Causes of Bronchiolitis
Bronchiolitis is inflammation of the tiny airway passages deep in the lung. Bronchiolitis causes obstruction by virtue of four different effects:
The exudates, like the normal secretions of the respiratory tract, are moved up toward the mouth and swallowed. In COPD, the excessive quantity of exudate or mucopus often appears at the nostrils and is recognized as a dirty nose or a nasal discharge, usually from both nostrils. The quantity of mucopus that appears at the nostrils is often increased when the horse grazes grass or eats hay off the ground because the effect of gravity helps it to drain down the airways to the nostrils. Coughing occurs because clumps of mucopus act to activate cough receptors in the wind pipe.
COPD has also been recognized as a complication of smoke inhalation (in horses that survive barn fires) and lungworm infections. It is possible that any provocative factor that leads to airway inflammation could, given the appropriate circumstances (concomitant mold exposure) lead to a state of immunological hypersensitivity in susceptible horses.
There are two distinct variants of bronchiolitis in horses.
COPD is very common in horses that are over six to seven years of age. In these horses, clinical signs of COPD are directly attributable to exposure to allergenic dust in the environment (aeroantigens). Clinical signs of COPD arise whenever horses inhale these allergens, and if the horse is not exposed, signs of COPD may be completely absent. The most common allergens include molds that grow on hay and straw.
When horses begin to develop COPD, the clinical signs usually occur seasonally during the first few years and are usually related to the horse's environment. As a rule, these mature COPD-affected horses are more likely to develop symptoms when accommodated indoors and fed hay, especially when bedded on straw. These same horses may become completely symptom-free when accommodated outside at pasture.
The exact reason that mature horses develop immune-mediated hypersensitivity to inhaled aeroantigens is not completely understood. In light of the fact that it is so common in domesticated horses, it is likely that a combination of feeding hay, bedding on straw, and the use of indoor accommodation leads to a suitable level of exposure to potential aeroantigens.
The second variant of bronchiolitis occurs in younger horses, typically aged between three to six years. These horses are often accommodated in barns at race or race training facilities. This variant of bronchiolitis/COPD is also known as inflammatory airway disease or small airway disease (SAD) and is more directly related to an immune reaction to viruses in the small airways.
The clinical signs of bronchiolitis associated with SAD are similar to those attributed to inhalation of aeroantigens. Some researchers have suggested that the "young horse/viral" variant of bronchiolitis always precedes the "mature horse/aeroantigen" variant and that one is simply an extension of the other. However, this hypothesis remains to be proved and has not yet been universally accepted.
Effects of Outdoors
Symptoms of COPD often become less prominent or less noticeable when the horses are accommodated at pasture. In rare cases, COPD-affected horses are found in a sudden state of severe respiratory distress. Such horses may have appeared normal to the horse owner; however, on recollection, these horses have a recent history of an occasional cough.
Signs of acute-onset severe respiratory distress include:
In one exceptional type of COPD, the affected horse exhibits respiratory symptoms when accommodated outside at certain times of the year. It is believed that these horses are "allergic" to inhaled pollen (like hay fever in children). This COPD variant is known as "summer pasture-associated obstructive pulmonary disease" or SPAOPD, for short. For these horses, the symptoms occur in the summer and fall when they are accommodated at pasture. SPAOPD is more common in the Gulf coast states.