Equine Asthma: Muscular Changes in Airways

Equine asthma causes coughing and exercise intolerance when mild but often distress and debility when severe. While researchers have known for some time that airway remodeling occurs in advanced asthma, new evidence shows it also happens in mild cases. Understanding airway remodeling in mild cases may help veterinarians develop biomarkers to better assess disease progression and response to therapy.

Airway remodeling refers to changes in the walls of respiratory structures, such as the trachea, bronchi, and bronchioles. In cases of severe asthma, remodeling typically describes the thickening of the walls due to an increase in smooth muscle mass. Smooth muscles line the airway walls and are responsible for increasing and decreasing the diameter of the air passages.

“Severe smooth muscle constriction, or bronchospasm, decreases the diameter of the airways, making it difficult for large volumes of air to pass in and out of a horse’s lungs,” explained Kathleen Crandell, Ph.D., a Kentucky Equine Research nutritionist.

An increase in the size of individual muscle cells and an increase in number of muscle cells are thought to contribute to smooth muscle augmentation during remodeling in severe cases of asthma. In addition, a novel type of smooth muscle protein that contracts about two times faster than the protein normally found in airway smooth muscle appears to be more common in horses with severe asthma. This protein is called the “(+) insert smooth muscle myosin heavy chain isoform.”

To determine whether smooth muscle changes were limited to severe cases of asthma or if they also existed in milder cases, biopsies from 18 mildly asthmatic horses were collected. Biopsy analysis revealed that expression of the (+) insert smooth muscle myosin heavy chain isoform was about one and a half times higher in mildly asthmatic horses than the protein levels in healthy horses, meaning narrowing of the airways occurs more easily than in healthy horses. The structural changes noted in severe asthma, such as an increase in size and number of smooth muscle cells, were not observed in cases of mild asthma.

These findings contribute to an improved understanding of equine asthma and could lead to “a better comprehension of the initial changes that occur in the development of the disease.” In turn, this may permit targeted therapy plans and, ultimately, prevent development of severe forms of this disease.

“Current therapeutic approaches for equine asthma of any severity center on environmental control and allergen avoidance. Airborne allergens, primarily from forage, are a major trigger for asthma flare-ups,” Crandell said.

With regard to treatment, she added, “Medications such as corticosteroids and bronchodilators ease airway inflammation and bronchospasm, as do omega-3 fatty acids, especially DHA.”

EO-3, an omega-3 fatty acid supplement formulated by Kentucky Equine Research, contains both DHA and EPA. The American College of Veterinary Internal Medicine recommends DHA be fed to horses with any form of asthma.^**

*Dupuis-Dowd, F., and J.-P. Lavoie. Airway smooth muscle remodelling in mild and moderate equine asthma. Equine Veterinary Journal. In press.

^**Couëtil, L.L., J.M. Cardwell, V. Gerber, J.-P. Léguillette, and E.A. Richard. 2016. Inflammatory airway disease of horses—Revised consensus statement. Journal of Veterinary Internal Medicine 30:503-515.