Heaves in horses make it difficult for horses to breath. Heaves are caused by dust, molds and other air pollutants. It is a severe form of bronchitis. Air ways are inflamed enough making it difficult to breath.
According to an article on Horsetalk:
At the end of each exhalation, the horse pushes so hard with its abdominal muscles that with chronic overuse, these muscles enlarge and form a “heave line” that runs diagonally from the point of the hip forward to the lower edge of the ribs.
Horses with heaves first show signs when they are around eight years old. Signs include coughing and exercise intolerance and are more likely first observed when the affected horse is in closed environments such as a stable. The cough persists, and after one to two years the horse may show difficult breathing at rest. If the condition is untreated and management of the horse is not changed, airway obstruction becomes more severe and breathing becomes progressively more difficult. An affected horse loses weight because it cannot eat sufficient food when gasping for air and because it uses lots of muscular energy in the effort to breathe.
Heaves is classically a disease of horses that are in dusty environments with poor ventilation. Breathing dust particles that originate primarily in hay can cause airway inflammation. The important role played by the environment in the development of heaves has been known at least since 1640, when green grass was described as the best treatment for heaves in Britain. It was recognized during the 1800s that horses with heaves in New York City stables would become healthy if moved to an environment with clean air.
However, horses kept outdoors also can develop heaves. In the southern United States, a common heaves-like disease known as summer pasture-associated obstructive pulmonary disease appears to be due to inhalation of mold spores originating from pasture soil. Meanwhile, evidence of a genetic susceptibility of horses to chronic airway disease is accumulating. For example, the foal of two affected parents is more likely to develop chronic airway disease than the offspring of unaffected parents.
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