Beta-Agonist Drugs: Effect on Respiratory Function in Horses

Horses in intense exercise, such as racing or three-day eventing, need full lung capacity to perform to the to
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Horses in intense exercise, such as racing or three-day eventing, need full lung capacity to perform to the top of their abilities. Hoping to give their horse every advantage, some racehorse trainers medicate with beta-agonist drugs, like clenbuterol (a drug commonly used to treat chronic obstructive pulmonary disease), in an attempt to improve airway function. But is such medication actually effective in horses? At the 2011 America College of Veterinary Internal Medicine Forum, held June 15-18 in Denver, Colo., Rose Nolen-Walston, DVM, Dipl. ACVIM, assistant professor in the department of clinical sciences at the University of Pennsylvania’s College of Veterinary Medicine, discussed the effects of beta-agonist drugs on the equine lung.

Nolen-Walston explained that what these medications can do is often misunderstood and, therefore, the drugs are often misused. Beta-agonist drugs are designed to inhibit bronchospasm (constriction) of the airways in horses affected by airway inflammation, she noted, and healthy horses typically don’t achieve dilation of the small airways in response to treatment with clenbuterol.

Clenbuterol is the only beta-agonist drug currently approved for use in horses and is primarily used to treat breathing problems. Clenbuterol also has anaboliclike effects that shift a horse’s body mass from fat to muscle; however, Nolen-Walston noted that its use has not been scientifically associated with any increase in performance. In fact, she added, its ergonomic effects could negatively affect performance and include:

  • Decreased aerobic capacity;
  • Decreased time to fatigue;
  • Decreased cardiac function; and
  • Decreased maximal oxygen intake.

"One potential issue with using beta-agonist medications is the development of tachyphylaxis, which is the tolerance or desensitization to effects of a drug after chronic administration," she cautioned, referencing one study that revealed that by Day 21 of clenbuterol administration, some horses’ airways had become hyper-reactive not just due to loss of effect but also from worsening of pulmonary function. The same study revealed that a peak effect is generally seen by Day 14, so Nolen-Walston recommended limiting its use to two weeks

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