New research from Canada has identified a troubling side effect of uncontrolled respiratory disease in horses—changes in the pulmonary arteries that could lead to heart failure.

Thickening of the pulmonary arteries and the associated increase in blood pressure is known as pulmonary hypertension. It’s a well-documented finding in people with severe asthma but until this study had not been detected in horses. Severe equine asthma, previously known as recurrent airway obstruction, is characterized by airway inflammation, coughing and labored breathing. Commonly call heaves, the condition is triggered by dust and mold in hay and other environmental factors, but when a horse is managed to reduce exposure to these irritants the clinical signs usually subside.

At the beginning of the University of Montreal study, the researchers took post-mortem lung samples from 18 horses from the Equine Respiratory Biobank: six that were in the midst of heaves flare-up when they died, six with heaves in remission at the time of death due to being kept in a dust-free environment and six horses that had no history of heaves to serve as controls. Multiple samples were taken from various places in the lungs of each horse.

Examining the tissue samples under a microscope, the researchers measured the thickness of the arteries that serve the lungs. They found that horses with uncontrolled equine asthma had thicker arteries than those whose condition was managed or the control horses. This finding was replicated in the second part of the study, where similar measurements were found in lung samples from six asthmatic horses and six control horses collected for an earlier research project.

Why arteries thicken when a horse has respiratory troubles isn’t clear, says Jean-Pierre Lavoie, DMV, DACVIM. “The exact cause is unknown, but experimental work in rodents suggests that both the low oxygen content—a condition called hypoxemia—and inflammation may cause an increased amount of smooth muscle in the arterial wall. Hypoxemia and inflammation are both present in severe equine asthma and likely contribute to this process.”

The ultimate effect of arterial thickening is known, however. “Thicker arterial wall decrease the lumen (the interior space) of the artery, making it more difficult for the blood to move out of the lungs,” explains Lavoie, “This may be due to increased muscle contraction caused by the increased mass of the tissues, and by the reduced lumen physically caused by the thickening of the airway wall. Together, this results in pulmonary hypertension, with a negative impact of the blood flow dynamics and cardiac function.”

Eventually, says Lavoie, thickening of the arteries in horses could lead to a condition called cor pulmonale, the enlargement and failure of the right ventricle of the heart.

Potential treatments for pulmonary hypertension in horses was the focus of the final portion of the study. For that, researchers measured pulmonary artery wall thickness in lung tissue samples previously collected from 11 asthmatic horses divided into two groups and treated for 12 months with two different protocols. The first protocol focused on reducing the horse’s dust exposure for the entire year, primarily through the use of hay alternatives. The second protocol utilized only a corticosteroid medication called fluticasone for the first six months, then added dust-control management measures for an additional six months.

The researchers found that both treatment protocols lead to a reversal in the thickness of the artery walls, but that trend was not evident until the dust-control measures were taken. “For most horses, hay is the main trigger, and keeping horses in the stable with a hay alternative and clean wood shavings will often suffice,” says Lavoie. Keeping a horse outdoors but still feeding hay, he adds, may not control the airway inflammation.

“In this study, we showed that the arterial wall remodeling is reversible but only with strict control of dust exposure—not just with inhaled corticosteroids, even after one year of treatment,” continues Lavoie, “This study again highlights the necessity of improving the environment of horses to control equine asthma.”

Lavoie adds that corticosteroids can be helpful in making short-term improvement in respiratory function while reversal of the airway thickening beings. “In horses with severe asthma, it may take more than 3 months before the lung function normalizes when only environment dust control is used. This often leads the owners to think that their horses do not respond to this strategy. The administration of corticosteroids at the onset of treatment is therefore indicated to rapidly improve the welfare of horses, but should be combined with an improved environment. With corticosteroids, a marked improvement in the lung function is expected within a week.

Reference: “Severe asthma is associated with a remodeling of the pulmonary arteries in horses,” PLOS One, October 2020

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To read about how to recognize heaves in a horse, click here.