Inhaled Corticosteroids, Management Changes Help RAO Horses

Imagine horses with recurrent airway obstruction (RAO, or more commonly heaves) wearing Darth Vader-like masks and breathing in corticosteroids every day sun, rain, or snow. That was essentially the scene at at the Université de Montréal in Quebec, Canada, during a yearlong study investigating the effect of inhaled corticosteroids (ICS) and antigen avoidance in horses with RAO.

Mathilde Leclere, DVM, PhD, Dipl. ACVIM, presented results of that study “Inhaled Corticosteroids and Antigen Avoidance in Heaves: A Yearlong Study” at the 2013 American College of Veterinary Internal Medicine Forum, held June 12-15 in Seattle, Wash.

RAO is a chronic and debilitating airway disease of the horse formerly known as chronic obstructive pulmonary disease. It describes a condition similar to asthma in humans in which the horse is seemingly unable to “catch its breath,” leading to the following clinical signs described in the TheHorse.com’s Fact Sheet “Recurrent Airway Obstruction in Horses”:

  • Cough at the start of exercise (early stage of RAO);
  • Frequency of coughing increases;
  • Increased respiratory effort (even at rest);
  • Nasal discharge but no fever;
  • An obvious abdominal lift at the end of exhalation;
  • A heave line (a line running diagonally from the point of the hip forward to the lower edge of the ribs in the external abdominal oblique muscle caused by the persistently increased respiratory effort); and
  • Weight loss due to the difficulty of eating while trying to breathe.

Leclere’s team found ICS treatment improved the clinical signs of RAO, much like it does for human asthma, even in the face of persistent exposure to hay dust. Antigen avoidance (limited exposure to hay, dust, mold, pollen, and ammonia) also helped reduce clinical signs. But, horses got the most relief from RAO when both receiving ICS and avoiding environmental antigens.

Study Structure

To create a research baseline, the researchers employed 11 heaves-afflicted horses stabled and exposed to hay to induce RAO clinical signs, including coughing, increased respiratory effort, nasal discharge, and abdominal lift at the end of exhalation.

Once they established a baseline, the researchers assigned five of the horses to the antigen-avoidance group, meaning they lived in 24-hour turnout without stabling (aside from shelters providing a weather break). These horses ate a grass and pellet-only diet to limit dust and mold exposure but received no other treatment.

The second group of horses had no housing or feed changes and remained stabled with a hay-based diet. However, this group received daily ICS treatments of fluticasone propionate through a respirator mask covering both nostrils. This drug is commonly used in humans to treat asthma, hay fever, sinusitis (swelling of the sinus), and rhinitis (swelling in the nose) and is distributed in the United States under the brand names Flovent and Flonase.

The researchers administered the Group 2 horses a one-month, weight-based loading dose consistent for all the animals. After the first month, they customized each horse’s dosage to control that individual’s clinical signs.

Researchers measured lung function and inflammation via bronchoalveolar lavage (BAL, a procedure in which the veterinarian passes a bronchoscope through the nose into the lungs to flush fluid into a small part of the lung, recollecting the fluid for examination) and blood samples collected at baseline, and after one, six, seven, and 12 months of treatment.

They also took lung biopsies from all horses at baseline, six, and 12 months to record changes in inflammation and airway remodeling (or a structural change to the airway associated with chronic inflammation), Leclere explained.

Results

Horses treated with ICS but still eating hay showed improved clinical signs of RAO, but persisting lung inflammation. Once moved outside, the ICS-treated horses' lung function continued to improve and the inflammation resolved.

The biopsy results also indicated that the thickening of the smooth muscle around the airways is partially reversible, even in animals that have been affected for many years, Leclere said.

“From a clinical point of view, we could summarize that the benefits of using inhaled corticosteroids in the treatment of heaves include a more rapid relief of airway obstruction and a more rapid decrease in airway smooth muscle compared to antigen avoidance alone, without inducing detectable side effects with prolonged use,” Leclere explained. “However, avoiding hay and barn dust leads to a better control of lung inflammation and has additional beneficial effects on pulmonary function, even when clinical signs are apparently controlled by medication.”

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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