Low-Dust Forages Essential for Asthmatic Horses

Researchers recently revealed that feeding Thoroughbred racehorses in training and racing  certain alternatives to dry hay resulted in 30% fewer respirable dust particles in horses’ breathing zones and decreased airway inflammation.

Laurent L. Couëtil, DVM, PhD, professor of Large Animal Internal Medicine and director of Equine Research Programs and Equine Sports Medicine Center at Purdue University’s College of Veterinary Medicine in Indiana, presented data showing the benefits of avoiding dry hay in horses with mild airway inflammation, also known as equine asthma (EA), during the 2020 American Association of Equine Practitioners (AAEP) Convention, which is being held virtually.

“Poor performance with or without a cough together with excess tracheal mucus visible via endoscopy directly after exercise suggest the presence of mild EA,” he said. “A bronchoalveolar lavage (BAL, which involves introducing sterile fluid into a horse’s lung before drawing it back out again for testing) can confirm the presence of airway inflammation, and airway endoscopy can rule out other airway conditions such as infectious diseases, exercise-induced pulmonary hemorrhage, and upper airway obstructions.”

Identifying horses with mild EA allows appropriate management changes to eliminate poor performance associated with airway inflammation.

Mild EA occurs very commonly, said Couëtil. For example, in one study he cited, researchers found that approximately 13% of more than 1,900 endoscopic examinations confirmed the presence of excess tracheal mucus after racing, and this accumulation was associated with decreased performance.

“Exposure to dust from forage is probably the most important trigger of mild EA in racehorses. Specifically, hay generates microscopic respirable particles measuring less than 4 microns that are undetectable to the naked eye but highly inflammatory when inhaled by horses,” said Couëtil.

He and colleagues worked with local Thoroughbred trainers to determine if steamed hay or haylage would be better alternatives for racehorses. They randomly assigned clinically healthy racehorses actively training and racing to be fed one of three forages for six weeks: 17 horses were fed dry hay, 18 horses were fed steamed hay, and another 18 horses were fed haylage.

The researchers found:

• Both steamed hay and haylage lowered horses’ exposure level to respirable dust particles compared to that experienced with dry hay. They identified no difference in particle levels present in steamed hay and haylage;
• Mucous scores (assessed via endoscopy) were significantly increased by Week 6 of the study in horses fed dry hay; and
• The number of neutrophils (a type of white blood cell) in BAL fluid had significantly decreased from dry hay levels in horses fed haylage at Weeks 3 and 6 of the study.

“This study clearly demonstrated the detriments to feeding dry hay to healthy horses,” said Couëtil, adding, “This study also highlighted that a BAL can be done safely in performance horses without interrupting their training and racing schedule.”