Equine chronic obstructive pulmonary disease (COPD) is a non-infectious respiratory disease in which inflammation in the small airways of the lung leads to impaired ventilation. COPD is possibly the most common medical condition of mature horses and is a common cause of a premature end to their athletic careers.
Normally, the lungs supply the blood with oxygen inhaled from the outside air, and dispose of waste carbon dioxide in the exhaled air, a process known as respiration. Air is breathed into and out of the lungs through the airway, which begins at the nostrils and passes down the windpipe (trachea) into the lungs. The final smallest branches of the airway, where the oxygen-carbon dioxide exchange takes place, are known as terminal bronchioles.
Most commonly, COPD occurs in cold climates where horses are kept in barns for prolonged periods of time and where hay is moldy. It is believed that COPD is caused by an allergic reaction to these molds when horses inhale them, causing an inflammation in the terminal bronchioles ("bronchiolitis").
COPD is also known by a number of other names. Each of these terms refers to some different aspect of the clinical manifestation of this important and common problem of mature horses. Other terms that are (broadly) used to refer to the same (very similar) clinical syndrome include:
A diagnosis of COPD should be considered for many problems associated with the respiratory system of mature horses. Oftentimes, observant veterinarians will detect signs of COPD in horses presented for examination for other problems. For example, during routine vaccinations, a horse may breathe with slightly increased rate and effort at rest. Clinical problems such as coughing, exercise intolerance, inappropriately increased respiratory effort following exercise, nasal discharge, and sudden-onset severe difficulty with breathing should all warrant consideration of COPD. In horses that are losing weight as a result of COPD, the weight loss is usually not as prominent as the abnormalities associated with breathing.
In many instances, the diagnosis of COPD can be made based on assessment of the horse's respiratory medical history and the results of physical examination.
Treatment for COPD is primarily aimed at controlling the enviroment and preventing worsening of signs. Some medications but be affective in reducing the signs of illness.
What to Watch For
The most common symptoms of COPD are directly related to increased resistance to air movement through the airway passages as a result of bronchiolitis. All of these clinical signs are more prominent at times when horses are exposed to antigens in the atmosphere, typically during periods of indoor accommodation.
Although there are numerous other causes of weight loss and exercise intolerance, COPD is generally the most common cause of coughing, nasal discharge, and respiratory embarrassment in adult horses. COPD is so common that, in many instances, the significance of signs of mild disease (such as intermittent coughing or slight nasal discharge) is not properly appreciated. All too often, intermittent coughing in barn-accommodated horses is attributed to an "innocuous" barn cough. In those cases, recognition and early treatment for COPD would be more effective than, as often happens, waiting until severe lung damage (irreversible lung scarring) has occurred.
Horses with COPD usually have a history of respiratory disease that has a seasonal occurrence. However, these earlier signs have typically been mild and intermittent.
Affected horses tend to manifest coughing and exercise intolerance when accommodated inside barns, bedded on straw, fed hay (especially big bale hay), or when worked in very dusty arenas. It should be remembered that the clinical manifestations of COPD are directly related to exposure to inhaled aeroantigens; therefore, a link between the time of onset of symptoms and exposure to "dust" can often be made.
Similarly, the symptoms often regress and the horse returns to normal after the provocative exposure has been removed (either deliberately or inadvertently). For horses affected with SPAOPD, the clinical expression of disease is much more likely in the summer/fall when these horses are accommodated at pasture.
Another common and misleading feature of the history of affected horses refers to the use of antibiotics. Many COPD-affected horses have been treated with antibiotics after developing symptoms of respiratory disease (cough and nasal discharge) that resemble a "cold." Although the horse may improve during treatment with antibiotics, the reason for improvement has more to do with the natural variation of symptoms of the disease than it does with the effect of the antibiotic.
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.
Your veterinarian may recommend specific tests in order to detect the presence of bronchiolitis or to rule out other diseases that may cause similar symptoms:
Some specialized equine respiratory diagnostic centers are able to perform some more advanced lung function tests on horses, although these tests are probably not useful in most routine cases of COPD. Some veterinarians elect to measure the oxygen content of the affected horse's blood (it may be quite low in COPD). Then the horse is treated with a bronchodilator and the oxygen content is re-evaluated. The prognosis for horses in which the blood oxygen content increases after treatment is often better than for those horses in which the bronchodilator has minimal effect.
The treatment for COPD falls into two broad categories:
Changes in Management
The most important aspect of treatment for COPD is clearly the avoidance of aeroantigens. For most COPD-affected horses, a complete recovery from the problem can be achieved simply by eliminating inhaled aeroantigens from the horse's environment. Several management adjustments can reduce the risk of exposure to inhaled aeroantigens. These include:
Although an expense would be involved, you might consider special adaptation of a single stall or loose box. The stall would be sealed-off from the common air-space of the barn environment and ventilated through a high efficiency filter in association with an air-conditioner.
These are intended to promote airway function. As a rule, without resort to some improvement in management, the drug strategies are rather ineffective. However, with the exception of the most severely affected horses, the use of drugs is not usually needed.
Drug strategies include the use of the following categories of drugs:
It is currently recommended that bronchodilators and anti-inflammatory agents (usually steroids) should be administered via inhalation.