Management Key to Preventing Heaves in Horses
RELEASE: August 4, 2009
AUTHOR/ADMINISTRATOR: By N. Edward Robinson, BVetMed, PhD, MRCVS, Department of Large Animal Clinical Sciences, and
Heaves (also called recurrent airway obstruction) is a severe form of bronchitis/bronchiolitis. The airway passages of a horse with heaves are so obstructed by inflammation, mucus and bronchial muscle contraction (bronchospasm) that the animal has difficulty breathing even at rest. 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. Diagnosis of Heaves
Veterinarians generally diagnose heaves based on the clinical history of chronic coughing, exercise intolerance and the typical difficult breathing even at rest. Wheezing sounds in the lungs can be heard through a stethoscope or sometimes even by standing close to the horse's nose. Heaves is differentiated from acute pneumonia by the absence of fever and by lack of increased numbers of inflammatory cells in the blood. An endoscopic examination of the air passages generally reveals accumulations of mucus within the bronchi and trachea. A sample of this mucus will contain numerous mature inflammatory cells known as neutrophils and no pathogenic bacteria.
Radiographs or ultrasound examination provide no additional diagnostic benefit but may be used to rule out other chronic lung diseases such as pulmonary fibrosis, chronic pneumonia and pleuritis, or tumors. Because heaves is thought to have an allergic basis, veterinarians may recommend testing to determine the cause of the allergy. These tests rarely identify the specific cause of the disease.Environmental Management
Decreasing the horse’s exposure to dust is essential to treat heaves and prevent progression of the disease. This can be done as follows:
• Allow horses continual access to pasture if possible.
• Remove access to hay and replace it with complete pelleted feed, soaked hay or other low-dust feed.
• Do not allow horses to eat hay from a round bale because they frequently b