Even Mild Cases of Asthma Can Slow Down Elite Racehorses

Horse races often come down to neck-and-neck sprints to the finish line, as was the case in the 1933 Kentucky Derby when two horses, Head Play and Brokers Tip, were in a dead heat in the final sprint. The run-off was so close that the two jockeys began throwing fists while still in their saddles. The fight was captured in a head-on photo by a Courier Journal photographer, however the race predated photo-finish cameras. Brokers Tip was declared the champion “by a nose,” but with no hard evidence. Head Play’s jockey Herb Fischer spent the rest of his days clinging to the race, believing his horse was the true winner of the 59th Kentucky Derby. These equine athletes need any edge they can get, however, foul play isn’t tolerated—both jockeys received 30 day suspensions for rough riding.

In modern horseracing, such as the 145th running of the Kentucky Derby on May 4, 2019, slight physical limitations in the animals might make or break a race. But diagnosing the causes of poor performance isn’t always an easy task.

However, as a team at Purdue University in Indiana found, if a seemingly healthy horse is off its game with no other visible symptoms, mild asthma may be to blame. “The likelihood of finishing first dropped nine percent with each one percent increase” in inflamed cells associated with asthma in the lungs, according to a paper the researchers published last fall in the Journal of Veterinary Internal Medicine.

“It’s a pretty silent disease, except that the horses aren’t performing well,” says study coauthor Laurent Couëtil, a veterinary scientist at Purdue.

The Most Sensational Racing Picture ... Ever! (1933)

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Even in healthy racehorses, a major limiting factor to peak performance is the animal’s lungs. Whereas a horse’s muscular system, including its heart, can be built up over time, the respiratory system cannot be strengthened through training.

“Horses evolved a large and powerful heart. They have all the ingredients to be great athletes. All they need is to be able to use oxygen from the air,” Couëtil says. A 1,000-pound horse, for example, has a heart and lungs about twice the size of a 1,000-pound cow, which is perhaps one reason we don’t race bovines.

Several factors make a horse’s breathing more complicated than other organisms’ even without the presence of asthma. For one, horses only breathe through their noses. A horse’s breathing is also controlled by respiratory-locomotor coupling, meaning a racehorse always inhales as they stride ahead and exhales when they pick their feet back up. But make no mistake: horse lungs are tough.

Lung capacity increases as a horse picks up its pace. When they reach full speed, horses will take two breaths per second, inhaling 16 liters of air per breath, Couëtil says. In other words, by the end of a five-furlong (1,110-yard) race, a horse would fill six bathtubs, or 1,800 liters, with air, as David Marlin, an equine respiratory specialist, explains for The Horse magazine.

Equine asthma is a relatively new term to describe cases of chronic airway inflammation typically caused by allergens, like dust, mold and mildew. In the past, most research has focused on severe cases with symptoms including coughing, nasal discharge and swelling. However, nearly 40 percent of horses in the study that ended up having internal signs of mild asthma didn’t cough or have other visible symptoms.

“[Mild equine asthma] was not as well studied because it’s more difficult to detect. It’s subtle. They may not show clinical signs except for not performing well,” Couëtil says.

The team studied a total 68 individual thoroughbred racing horses at Indiana Grand race course. They compared each horse’s race scores—adjusted for distance and track conditions with a program called Equibase—to the results of a dust exposure assessment. To measure dust exposure, the horses received what’s called a lung wash after each race. While considered a serious procedure in humans, a lung wash is rather non-invasive in horses. While the horse is sedated, its lungs are flushed with fluid, which is then suctioned out and tested for inflammation in two types of cells that would indicate asthmatic symptoms.

“In horses, it’s minimally invasive and the horses tolerate it well. We can do it in the barn,” says the study’s lead author Katy Ivester, also a Purdue veterinary scientist. “We can do the test and it does not impair their success as racehorses.”

The research team found evidence of mild asthma in 80 percent of the horses they tested. In addition, for every percent increase in inflamed cells, they found the horses’ Equibase speed score dropped by 1.5 to 2.9 points.

Digging deeper, the team confirmed a link between one type of inflamed cell they tested, mast cells, and a certain kind of dust called respirable dust that is so small that horses can breathe it deep into their lungs. Respirable dust is smaller than one-tenth the width of a human hair. (In comparison, slightly larger particles called inhalable dust might be seen when shoveling hay, but this dust usually gets filtered out in a horse’s nasal passage; these particles can still cause irritation in the second type of cell tested, neutrophil, but the team found it to be less damaging.)

Because the dust that hinders the horses’ mighty bellows is so small and unseen, Ivester calls it an “invisible problem” that could have impacts beyond racehorses.

“This isn’t just a problem that racehorses have,” Ivester says. “Any horses fed dry hay or really any horses kept in stalls, we would see a high rate of mild asthma as well. It’s an invisible problem that affects any horses needed to work hard.”

If mild cases of asthma turn severe, horse owners can face steep medical bills. There’s no FDA-approved asthma medication specifically for horses, and while it’s possible to treat horses using human medication, the treatment can get expensive fast. A horse’s lung capacity is about ten times larger than our own, Couëtil says, so their dosage would also be ten times higher—and therefore, ten times pricier.

Luckily, the team also identified several inexpensive prevention measures for horse owners. In a previous study using breathing sensors placed on the horses’ harnesses, Ivester found that high levels of respirable dust enter a horse’s lungs when their hay is hung in nets at eye level in their stall, rather than fed to them from the ground like they would eat in pasture, which is ideal.

“Unless there is a medical reason, feed horses from the ground. The exposure from the ground was same level at pasture. Elevating the hay leads to four times higher dust exposure,” Ivester says, adding that alternative options include steamed hay or haylage, which is specially treated and wrapped for preservation.

The team isn’t just thinking about horses, however. They emphasize that it’s important to study asthma in horses because people who care for horses also have higher instances of asthma. Horses may be a prime animal model in asthma research because the disease occurs naturally in the species, whereas lab mice have to have asthma induced.

Couëtil says that it’s rare to see a new breakthrough treatment for asthma in humans and that many options fail to make it to clinical trials, meaning research dollars are lost in the process.

“We are hoping maybe the horse can contribute and help triage what can be used in human trials,” he says. “You can’t afford to have so many drugs fail the approval process. But if you can use horses, which can develop asthma naturally, it can benefit both horse and human research.”

Rachael Lallensack | READ MORE

Rachael Lallensack is the former assistant web editor for science and innovation at Smithsonian.