Breathing properly is a crucial task for a sport horse. If he can’t breathe well, he isn’t going to be successful. All horses working at 60% of their maximum capacity experience hypoxemia—low levels of oxygen in the blood—but those with respiratory disease become even more hypoxemic.

“It’s the way they are built,” says Emmanuelle van Erck-Westergren, DVM, PhD, Dipl. ECEIM, ECVSMR, owner of Equine Sports Medicine Practice, in Waterloo, Belgium. She spoke about respiratory challenges in sport horses at the 2025 IGNITE Sports Science for the Olympic Disciplines Seminar, held Jan. 12-13, in Ocala, Florida.

How Horses Breathe

The horse’s respiratory system differs from those of many other animals. They can only breathe through their nose, so it’s more difficult for them to shift air in and out of their lungs than it is for humans, who can switch to mouth breathing when necessary. They also have a small larynx, and moving a column of air through it consumes energy.

Racing trotters or pacers working at speed take large breaths every few strides, but horses cantering or galloping can’t. They must breathe in and out at the same rate as their strides. This is due to respiratory-locomotor coupling, the synchronized relationship between breathing and stride, where one breath is taken with each stride at a canter and gallop.

How Do Respiratory Issues Affect Sport Horse Performance?

“Horses are massively muscular,” said van Erck-Westergren, making them built to be oxygen deficient. When you lack oxygen, you get tired, she said. The resulting muscle fatigue leads to a decline in performance.

It takes a lot of energy to move oxygen as the horse’s speed increases. The negative pressures applied to the throat can become considerable when the horse canters because no bony structures exist to keep the throat open. The larynx, located at the back of the throat, essentially serves as the dividing mark between the upper and lower respiratory tracts. When the horse is breathing, but not swallowing, there is an open channel into the trachea. After passing through the trachea, inspired air arrives at the bronchial trees within the lungs.

In horses with partial paralysis of the larynx, the more the horse needs air, the more the larynx closes, van Erck-Westergren explained.

Signs of respiratory conditions are not always obvious. There might not be loud breathing noises, cough, or nasal discharge. The only signs could be premature fatigue during exertion or defensive behavior such as exercise intolerance or a reluctance to work. The environment and competitions can challenge horses, making diagnosis tricky, she said. Veterinarians commonly use controlled exercise testing to investigate a problem, observing how a horse recovers. If they suspect a respiratory condition, veterinarians can conduct further diagnostics, including bloodwork and an endoscopy while the horse is at rest or exercising.

Take-Home Message

The horse’s respiratory system naturally operates near its limits, so even a mild breathing problem in a sport horse often results in poor performance. Proper diagnosis and treatment are required for the horse to be able to return to peak form.