Submitted to: Journal of Equine Veterinary Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/30/2007
Publication Date: 1/1/2008
Citation: Wagner, S.A., Mostrum, M.S., Hammer, C.J., Thorson, J.S., Smith, D.J. 2008. Adverse effects of zilpaterol administration in horses: three cases. Journal of Equine Veterinary Science 28(4):238-243. Interpretive Summary: Zilpaterol is a newly developed feed additive that is used in finishing cattle to increase the relative proportion of carcass muscle and to decrease the relative proportion of carcass fat at slaughter. Because of this “anabolic” (muscle growth) effect, zilpaterol has the potential to be used as a doping agent within the horse racing industry. Three horses were used in a study to determine the length of time that zilpaterol could be detected in the urine after oral zilpaterol administration. Shortly after a single zilpaterol dose, each horse began to sweat profusely; in addition horses began to have muscle tremors and heart rates increased dramatically. Analysis of blood revealed that indicators of muscle damage were elevated for as long as two weeks in some horses. Although the side-effects of zilpaterol were not unexpected, their duration and severity greater than expected. Each horse recovered on its own without medical intervention. Veterinary practitioners should be aware of the side-effects of zilpaterol and other beta-agonists as they evaluate performance horses for fitness.
Technical Abstract: Three healthy horses were fed 0.17 mg/kg body weight of the beta-adrenergic agonist zilpaterol to determine zilpaterol elimination kinetics. Shortly after treatment, each horse developed skeletal muscle tremors, tachycardia, and serological abnormalities lasting several days. A 75% to 87.5% reduced dose of zilpaterol was fed to the horses 24 hours after the initial dose; treatment was discontinued thereafter. Horses exhibited nervous behavior, muscle tremors, and profuse sweating 20 to 25 minutes after consuming zilpaterol. Tachycardia developed within 40 minutes and took up to two weeks to resolve. Muscle tremors lasted up to one week. The most pronounced serological derangements were increased lactic dehydrogenase, creatine kinase, and aspartate transferase, indicating muscle damage. The most severely affected horse also had transient azotemia, hematuria and proteinuria, suggesting hemolysis and transient renal damage. All three horses recovered without treatment and were clinically normal two weeks after the initial dose of zilpaterol. Because of their anabolic properties, beta-adrenergic feed additives are considered a risk for abuse in performance horses, despite the absence of FDA approval for such use. Oral administration of zilpaterol to horses at the dosage indicated for use in cattle may result in prolonged adverse effects including tachycardia, muscle tremors, and the risk of hemolysis and renal damage. These effects appeared to be more severe in the female horses.