust. This requires significant adjustment to stable management.

What are the symptoms of RAO?

In early cases, the only clinical sign may be a slight nasal discharge or dry cough that may go unnoticed. If untreated, the horse may start to find faster work more difficult. The cough will become more noticeable. As the disease progresses, the horse will cough with only slight exercise. In severe cases, the horse will have difficulty breathing even at rest, causing increased respiratory rate and effort. In very long-standing cases, the horse has to make a double effort to breathe out, using both the chest and abdominal muscles, developing a noticeable 'heave line'.  These horses used to be called 'broken winded'.

Symptoms are often mild for years and the condition may only progress slowly with age. In some horses, however, acute attacks of respiratory distress accompany repeated exposure to dust or pollens. This means that the condition may be seasonal (especially when associated with crops such as oil seed rape) or associated with stabling or feeding conditions.

What causes RAO to develop and how is it diagnosed?

For a horse to show signs of RAO, it must have developed an allergy to inhaled dust, spores or pollen. A horse may be allergic to pollen but not spores or vice versa. Fungal spores are present in hay and straw, moldy bedding or feed, and other organic material. Pollens are found just about everywhere but levels in the air fluctuate greatly with season, location and weather conditions. As with human hay fever, the higher the spore count in the air, the worse the condition becomes. Air quality in stables is therefore important for maintaining equine respiratory health.

The diagnosis of RAO is based on history, management conditions and clinical signs. Endoscopy ('scoping') and the collection of samples from the horse's lower airway i.e., by tracheal washing or bronchoalveolar lavage (BALs) for microscopic (cytological) examination and bacterial culture, helps to distinguish between RAO and other causes of chronic cough such as infection. Infection may of course be a secondary complication of RAO.

What treatments are available?

recurrent_airway_obstruction_rao-2Many early stage cases respond to changes in management that remove the cause of the allergy. Horses with RAO should be turned out of the stable to benefit from fresh air and when indoors, kept under 'dust-free' management, designed to keep environmental dust and spore levels as low as possible. Bedding should be paper, shavings or other non-organic material and should be kept scrupulously clean. Hay should be soaked before being fed or haylage should be used. Feeding these materials from the floor also encourages gravitational drainage of mucus from the lower airway. 'Dry' feed should be fed dampened to reduce dust. Horses should be stabled away from other horses bedded on straw and away from hay and straw stores, manure piles and other sites where dust and moulds may be produced. 'Run-in' sheds should also be kept clean and free from remnants of dusty/moldy hay. The need for good air quality and efficient ventilation cannot be over-emphasized.

Horses affected out of doors by pollens should be moved from high pollen areas until the season has passed. However, managing such cases can be quite problematic, especially when several pollens are involved over a prolonged period of the year.

More severe or long standing cases often require medication. Medicines that dilate the airways (bronchodilators) such as clenbuterol may help the horse or corticosteroids can be used to reduce the allergic reaction. Such medication is often used for pollen-sensitive horses, when removing the horse from the source of the allergens can be very difficult or impossible.

These medicines may be given by mouth, by injection or by inhalation using a nebulizer. The advantage of a nebulizer (which looks like a face mask) is that the medication is delivered straight to the lungs so the effect is quicker and lower dosages of medicine may be used. Many cases require long-term treatment. Desensitization treatment is successful in some cases although it can be difficult to know exactly what agent a horse is allergic to. Only a few specialized clinics offer this service and results appear better with some horses than others.

What is the best advice?

Good management is the key to owning a horse with RAO. Maintaining a clean, dust-free, well-ventilated environment, correct storage and maintenance of feeds and bedding and the use of hypoallergenic bedding materials all help reduce the incidence and the severity of this chronically-incapacitating condition.

Related Articles