Researchers who examined lung tissue from 95 racehorses which had actively raced or trained before their deaths found that most had inflammatory airway disease (IAD).
The horses were a mix of Thoroughbreds, Standardbreds and Quarter horses.
The research at Canada’s Ontario Veterinary College, part of the University of Guelph, shows the disease is much more common than previously thought.
“We looked microscopically at the lung tissue of horses that died during or just after races, and quantified the inflammatory cells within their airways,” explained Professor Luis Arroyo, who is in the college’s Department of Clinical Studies.
“We expected to find that the majority of the animals would have normal airways, with only a small number actually affected with the disease, but that was not the case.”
Graduate student investigator Federika ter Woort, under the supervision of professor emeritus Laurent Viel, collaborated with pathobiology professor Jeff Caswell and Arroyo in the discovery that most of the horses had some degree of IAD, with mild to severe airway changes.
The researchers said inflammatory cell infiltration in peribronchiolar tissues was found in 86% of the horses, smooth muscle hyperplasia (enlargement) in 98% of cases, and hemosiderin, arising from the breakdown of hemoglobin, was found in 80% of the horses.
Previous research suggested IAD occurred in up to half of equine athletes.
“The disease was known to be common in racehorses, but not as widespread as this study reveals,” Caswell said.
“The findings suggest that IAD does not result from unique exposure of an affected horse to the stimulus that causes the disease. But rather the research suggests that all racehorses may be exposed, with inflammation of the airways experienced by many.”
The study, the findings of which have been reported in the American Journal of Veterinary Research, examined lung tissue from 95 deceased racehorses that had actively raced or trained before their deaths.
This was the first study to assess inflammation on a tissue level and the first to discover airway inflammation in horses not specifically selected for poor performance.
“None of the deceased horses showed obvious signs of airway inflammation in their final three races,” said Arroyo. “The research shows that inflammation is always prevalent in racehorses, even those that may or may not have respiratory signs.”
Unlike equine asthma in older horses, IAD causes no observable symptoms at rest but only during exercise. It most readily shows itself in poor race times, Caswell said.
Possible causes of IAD include recurrent pulmonary stress, deep inhalation of dust, atmospheric pollutants and persistent respiratory viral infections. Young horses have higher risk of exposure to these factors because of frequent transport, intense exercise and time spent in stables.
Little is known about how IAD changes an affected horse’s lungs, Arroyo said.
“At this stage, the findings are mainly relevant to understanding the nature of the disease and how it develops. Until now, there was no knowledge about a potential correlation between the classification of the inflammatory cells in the airways and the lung tissues.”
Ontario racing authorities require a mandatory autopsy when a horse dies in or soon after a race. That meant experts knew a lot about what causes racehorses to die. However, since IAD is not fatal, it has not been closely examined until now, Arroyo said.
“This project gives important information regarding the health status of the performing horse,” he said. “Developing a better understanding of IAD could lead to better health in horses and a more competitive horse racing industry.”
Fe ter Woort, Jeff L. Caswell, Luis G. Arroyo, Laurent Viel. Histologic investigation of airway inflammation in postmortem lung samples from racehorses. American Journal of Veterinary Research, 2018; 79 (3): 342 DOI: 10.2460/ajvr.79.3.342