Adverse effects of zilpaterol administration in horses: Three cases

Three healthy horses were fed the beta-adrenergic agonist feed additive zilpaterol at a dosage of 0.17 mg/kg body weight to study zilpaterol elimination kinetics. Soon after ingestion of zilpaterol, the horses developed skeletal muscle tremors and tachycardia. A 75 to 87.5% reduced dose of zilpaterol was fed to the horses 24 hours after the initial dose; administration was discontinued thereafter. The horses exhibited restlessness, muscle tremors, and profuse sweating 20 to 25 minutes after ingestion of zilpaterol. Tachycardia developed within 40 minutes and took up to 2 weeks to resolve. Muscle tremors lasted up to 1 week. The most pronounced derangements in serum biochemistry were increased activities of lactic dehydrogenase, creatine kinase, and aspartate transferase, indicating muscle damage. The most severely affected horse also had transient azotemia, hematuria, and proteinuria, suggesting renal damage. All three horses recovered without treatment and were clinically normal 2 to 3 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 Food and Drug Administration approval for such use. Oral administration of zilpaterol to horses at the dosage indicated for use in cattle may result in prolonger adverse effects, including tachycardia, muscle tremors, and renal damage.