Please take a deep breath. Excuse me, I said could you take a deep breath please? No, stop nuzzling at my backside. Could you please just take a deep breath?

This might be a typical one-sided conversation a veterinarian would want to have with one of his or her equine patients (highlighting a slightly different meaning to the word patient) when attempting to evaluate the respiratory system. This is the point where veterinary medicine becomes an art in an effort to get the patients to "tell" you what they are feeling (and cooperate with a few basic diagnostic aids—like taking a deep breath) when you don't speak their language. One of my favorite veterinary professors (Dr. Francis Fox) always said, "If you want to know what's wrong with the animal, just ask them."

This, of course, requires that you have made an effort to learn the language and have powers of observation attuned enough to see the pertinent information. With respect to the respiratory system, the language includes the rate and character of breathing, the "position" of the upper airway—head and neck and, therefore, the nasal passages, the junction between them and the trachea (wind pipe), and the trachea—and what the rest of the body is doing during each breath. You should note that most of this information can be acquired without even touching the horse and can be evaluated from outside the stall. In addition, a veterinarian will listen to the lungs with a stethoscope while the horse takes a deep breath, generated with the use of a simple kitchen plastic trash bag. Because I have yet to have a horse take a deep breath when asked, the plastic bag is placed over the nose until the animal is very short of breath so that when the bag is removed, the horse breathes deeply. In addition to greatly increasing the lung sounds in all areas of the lungs, the "stress" of the bag can elicit hidden coughs. Also, the horse's inability to breathe into the bag for a normal period of time and/or the horse's not recovering quickly after the bag is removed are more subtle signs of respiratory disease.

Chronic obstructive pulmonary disease (COPD) or "heaves," also is called broken wind. The disease primarily affects the airways or bronchi and is somewhat similar to human asthma.

The respiratory system starts at the nostrils, passing through the head as the nasal passages. Just behind the eyes there is the junction between the nasal passages and the trachea. Between the trachea and the nasal passages is the larynx, which consists of the epiglottis, arytenoids (flappers), vocal cords, and a complex series of small muscles that control the entire area. At this junction is the area where the esophagus joins with the oral cavity.

One of the functions of the larynx is to coordinate the closing of the airway during swallowing so that food material travels down the esophagus and not down the wind pipe. It is the epiglottis that flips up to cover the tracheal opening during swallowing. The arytenoids are the structures that can be involved in roaring (See The Horse of September 1999, article #369). The trachea courses down the neck, splits into the right and left main stem bronchi in the area of the heart, and connects the right and left lung lobes to the respiratory system. The bronchi then split many times, becoming smaller and smaller as they disperse throughout the lung tissue. At the very smallest connection, the bronchi (called bronchioli at this level) enter the air sacs in primary lung tissue. It is at the level of the air sac (or alveoli) that the fine blood vessels carry blood from the right side of the heart exposing it to oxygen in the air sac. Oxygen (room air is 21% oxygen) then is picked up by the oxygen-poor blood before it returns to the left side of the heart and is pumped throughout the body. The bronchial system can be looked at as all of the branches of a tree (with the trunk being the trachea) and all of the leaves being the alveoli.

Surrounding the bronchi is a substantial amount of smooth muscle that regulates the size of the small airways. If the muscle contracts and the size of the airway decreases, the resistance to airflow within the bronchi increases; as the airways become narrower, the resistance to airflow within them becomes greater. Remember, too, that the airways are lined with millions of small hair fibers coated with a mucous layer that constantly collect inhaled debris and transport it back up the trachea (and away from the lungs) to be coughed up or swallowed—the "mucocilary escalator" or "elevator." This mucocilary escalator is an extremely important part of the local protection of the respiratory system.

Chronic obstructive pulmonary disease primarily involves the bronchi and bronchioles (i.e., the airways) rather than the lung tissue itself. In its most severe form, COPD is called "heaves" or, in the United Kingdom, "broken wind." It is an inflammatory disease that leads to obstruction of the air passages and is somewhat similar to human asthma. Inflammation of the airways (bronchi and bronchioles) is known as bronchitis or bronchiolitis. This inflammation can have many causes, including inhalation of irritant gases, dusts, endotoxin, allergens, viruses, and bacteria.

N. Edward Robinson, Bvet Med, MRCVS, PhD, of Michigan State University and one of the airway "gurus" of North American, says that while infections can cause airway inflammation, infection is not important in heaves. "There is no evidence that infection causes the problem or that horses with heaves are more susceptible to infection," noted Robinson. "This is a misconception that leads to the inappropriate treatment of horses with heaves."

During the acute COPD episode known as heaves, the muscle surrounding the bronchi and bronchioles goes into spasm and the clearance of the mucous is delayed because of an increasing mucous viscoelasticity. Muscle spasm and mucous accumulation leads to airway obstruction.

The fact that COPD primarily involves the bronchi also yields diagnostic clues. The obstruction of the small airways leads to characteristic sounds being produced within the lung tissue when you are listening with a stethoscope. The increase in resistance to airflow creates a whistling or wheezing sound. However, an infection within the air sacs leads to a build-up of inflammatory fluid and the production of a crackling sound as the air sac fills with air. So, the particular sound your veterinarian hears while listening to the lungs gives important clues to the disease that is occurring.

Cause And "Affects"

There is still some degree of debate regarding the specific cause(s) of COPD in the horse, but a hypersensitivity/allergic cause is highly suspected in most cases. COPD is mostly an "indoor" disease, meaning that most of the stimuli for the hypersensitivity reaction are associated with things in the barn (hay, straw, bedding, barn dust, etc.). "Dusts" contain a variety of substances known to cause airway inflammation, and there most likely is an interaction of several factors that cause COPD. In the southern U.S., pasture can be associated with COPD.

COPD is observed worldwide, but has its highest incidence in the Northern Hemisphere, where summers are damp and winters are long. Under these conditions, horses are stabled and eat the poorly cured hay that is rich in dust. The average age of onset is reported to be eight. The primary agents suspected in causing the hypersensitivity are molds. The two most frequently implicated molds, Aspergillus fumigatus and Micropolyspora faeni, are found in hay. The pasture-associated allergens have yet to be positively identified, but are likely to be a variety of pollens similar to those that set off millions of cases of summer allergies in people. COPD can probably follow chronic exposure of horses to a variety of antigenic agents. For example, there is a description of heaves in a horse exposed to chickens.

The constriction of the smooth muscle surrounding the bronchioles and the accumulation of mucus cause periods of airway obstruction and subsequent difficulty in moving air in and (to a greater degree) out of the lungs. These changes are a response to a variety of inflammatory chemicals released in response to the hypersensitivity reaction occurring within the respiratory system.

The clinical signs of COPD include a chronic cough, the presence of a cloudy (mucopurulent) nasal discharge, potentially an increased respiratory rate (the normal rate is eight to 12 breaths per minute), and an increase in the effort of expiration. In severe cases, there can be exercise intolerance, weight loss, and lack of eating (some horses work so hard at breathing they stop eating). In more chronic cases, there is a build-up of the muscles in the abdominal wall (the abdominal muscles are used to force air out of the lungs) due to "abdominal breathing." This results in the presence of the classic "heave line" along the body wall. During a severe crisis, the horse can be extremely stressed, concentrating intently on every breath. It will have a flaring of the nostrils due to increased breathing effort (see photo pg. 36) and an increased heart rate and respiratory rate. The horse will refuse feed and water and have a pronounced abdominal effort associated with each breath. Should such a crisis occur, your veterinarian should be contacted immediately.

The diagnosis of COPD is based on clinical signs and history. There might have been a recent change in management such as hay, bedding, stabling, or the season with some or all of the aforementioned clinical signs.

Treating COPD

The primary treatment for COPD must be a change in management to reduce the exposure of the horse to dust. Hay is the main source of dust, and elimination of contact with hay dust is essential to management of a horse with COPD. Simply reducing the exposure to hay, however, might not be enough in many cases. If the case is "pasture" related, then moving the horse indoors might be necessary. If the horse is bedded on straw, a switch to wood shavings might help. If the horse is already on wood, then a switch to peat moss or shredded paper or cardboard might be beneficial—the goal is to remove exposure to any potential allergen causing the hypersensitivity reaction.

With respect to reducing dust exposure, the first step can be to soak hay in water. For this, I prefer actually soaking it for several hours before feeding (a clean manure tub works well for this). In many cases, it is necessary to remove hay from the horse's diet all together. The hay replacement will be either a complete feed pellet, hay pellet, or hay cube. Researchers at Michigan State report that they have removed hay and immediately switched horses to pellets many times in the last 15 years without any associated problems.

There also are fermented hay products available that are made of oat or wheat. I have had good success feeding the fermented hay products. These products are of good nutritional value and have virtually no dust. Great care must be taken when switching over to the fermented hay products—it must be done very gradually to avoid gastrointestinal upset. The total nutritional picture must be considered and appropriate supplements must be used to maintain calories, vitamins, and minerals. Your veterinarian should be consulted prior to making any of these changes.

While you can make management changes based on stabling and feedstuffs, your veterinarian should examine the horse before treatment begins to make an accurate diagnosis of the horse's problem. If the horse truly has COPD, infection is unlikely, but your vet will want to check this.

The two main therapies for COPD are drugs that dilate the bronchioles and those that decrease the inflammation within the lungs. The drug clenbuterol (Ventipulmin) recently was approved for use as a bronchodilator in the horse and has been shown to be of substantial therapeutic value. Similar older drugs still are available, but these were of unproven efficacy and relatively non-specific. They also have either a very short duration of action or a high incidence of central nervous system side effects. Clenbuterol is administered orally twice a day.

The other group of drugs for COPD is the corticosteroids, which function to reduce the inflammation occurring with COPD. The other major class of anti-inflammatories (the non-steroidal anti-inflammatory agents such as phenylbutazone) does not work on the type of inflammation occurring with COPD. There is a risk of laminitis or founder when very potent steroids are used at high doses, so they should be used only on the recommendation and under the supervision of your veterinarian. Most of the steroids used to treat COPD are administered orally, but there are some available that are given to the horse via the respiratory system by the use of a puffer inhaler (the main way of treating allergic airway disease in people) or by nebulization.

The development of the Aeromask made it possible to use a variety of human asthma drugs in the horse. Prior to that, it was difficult to use the puffer (inhaler) delivery device in the horse (it just would not breathe deeply when told to!). There are a number of steroids and bronchodilators available in inhalers that have been shown to be beneficial in the treatment of COPD. One of the significant benefits to the inhaler use is the direct administration of the drug into the respiratory system and a reduced incidence of systemic side effects. The Aeromask forms a tight-fitting mask over the horse's nose and provides a mechanism for both the use of inhalant drugs and nebulization. The only downside of this route of administration is the expense of the human inhalers. With people, an insurance company picks up the cost, but for horses, the owner usually has to pay.

Nebulization is the process of turning water (and drugs dissolved in the water) into an extremely fine vapor to be inhaled by the horse. There is a nebulizer unit that can be connected to the Aeromask or other mask device covering the horse's nose. Many of the drugs used to treat COPD can be nebulized.

Robinson noted that, "The increased viscoelasticity of mucus is not due to the lack of water, but due to products such as DNA and actin that are released from inflammatory cells."

Once a horse has been diagnosed with COPD, it has the disease for life. With proper management, it can lead a useful and comfortable life. However, COPD "attacks" can and do happen, and they will vary in severity. It is now clear that the mentioned management changes are the most important aspect of treatment. New, powerful and specific drugs are also available to improve the horse's quality of life.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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