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 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.

Another commonly used beta-agonist in horses is albuterol, which has an onset time of about five minutes, but only provides positive effects for about 30-60 minutes, she said. It is administered via a nebulizer or with a metered dose inhaler sprayed into a specialized equine breathing mask. Nolen-Walston said that pretreatment with vitamin C can increase sensitivity to albuterol three- to tenfold and decreases tachyphylaxis. However, she pointed out that oral-administered albuterol achieves minimal bioavailability (the rate at which a drug is absorbed or becomes available at the site of physiologic activity after administration).

Clenbuterol also is a useful medication for horses experiencing recurrent airway obstruction (RAO, or heaves) and has 84% absorption when administered orally. Beta-agonist medications can have a beneficial effect in healthy horses by increasing the frequency of the beat of the cilia (hairlike projections that aid in clearing debris from the airways) within the trachea and upper airways. Researchers have observed the rate of mucociliary clearance, which is responsible for cleaning debris and particulates from the upper airways, to improve up to 25% in healthy horses.

Always consult with a veterinarian prior to administering any medication to a horse, especially a healthy one. As Nolen-Walston pointed out, although a particular medication might seem like a surefire way to give a horse an advantage, careful consideration is needed to determine if the horse will really benefit from the drug.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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