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Researchers at the University of Montreal study the effect of heaves on smooth muscle surrounding horses’ airways.

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Courtesy Dr. Jean-Pierre Lavoie

Heaves is an asthma-like equine respiratory disorder with chronic airway inflammation. In recent years, researchers have contended that horses suffering from this condition experience remodeling of the airways, preventing normal function of the lungs.

“It has long been suspected that the amount of smooth muscle (ASM) surrounding the airways is increased in heaves and contributes to the severe airway obstruction affecting these animals,” said Dr. Jean-Pierre Lavoie, DMV, Dip ACVIM, Professor, Chief, Equine Internal Medicine Service and Director, Cell and Molecular Biology Respiratory Laboratory Department of Clinical Sciences at the University of Montreal. “When constricted, this smooth muscle prevents the normal movement of air during respiration.”

In 2006, a study performed by a student of Dr. Lavoie’s laboratory, and in collaboration with Dr. Jim Martin at McGill University, revealed that horses with heaves had two to three times more ASM surrounding the airways when compared to age-matched control horses. Using quantitative analysis, the researchers assessed post-mortem tissue from five horses with heaves and five control horses.

“Interestingly, the most severe changes were observed in the small airways in the periphery of the lungs,” said Dr. Lavoie. “These novel and exciting results were published in a high impact human respiratory journal, and they helped us obtain a grant from the Canadian Institutes of Health Research and AllerGen NCE Inc. to study these changes and to determine whether they are reversible by common anti-asthma medication.

“With a team of veterinary surgeons, clinicians and support staff from the in the Equine Referral Hospital at the University of Montreal, we developed a cost effective and minimally invasive technique to sample the peripheral lungs of standing horses using thoracoscopy [the insertion of an endoscope through a small incision in the chest cavity].

“We thus confirmed our previous findings of an increased ASM in the peripheral airways of horses with heaves, and further reported that fibrosis [thickening and scarring] of the airways is also present. ‘We also found that, once established, these changes are unaffected by 30 days of stabling, and that six months of treatment, or more, with inhaled corticosteroids, is required to slightly decrease the ASM. Surprisingly, there was a better improvement in the airway fibrosis, which was almost completely reversible, however.”

Dr. Lavoie explained that “it is important to study airway remodeling, as the changes occurring within the airways, both in terms of composition and architecture, are believed to be responsible for progressive loss of airway function, leading to the marked difficulty of breathing in horses with heaves at rest.”

The researchers are now investigating possible blood markers that would allow veterinarians to identify horses susceptible to developing heaves so that, in the future, treatment for such horses can begin earlier, thereby preventing the development of pulmonary lesions.

The team’s findings have helped them secure a $500,000 grant from the Canadian Foundation for Innovation to purchase state of the art equipment to bring their research to the next level. “We are continuing our characterization of changes occurring within the airways, and the molecular pathways associated with them, and assessing novel non-invasive techniques to study the alterations occurring within the airways that, if successful, could be used for the early detection of susceptible horses,” said Dr. Lavoie.