A case of the heaves: Veterinarians deal with recurrent airway obstruction in horses - DVM
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A case of the heaves: Veterinarians deal with recurrent airway obstruction in horses
Vets find stabling and eating hay are two of many factors that can contribute to this life-threatening disease.


Airway remodeling

A critical portion of Lavoie's research involves the physical changes that take place within the lungs of horses with heaves—a buildup (which gets more severe over time) of smooth muscle surrounding the airway tissue. Eventually, a lot of these horses are euthanized because they're difficult to manage.

"We think the reason they become increasingly severe is because of the changes occurring within the lungs surrounding the airways, as is true in human asthma," Lavoie says. "We reported that horses with heaves have two to three times more smooth muscle surrounding the airways than age-matched control healthy horses. The more muscle present, the more likely the contraction is going to be. The lumen of the airways is reduced in diameter, therefore air is not moving freely through the airway. And with the increased smooth muscle, there's increased contraction of the airway, making breathing even more difficult."

In further research, Lavoie studied airway remodeling, taking lung biopsy samples from deceased horses and from horses with heaves over time (a few months or years). In the live horses with severe cases of heaves, they documented the same thing they saw in the necropsied samples.

Lavoie tried to reverse this increase in smooth muscle remodeling. His team studied a group of horses with heaves—half were pastured on pelleted feed for a year and the other half were treated with inhaled corticosteroids for a year. Initially, the horses treated with corticosteroids were stabled. After six months they continued the treatment and put them on pasture. They measured smooth muscle surrounding the airways before the treatment, at six months and again at a year. Mathilde Leclere, DMV, a graduate student in Lavoie's lab, showed there's a decrease in the amount of smooth muscle surrounding the airway after treatment with inhaled corticosteroids, but they were very far from reversing it even after a year. The researchers also showed if they kept horses with heaves stabled for a few weeks, smooth muscle mass didn't change, since remodeling is inherently a very gradual process.

Leclere also looked at what was responsible for the decrease in airway smooth muscle, as they hope to eventually develop targeted therapy that may allow for restoration of lung tissue to normal status, if possible.

Another Université de Montréal graduate student, Emilie Lanctot-Setlakwe, DMV, is looking at the collagen content present between the epithelium and the smooth muscle. "She found there's an increase in collagen, so there is also some fibrosis taking place in the lung tissue of horses with heaves. We're trying to determine if this is reversible or not," Lavoie says.

These studies show airway remodeling, or the change in airway architecture, takes place during a period of several years. "That's probably why we rarely see the severity of clinical signs of airway disease in young horses," Lavoie says. "Coughing may be present, though the horse lung has an incredible reserve compared with the human lung. The severe disease process takes years to develop, and we rarely see the lung damage of heaves in young horses at rest."


Source: DVM360 MAGAZINE,
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