Equine Pulmonary Disease

Pinch or put a kink in a hose, and water doesn’t flow freely. The same can be said for a horse’s airways, which supply his body with the oxygen it needs for energy production. And while other body systems generally adapt well to exercise, the respiratory system is less capable of doing so, making it an important one for veterinarians to understand and better treat.

“Horses need 100% respiratory capacity to perform at their best,” said Laurent Couëtil, DVM, PhD, Dipl. ACVIM, Couëtil, professor and section head of Large Animal Internal Medicine at Purdue University’s College of Veterinary Medicine, in West Lafayette, Indiana. Couëtil, who is also director of Purdue’s equine research programs and the equine sports medicine center, reviewed with veterinarians how pulmonary disease can impact performance at the 2017 American Association of Equine Practitioners Convention, held Nov. 17-21 in San Antonio, Texas.

Energy required for muscle contraction is produced in two major ways, said Couëtil—via anaerobic or aerobic metabolism.

Anaerobic metabolism requires stored substances such as glycogen or glucose and, while it produces bursts of energy, it’s not sustainable. It also produces lactic acid, which results in muscle fatigue and soreness. Aerobic metabolism requires oxygen; while it’s produced at a slower pace, it lasts longer. In most cases, he said, horses use aerobic energy rather than anaerobic energy and, so, must be able to consume oxygen optimally.

Fortunately, he said, horses’ large hearts and lungs are geared toward taking in and utilizing oxygen for energy production. Yet, the respiratory system is “the limiting factor for performance,” Couëtil said.

“Even a mild degree of airway obstruction or pulmonary disease may further compromise oxygen delivery to tissues and potentially lead to decreased performance in horses exercising at or above VO2max (maximum oxygen consumption rate), such as racehorses,” he added.

Veterinarians can assess the importance of these conditions disease using a performance evaluation. Objective measures, such as racing time, speed, and earnings, and subjective measures, including rider or trainer’s opinion and clinical signs, can help them evaluate performance. They can also use physiologic means, such as heart rate, blood lactate, or VO2max, to assess performance, but these measures are more easily collected in a clinic than in the field.

Pulmonary diseases that can reduce performance include:

  • Mild equine asthma (inflammatory airway disease) One effect of mild asthma is increased mucous accumulation in the airways. Researchers have found that mucous scores of 2 and higher and 3 and higher are associated with decreased performance in racehorses and sport horses, respectively.
  • Severe equine asthma (recurrent airway obstruction, or heaves) Studies have shown that horses with severe asthma fatigued sooner than unaffected horses. They exhibited signs of decreased respiratory capacity. Researchers learned that feeding affected horses a low-dust diet (such as a complete feed) and adding omega-3 polyunsaturated fatty acids (PUFAs) can help improve respiratory function.
  • Exercise-induced pulmonary hemorrhage (EIPH) Scientists have long studied EIPH’s impact on racehorses. Important findings include:
    • As EIPH score increases, so does the affected horse’s distance behind the winner;
    • Horses are less likely to win races as their EIPH scores increase; and
    • Grade 4 EIPH is most likely to impact finishing position compared to other grades.

Take-Home Message

Understanding how pulmonary disease impacts performance can help veterinarians diagnose and treat issues that could be impairing horses’ athleticism.