Why is even a small cough a big problem in a racehorse?

Professor Melissa Mazan, V93, a board-certified internist and pulmonology expert at Tufts Equine Center, explains

Coughing is really common in horses, but it’s not normal. That’s true for all horses, but racehorses demand a different level of respiratory excellence than other horses. If you look at the way racehorses use oxygen and the demands they put on their body, they are living on a cliff edge—it’s phenomenal that they can even survive a race.

Racehorses move an incredible amount of blood and air every minute thanks in part to their enormous hearts (which can pump a stunning amount of blood to their muscles), a large number of capillaries in those muscles (allowing them to receive more blood), and more mitochondria (the parts of cells that use oxygen to produce energy) than most other species. Racehorses also have very big lungs—about twice the size of a Holstein cow’s, which typically weighs a little more than a thoroughbred racehorse.

If you were to do a stress test to size up a human athlete’s ability to perform aerobic exercise, you’d measure their VO2—or the uptake of oxygen from the bloodstream. Well, an elite racehorse will have a VO2 that’s more than twice that of the best human athletes in the world. (Gazelles, for the record, have an even higher VO2 uptake than racehorses.)

However, this enormous VO2 means racehorses can see up to a 20 percent dip in their blood-oxygen levels while galloping at race speed. (Even elite human athletes, by stark comparison, only see their blood-oxygen levels dip by 2 to 5 percent while exercising to the maximum of their ability.) A racehorse’s blood-oxygen lows are almost pathologic—if Michael Phelps’ level dipped to those levels, he’d be put on a stretcher and wheeled down to the ICU. A mammal shouldn’t be able to suck that much oxygen out of its blood. But racehorses do.

So racehorses have absolutely no room for error when it comes their respiratory system. It has to be perfect. Yet you can’t train the respiratory system to become stronger the way you can with muscles—like the heart or the legs—or bones.

Anything that makes a racehorse cough is going to impede its performance in a big way, and increase its likelihood of getting hurt, because if you can’t bring enough oxygen to the rest of the body, it will be weaker and more prone to injury.

The most common reason for cough in horses is equine asthma, which—just like human asthma—is caused by inflammation in the lungs. Even the best and most pristine barn is full of hay, animal dander, manure, wood shavings—meaning there’s infinite production of particulate matter that can cause asthma, with coughing from the resulting mucous and easily irritated, narrowed airways.

Horses also are obligate nose breathers, which means they can only breathe through their noses, not their mouths. Having separate routes for breathing and eating is evolutionarily a good thing for, say, a wild horse that might suddenly have to go from 0 to 40 mph to escape from predators without accidentally sucking food into its lower airways.

But this unusual anatomical arrangement provides an opportunity for a whole host of other things to go wrong. And coughs can result from even minor anatomical abnormalities, such as a displaced soft palate or an entrapped epiglottis—the condition that 2019 Kentucky Derby favorite Omaha Beach had and why he ended up being scratched from that race.

Racehorses are phenomenal athletes. They make us humans look like we’re not even trying. But they pay a price. To function on the constant verge of oxygen deprivation, they physiologically cannot afford even the tiniest reduction in their respiratory abilities when they’re performing at the very peak of their ability. The difference between winning and being at the back of the pack can be less than a hundredth of a second—and that hundredth of a second can be determined by a horse’s cough. 

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