Pinpointing the cause of poor performance in athletic horse is often a challenge. If it turns out there could be a respiratory cause, then mild equine asthma (EA) could be to blame. Luckily, the common saying that “prior preparation prevents poor performance” can be taken to heart in such situations. While hay steamers have been marketed to horse owners for several years, new research demonstrates that steamed hay and haylage can make measurable differences in a horse's
Mild EA, the preferred term that replaces inflammatory disease, describes horses with a chronic low-grade cough (defined as having gone on for longer than three weeks), decreased/poor performance, and the presence of tracheal mucous when the horse is scoped. Many underlying conditions can be confused with EA. Those include infectious causes (viral or bacterial), upper airway obstruction (dorsal displacement of the soft palate, for example), and exercise-induced pulmonary hemorrhage. Some veterinarians have even speculated that those conditions may even predispose horses to mild EA.
“The most important factor contributing to mild EA in Thoroughbreds is the small dust particles horses breathe in primarily as a consequence of feeding dry hay,” explained Dr. Laurent Couëtil, section head of Large Animal Internal Medicine at Purdue University, in West Lafayette, Ind.
Dry hay contains fungi, molds, mite debris, inorganic particles, endotoxins, and other inflammatory molecules. This microscopic particulate matter contaminates the horse's breathing zone, causing inflammation in the lower airways.
“Particulate matter measuring less than 4 microns in diameter results in a sharp and significant increase in the number of neutrophils in mucus collected from the lungs,” said Couëtil.
Particles this small cannot be seen to the naked eye but can be measured with specific, wearable equipment fastened to a horse's halter.
Mucus — a hallmark of EA — can easily be collected from horse's lungs via bronchoalveolar lavage (BAL) and microscopically analyzed. The presence of neutrophils in this BAL fluid indicates inflammation. Other inflammatory cells may also be appreciated, such as mast cells and eosinophils.
The amount of tracheal mucous, which can be scored on a scale ranging from 0 (no excess mucous) to 5 (defined as a profuse amount pooling throughout the trachea) can also be used to gauge the severity of mild EA.
According to Couëtil, studies in both Standardbred and Thoroughbred racehorses have demonstrated an association between severity of mucus score and poor performance. As mucous scores increase, speed of the horse decreases.
“A 2006 study performed by Sue Holcomb showed that horses with tracheal mucous scores of 2 or greater were significantly behind in finishing place than horses with a score of 0 or 1,” Couëtil relayed.
Because forage is the most important source of dust that triggers EA, various tactics designed to minimize dust have been explored. Recently, Couëtil and colleagues conducted a study at an Indiana Thoroughbred racetrack. They demonstrated that racehorses actively involved in training and competition that were fed steamed hay or haylage had reduced exposure to dust by approximately 30% when compared to horses fed dry hay.
In that study, Couëtil's team recruited 69 Thoroughbreds and divided them into three groups based on type of forage fed: haylage, steamed hay, and dry hay. All horses were fed this diet for a total of 6 weeks. On weeks 0 (baseline), 3 and 6 of the study, endoscopy was performed after coming back from the track to assess respiratory function and to grade mucous. In addition, all horses were equipped with sensors to measure respirable particles (less than 4 microns in diameter) for 3 hours after returning from training and being fed.
Haylage is grass that is cut and baled at a higher moisture content (about 30%) than regular hay (about 15%) and is package in sealed plastic films similar to shavings bales. This packing prevents molding of the moist forage and allows preservation of the nutritional value of fresh grass similarly to what is achieved with silage for cows. This moist forage results in a marked decrease in dust exposure when horses eat haylage. For the purposes of this study, trainers were each given a hay steamer provided by Haygain.
Key findings of the study were:
- Respirable dust particles (less than 4 microns in diameter) were significantly higher in the breathing zones of horses fed hay. Both the steamed hay and haylage generated the same, significantly lower level of dust particles;
- By the end of the study, mucous scores were significantly higher in the hay group. Both the steamed hay and haylage groups had the same, significantly lower mucous scores;
- BALF analysis showed that the number of neutrophils, an indicator of airway inflammation, increased significantly as the respirable dust concentration in the horse's breathing zone increased; and
- Over time, the number of neutrophils in BALF decreased in horses fed steamed hay and haylage but only reached statistical significance for horses fed haylage.
“In sum, our results clearly demonstrated the benefits of feeding low-dust forages on airway health in just 6 weeks,” Couëtil concluded.
Another conclusion that Couëtil highlighted was that BAL can be performed safely in Thoroughbred racehorses without interruption in racing or training.
“For some veterinarians, owners, or trainers, the idea of a BAL can be off-putting,” Couëtil said. “Many veterinarians are not familiar with the procedure, and others think that a BAL will require resting their horses for an extended period of time after infusing fluid in the lungs.”
The reality is that even if only 50 percent of the sterile saline solution administered is recovered, the rest is rapidly absorbed. Couëtil's study proved that a BAL can be performed without interfering with the training and racing schedules.
“Owners and trainers shouldn't hesitate to perform a BAL in any case of chronic cough, poor performance or when excess mucus is seen by endoscopy after the race,” Couëtil said. “This test can be highly beneficial especially when used in conjunction with the mucous score. The BAL rules in mild EA while endoscopy can help rule out other causes of cough and poor performance.”
One caveat worth noting is that medications are sometimes used for sedating the horse and to decrease coughing during BAL, and it is important to respect drug elimination times prior to racing.
In sum, identifying realistic ways of decreasing airway inflammation, such as a small change in hay preparation, is important because an estimated 80% of Thoroughbred racehorses have mild EA and are not living up to their potential.
Dr. Stacey Oke is a seasoned freelance writer, veterinarian, and life-long horse lover. When not researching ways for horses to live longer, healthier lives as athletes and human companions, she practices small animal medicine in New York. A busy mom of three, Stacey also finds time for running, hiking, tap dancing, and dog agility training.
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