ONE OF THE most common winter problems in horses is chronic obstructive pulmonary disease (COPD). Veterinarians now call the condition recurrent airway obstruction, but it is also known as heaves, recurrent airway disease, inflammatory airway disease, small airway disease, chronic bronchitis or bronchiolitis, equine asthma, emphysema, broken wind, chronic airway disease, and hay sickness. When a horse is confined in a barn, especially one with poor ventilation, or is fed dusty hay, he is at risk for respiratory problems.

According to Philip J. Johnson, B.V.Sc., M.S., a professor of veterinary medicine and surgery at the University of Missouri, heaves is possibly the most common medical condition of mature horses.

"COPD is a condition in which inflammation of the small airways of the lung leads to impaired ventilation of the lung," Johnson said. Swelling and thickening of the membranes always narrow the airways. "Most commonly, COPD occurs as a consequence of immune-induced inflammation in the terminal bronchioles [the smallest branches of the windpipe that are close to the actual blood/gas exchange interface in the tiny alveoli]."

When a healthy horse inhales, he expands his chest and draws air into the lungs with a powerful vacuum. The expanding chest wall causes lung tissue to stretch and pull apart, enlarging the flexible airways. Inhaling is an active process and exhaling is passive; collapsing of the chest wall pushes air out effortlessly as the lung tissue goes back to its unstretched shape.

In a horse with heaves, the lungs lose their elasticity and the smallest air passages (terminal bronchioles) are swollen and thickened; their inner diameter is greatly reduced. After inhalation, they collapse. The horse makes a wheezing sound as he forces air back out through the constricted passages.

Signs of obstruction

COPD is often characterized by chronic cough, inability to work hard from a lack of stamina, weight loss, lack of response to antibiotics, labored breathing, and sometimes a watery discharge from the nostrils. It is most common in horses older than five years because prolonged exposure to irritants brings on the reaction.

"In these horses, clinical signs of COPD are directly attributable to exposure to allergenic dust in the environment ," Johnson said. "Clinical signs arise whenever horses inhale these. If the horse is not exposed to aeroantigens, signs of COPD may be completely absent."

When horses begin to develop breathing problems, clinical signs could be seasonal in occurrence during the first few years and correlated with the environment. The horse might be noticeably worse in winter if he is kept in a barn, fed hay instead of pasture, or bedded on straw, Johnson said.

The characteristic sign, from which the term heaves is derived, is the forced effort to exhale, sometimes described as double expiration. Air is drawn in easily, but the horse has trouble pushing it out. In forcing air out, the abdominal wall makes two movements called a double expiratory lift.

Simple relaxation of lungs and ribcage will not empty the lungs; the horse has to immediately follow this with more effort by contracting his abdominal muscles and tensing his chest wall to push air out to give an exaggerated lift of the flank. Instead of a steady two-beat inhale-exhale pattern, his breathing has a three-beat pattern from adding that extra lift to force out each breath. Many horses develop an enlarged ridge of muscle, called a heave line, along the lower edge of the abdomen from overworking these muscles.

Decreased speed and stamina in the working horse might be the only signs of early heaves, but as the problem progresses, the horse has more trouble breathing and eventually develops a large chest and perhaps a potbelly because the diaphragm muscles enlarge. If the horse must breathe deeply, as when exercising, he coughs. Coughing spells are also brought on when he eats dusty hay.

Initial inflammation makes airways overly responsive to irritating dust or mold spores, causing small airways to constrict. Whenever a COPD-susceptible horse is in an environment such as an enclosed barn where he breathes dust particles, the airways become hyper-reactive, resulting in the vicious cycle of self-perpetuating airway obstruction. The compromised airway walls produce excess mucus that tends to block them, and the muscles in the airway walls contract to constrict the airways.

"The most common clinical sign of COPD is increased resistance to air movement through the passages as a result of bronchiolitis," Johnson said. "In the beginning, mildly affected horses exhibit slightly increased respiratory effort and respiratory rate following exercise. As the condition progresses, increased respiratory effort will be apparent in the standing horse at rest. Horse owners often miss these early changes in exercise capacity in mildly affected horses."

Eventually, clinical signs include intermittent coughing. All signs become more pronounced when the horse is exposed to dust or is stabled indoors and become less obvious when the horse is at pasture.

Occasionally a COPD-affected horse will develop a sudden state of severe respiratory distress. "Such horses may not have appeared abnormal to the owner, though on recollection, these horses are commonly reported to have had an occasional cough," Johnson said. Signs of acute-onset severe respiratory distress include increased respiratory rate, increased respiratory effort (with abdominal breathing), flaring of the nostrils, and cyanotic (blue-colored mucous membranes).

Horses with heaves do not have a fever, an important point to consider when differentiating this condition from infectious respiratory diseases. "However, in hot weather, the rectal temperature of a COPD-affected horse may be increased because reduction in airflow through the airways interferes with thermoregulation and reduces the ability of the horse to eliminate body heat," Johnson said.

Because COPD is the most common cause of coughing, nasal discharge, and respiratory distress in adult horses, signs of the disease are often overlooked.

"All too often, intermittent coughing in barn-accommodated horses is attributed to an innocuous barn cough," Johnson said. "Recognition and early treatment for COPD would be much better, however, than waiting until severe lung damage has occurred, with irreversible lung scarring."

Causes

The most common cause of heaves is airborne particles such as molds that grow on hay and straw. Therefore, exposure to dusty hay or bedding, especially in a barn, is likely to result in breathing difficulty. Allergic sensitivity to hay dust and molds could bring on an acute episode of breathing difficulty.

Some horses are more sensitive to respiratory irritants, just as some people have more allergic reaction to dust and pollen than do other individuals (there is some speculation about genetic predisposition in horses’ allergic responses to dust). When a normal horse is confined indoors, he might get a mild inflammation in his airways, but a horse with allergic tendencies could develop a very pronounced inflammatory response.

A heaves-susceptible horse does not need much inflammation to begin wheezing and coughing. Even if he is turned out into a non-dusty environment after an episode, he might require weeks for the inflammation to disappear. If he is returned to the barn, even for a short time, he will relapse, starting a chain of events that will cause days or weeks of breathing difficulty. Each time the horse experiences an episode of difficult breathing, he might become a little worse.

Normal secretions from the respiratory tract in a healthy horse move up toward the mouth and are swallowed, keeping the airway clear. In a horse with COPD, the excessive quantity of mucus might appear at the nostrils, creating a nasal discharge or dirty nose. The quantity of mucus that appears at the nostrils is often increased when the horse grazes or eats at ground level because gravity helps it drain down out of the airways to the nostrils.

"Coughing occurs because clumps of mucus act to physically activate cough receptors in the windpipe as a result of mechanical irritation," Johnson said. "The bronchiolitis is provoked by an immune-mediated reaction against inhaled aeroantigens. The reaction occurs in the terminal bronchioles because it is at this location that the tiny particles are filtered out of the inspired air. Horses are exposed to inhaled aeroantigens whenever they eat hay, are bedded on straw, or accommodated in barns--especially old barns or barns in which hay and straw are stored.

Johnson said COPD-affected horses may also show aggravation of signs when exposed to dusty paddocks or arenas, wood shavings, petrochemical pollution, or other non-allergic air contaminants. "The exacerbation of signs is related to the fact that the affected horse’s airway has already been sensitized by the presence of inflammation due to exposure to aeroantigens," he said.

Prevention and treatment

A horse with a breathing problem must be protected from dust as much as possible and fed dust-free feeds. Most horses with winter respiratory problems will completely recover without drug treatments if inhaled aeroantigens are eliminated from their environment. A horse with a respiratory problem should not be kept indoors. An open-sided shed, with no bedding, is much better.

Studies in the early 1990s that measured levels of dust in stalls found that the conventional management system of feeding hay and using straw for bedding resulted in three times more general dust in a stall and 30 times more dust in the breathing zone where the horse’s nose is located while he eats than a system using wood shavings as bedding and complete pelleted diet instead of hay.

Other considerations for caring for a horse with COPD include:

• Sawdust should not be used as bedding because it can be quite dusty.
• Compared with straw, peat moss, wood chips, shavings, damp sand, or shredded paper are less irritating to the horse’s respiratory system.
• Whenever hay or straw is stored in a barn, chances for dust are increased.
• Concentrations of dust particles in stalls are greater during the day, when people are moving around in the barn, than at night, when there is less activity. But when horses are fed hay, the concentration of dust particles in the horse’s breathing zone stays high both day and night. Even in a well-ventilated barn, air movement does not remove dust particles from the horse’s breathing zone when he is eating hay. Each time he noses the hay around or shakes it, there is dramatic increase in dust particles.
• If hay is fed, only dust-free hay should be selected, and it can be dampened with water to eliminate dust particles. For a horse with COPD, sprinkling to dampen hay may not be adequate; each flake should be soaked in a tub of water to become completely moistened, then drained before it is given to the horse. Grain can be fed as a mash or with molasses added to dampen it and reduce dustiness. Any dampened feed not cleaned up quickly should be removed because it might begin to mold.

  None of these efforts will help, however, if the horse is still kept indoors and other horses in the barn have straw bedding or hay; dust particles will continue to float about in the air.

  "If indoor accommodations must be used, all horses in that barn should be managed under the same precautions, and barn cleaning should not be undertaken when COPD-affected horses are in the barn," Johnson said.

• Non-dusty pelleted feeds can be substituted for hay. Pasture grazing in winter can be supplemented with oats, silage, haylage, pelleted feeds, alfalfa cubes, beet pulp, or complete pelleted rations such as those formulated for older horses.
• If hay must be fed, it should be at ground level and moistened, not at head level in a hay net where particles may fall into the horse’s face. Feeding large bales outdoors in a paddock or pasture also can aggravate a respiratory problem because horses will eat into the bale and bury their noses in dusty conditions.

  Drug treatment is not usually needed, except in a few of the most severely affected horses, Johnson said. Without improvement in management, drugs are ineffective.

  "Drug strategies include bronchodilators, anti-inflammatory agents, antibiotics, and immunological moderators," Johnson said. "It is currently recommended that bronchodilators [ipratropium bromide and albuterol] and anti-inflammatory agents [usually steroids such as beclomethasone or fluticazone] should be administered via inhalation." Alternatively, they can by given by injection or by mouth.