Mild asthma common among US racehorses

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Justify led from wire to wire to win the Belmont Stakes at the weekend. © Elsa Lorieul
US racehorse Justify winning the Belmont Stakes earlier this year. [File image] © Elsa Lorieul
Mild asthma is common among Thoroughbred racehorses, the findings of a study suggest, with levels of fine airborne particles that penetrate deep in the lungs having a measurable effect on performance.

“Equine asthma appears to be highly prevalent in racing Thoroughbreds,” concluded Kathleen Ivester, Laurent Couëtil and George Moore, writing in the Journal of Veterinary Internal Medicine.

The researchers from the Indiana-based Purdue College of Veterinary Medicine based their findings on a study involving 64 individual horses from eight stables, across a total of 98 race performances.

Mild to moderate equine asthma, also known as inflammatory airway disease, is commonly observed in poorly performing horses, the authors noted.

Because affected horses appear clinically normal at rest except for occasional coughing, diagnosis requires advanced techniques.

Their investigations included 79 dust exposure assessments, an analysis of the race performance of the horses involved, and an investigation of the contents of bronchoalveolar lavage fluid – that’s fluid flushed through their lungs.

On the day of racing, each enrolled horse underwent a physical examination, endoscopy of the respiratory tract, and was given bronchoalveolar lavage around an hour after racing, in which they were sedated and 250ml of sterile saline was flushed through the lungs.

Respirable and inhalable dust, respirable endotoxin, and respirable β‐glucan exposures were measured in their stalls at the breathing zone over the course of 4 to 6 hours on a single occasion within a week after racing.

The researchers found evidence of mild equine asthma in 80% of bronchoalveolar lavage fluid samples – that’s 78 of the 98 post-race samples collected. These samples came from 52 of the 64 horses involved in the study.

For each percent increase in mast cell and neutrophil proportions in the bronchoalveolar lavage fluid samples, speed figures were reduced by 2.9 and 1.4 points, respectively.

Neutrophils are a type of white blood cell which play an important role in the immune system. Mast cells are also a type of white blood cell, releasing histamine and other substances during inflammatory and allergic reactions.

American Pharoah and Victor Espinoza in full flight.
American Pharoah and Victor Espinoza in full flight. [File image] © Mathea Kelley
The respirable dust concentration in the lavage fluid was linked to neutrophil proportions, while mast cell proportions were linked to levels of respirable β‐glucan exposure.

β‐glucans are sugars that are found in the cell walls of bacteria, fungi, yeasts, algae, lichens, and plants, such as oats and barley.

“Mild equine asthma is common in racing horses and negatively impacts performance,” they concluded, adding that the data supported the view that respirable, rather than inhalable, dust exposure measures were pertinent to equine airway health.

Respirable dust is the fraction of inhaled airborne particles that can penetrate beyond the terminal bronchioles into the gas-exchange region of the lungs.

Discussing their findings, the trio said respirable dust and β‐glucan exposures appear to be important determinants in the type and degree of airway inflammation.

In otherwise healthy racing thoroughbreds, mastocytic airway inflammation significantly impairs performance, reducing both indices of speed and the likelihood of winning in a dose‐dependent manner.

Neutrophilic airway inflammation also negatively impacts performance, though to a lesser degree.

“As hypothesized, racing performance was negatively impacted by lower airway inflammation, with mast cells having the greatest apparent effect upon the speed of the horse.

“Similar to the effect on the speed figure, mast cell inflammatory cell proportions were found to influence the likelihood of winning: the likelihood of finishing first dropped 9% with each 1% increase in the mast cell proportion.

“Neutrophil cell proportions did not significantly affect the likelihood of winning, further corroborating the stronger effect of mast cells upon performance.”

“In our study, mean breathing zone respirable dust levels (0.089 mg/m3) were generally comparable to those levels reported in conventional equine management systems when horses were fed dry hay (0.064 mg/m3), and lower than those reported under low‐dust management when horses were fed haylage (0.22 mg/m3).”

The range of measurements was large, they noted – roughly a 100‐fold difference between lowest and highest readings, but consistent with a previous study in thoroughbred racehorses and likely affected by individual horse behavior.

The median respirable dust exposure (0.031 mg/m3) was also similar to that measured in the breathing zone of young Thoroughbreds fed hay from the ground (0.055 mg/m3).

“In that study, feeding hay from a net inside the stall increased respirable dust exposures 4‐fold, but respirable dust exposure did not differ between those fed hay from the ground and those fed from a net in the current study.

“This difference is likely because of the fact that hay nets were hung outside the stall rather than inside the stall, so horses in the current study were unable to bury their muzzles directly in the hay.

“Additionally, hanging the net outside the stall likely resulted in those horses spending more time with their heads outside the stall, though this variable was not measured.”

They continued: “Despite relatively low dust exposure levels, results from this study demonstrated a highly significant effect of respirable dust exposure upon bronchoalveolar lavage fluid neutrophil proportions.”

Management between trainers was similar in the study, they noted,, with all horses fed dry hay, bedded on sawdust, and fed the same textured sweet feed.

An observational study of environmental exposures, airway cytology, and performance in racing thoroughbreds. Kathleen M. Ivester Laurent L. Couëtil George E. Moore. 17 September 2018, Journal of Veterinary Internal Medicine. 

The study, published under a Creative Commons License, http://creativecommons.org/licenses/by-nc/4.0/ can be read here

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