New research from Canada has revealed 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 called heaves, the condition is triggered by dust, mold 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 postmortem lung samples from 18 horses at the Equine Respirator­y Tissue Biobank: six who were in the midst of a 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 with 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 did the control horses or those whose condition was managed. 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 the amount of smooth muscle in the arterial wall to increase. 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 walls 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 were 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 asthma.”

In horses with severe asthma, he explains, it may take more than three months for lung function to normalize when only environmen­t dust control is used: “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.”

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

October 2020

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 ??  ?? In addition to medication, successful treatment of equine asthma requires reducing exposure to dust and other environmen­tal triggers.
In addition to medication, successful treatment of equine asthma requires reducing exposure to dust and other environmen­tal triggers.

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