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Chronic respiratory irritation is most common in horses that spend some time in barns. In various horses, the irritation can be fairly mild or quite severe and performance-limiting. Known by a number of names such as chronic obstructive pulmonary disease (COPD), heaves, recurrent airway obstruction, small airway disease, or inflammatory airway disease, the condition is chiefly caused by an allergic response to organic dusts including molds in feed and bedding.

There is no cure for respiratory irritation. However, in all but the most severe cases, management and treatment measures can result in considerable improvement and minimize the number of further episodes. The most important component of treatment is environmental control to reduce exposure of the horse to the airborne allergens and other pollutants that cause airway inflammation.

Dust in horse stables contains over 50 types of molds, large numbers of forage mites, endotoxins, and other inorganic factors. The primary source of organic dust is hay and straw. Many of the molds contained in hay and straw are sufficiently small to reach the small airways of the lungs when breathed in by the horse. The concentration of molds and other organic dusts in the horse’s breathing zone, the airspace around the nose, is a critical factor in the development of irritation in susceptible horses. Characteristic feeding behavior, such as eating hay for long periods and shaking the hay, results in dust concentrations in the breathing zone that are much higher than in the rest of the stable.  Therefore, while bedding and barn ventilation are considerations in improving the environment of COPD-affected horses, it is most important to remove the hay. This helps to explain the occurrence of episodes of respiratory irritation in horses even when kept outdoors such as a susceptible horse given access to large round bales that contain significant mold contamination.

Exposure of susceptible horses to hay and straw dust causes an accumulation of mucus in the airways, edema or thickening of the airways, and bronchospasm (airway narrowing). During clinical episodes, the airways are also hyperresponsive to nonspecific stimuli such as ammonia and other air pollutants, resulting in severe bronchoconstriction. Importantly, these lung changes are reversible after removal of the offending allergens.

In mild cases, the horse may appear normal at rest. However, during exercise, the horse may cough and discharge mucus from the nostrils. Horses with more severe reactions have obvious signs even at rest including frequent coughing, nasal discharge, increased respiratory rate, and increased effort of breathing. These horses will be exercise intolerant. Forced expirations eventually result in overdevelopment of the abdominal muscles, recognized as a “heave line.” Sudden changes in management or feeding, such as a new batch of hay or straw, can precipitate very severe clinical episodes. In most cases, the severity of the clinical disease worsens with time unless prevented by treatment and management changes.

Keeping the horse at pasture, without exposure to hay and straw, is by far the most effective means of environmental control, and horses can show a significant improvement in lung function within three days of changing from a stable to a pasture environment. If the horse must be stabled, removing dry hay from the diet is the most important aspect of environmental control. In mild cases, soaking the hay prior to feeding is often effective in reducing dust levels in the horse’s breathing zone. The hay must be thoroughly soaked, preferably by immersing the hay in a large tub of water for 10 to 15 minutes. Dry areas in a poorly soaked hay portion can release enough mold spores to cause an allergic reaction. This process should be done no sooner than 20 to 30 minutes before feeding to avoid leaching of water-soluble nutrients from the hay.

A much less labor-intensive approach involves the use of alternative forages such as hay cubes, haylage, or complete pelleted feeds. This measure may be the only alternative that is successful in severely affected horses. Owners should not use broken or damaged haylage/silage bags and should use open bags within two to three days of opening, as mold counts in haylage or silage can increase rapidly after exposure to air.

Bedding is the other major source of dust and mold spores in a barn. Horses susceptible to respiratory irritation should not be bedded on straw. Shredded paper, wood shavings, rubber mats, or peat moss should be used. If possible, it is also preferable to bed the stalls adjacent to the one used by the affected horse with the same bedding. Horses should be taken outdoors during stall cleaning and rebedding to minimize exposure to dust.

Barn ventilation is another consideration in optimizing the horse’s environment. The buildup of ammonia, endotoxins, and other noxious agents can worsen the disease and delay recovery, even when other treatment measures have been instituted. In mild cases, environmental management alone may effectively control the disease. However, horses with moderate to severe signs of irritaiton will also require medical treatment, at least initially. Treatment will often involve combined use of bronchodilator and corticosteroid drugs. Bronchodilator drugs (e.g., clenbuterol) are particularly indicated for horses experiencing an acute attack. However, this relief is short-lived and longer term drug therapy must be directed at reducing airway inflammation. This is achieved through use of corticosteroid drugs.

Although environmental management will always be the most important factor in the control of COPD, some horses will require long-term treatment with corticosteroids to control the underlying airway inflammation.

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