Respiratory Disease: Not So Easy Breathing

The terms recurrent airway obstruction (RAO or heaves) and inflammatory airway disease (IAD) are often wrongfully used interchangeably to describe horses with non-infectious respiratory disease. Bonnie Rush, DVM, MS, Dipl. ACVIM, professor of equine medicine at Kansas State University, explained basic differences between the two.

RAO is a common respiratory disease in horses that is characterized by chronic coughing, nasal discharge, and respiratory difficulty. The average age of horses developing RAO is around nine years old. Thoroughbred mares over the age of seven are at the greatest risk for developing RAO. However, all breeds and genders are affected by the disease.

Rush said, "Most of these (RAO) horses I diagnose by physical exam alone, and look for response to treatment. If I treat them (for seven days) and they are unresponsive, I'm going to take chest films."

Because of increased respirations caused by respiratory diseases, using X rays to diagnose RAO is very difficult. But, she said chest X rays will help rule out other types of diseases, such as interstitial pneumonia.

Horses with RAO usually have difficulty breathing, low body condition scores due to the disease, and excessive mucus production. Air flow is restricted by bronchocontriction, excessive mucus production, and cellular debris.

Neutrophilic inflammation is the most common inflammatory cell type associated with RAO. In an RAO horse, they make up 50-70% of the total cell count in a bronchoalveolar lavage (a diagnostic procedure conducted by placing a bronchalveolar lavage catheter into the lung of a patient, injecting sterile saline into the lung, and removing the fluid for testing). A normal horse would have less than 5% neutrophilic cells.

"Most evidence suggests that RAO is the result of hypersensitivity to inhaled agents," Rush said. "Inhalation challenge of horses with RAO with organic dust induces pulmonary dysfunction within 90 minutes and hypoxemia and neutrophilic inflammation within five hours."

On the other hand, IAD affects a younger population of horses (2-year-olds and younger) and is less severe that RAO. Horses with IAD usually have low-grade pulmonary inflammation.

Clinical sign of IAD generally include a chronic cough, nasal discharge, exudates (cellular waste product) in the lower respiratory tract, and poor performance. Horses with IAD rarely have radiographic pulmonary abnormalities or signs of systemic disease (fever).

Diagnostic testing for IAD can include endoscopic examination, bronchoalveolar lavage, and bronchoprovocation challenge tests (which measure the degree of airway hyperresponsiveness to inhaled histamine).

Researchers maintain a number of theories regarding the origin of IAD, and in fact, low-grade pulmonary inflammation might be the result of a number of inciting causes including recurrent pulmonary stress, inhalation of particle matter (i.e., dust, endotoxin), exposure to noxious gases and pollutants, and/or bacterial or viral respiratory infections. The inflammatory cells in the lungs of IAD affected horses appear to fall into 3 specific categories, indicating the cause of airway inflammation is unlikely to be the same in every case.

What both diseases have in common is that they can severely affect the horse's performance, and both need to be addressed by a veterinarian.


About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for .

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