Heaves is a respiratory disease of horses 6 years of age or older, characterized by difficultybreathing, chronic coughing, abnormal lung sounds and decreased exercise tolerance. These signs manifest themselves as a result of narrowing of the small airways of the lungs. This narrowing is caused by inflammation and thickening of the lung tissues, constriction of the smooth muscles surrounding the lungs and accumulation of mucous and exudate within the lumens of the lungs. The end result is known as emphysema or a trapping of air in the lungs.
With the narrowing of the small airways of the lungs, horses with heaves experience difficulty pulling air into the lungs as well as pushing the air out again. As the condition worsens, severely affected horses are seen to contract the muscles of the abdominal wall during the last phase of exhalation, causing the floor of the abdominal wall to lift up at the very end of exhalation. As a result, the lungs tend to remain over-inflated causing a condition referred to as functional or reversible emphysema.
In severe cases, the disease can progress to the point that permanent, non-reversible damage occurs in some portions of the lung tissue. In many cases, however, much of the loss of lung function that occurs with heaves is reversible with careful management of the horse and it’s environment.
Although heaves has long been recognized as a disease of horses, it’s exact cause remains uncertain. Most evidence suggests that the inflammation of the small airways occurs as a result of an allergic reaction to dusts and molds, especially those found in poorly cured hay.
Common risk factors include:
1. Exposure to improperly cured, moldy or dusty feeds
2. Confinement to a stable environment
3. Inadequate stable ventilation
4. Straw bedding
5. Being 6 years of age or older
Most modern treatments of heaves in horses involves the use of drugs to decrease the amount of inflammation and the accumulation of inflammatory exudate in the respiratory tract, to dilate the airways and to speed up the clearance of mucus and inflammatory debris from the respiratory tract. The duration of the disease depends largely upon the amount of effort given to improving the conditions under which the horse is kept.
Complete or near complete recovery from the symptoms of heaves has been reported in horses turned out to pasture or moved into a well-ventilated stall, fed cubed or pelleted roughage with dampened grain and provided with bedding that is virtually dust and mold free, such as shredded paper or high-quality wood shavings.