Complications with the use of carfentanil citrate and xylazine hydrochloride to immobilize domestic horses.

Carfentanil citrate, the only opioid approved in the United States for immobilizing large exotic animals, increasingly has been used to chemically restrain exotic horses, such as Prezwalski's horses (Equus przewalskii) and wild horses (E caballus). Because carfentanil's duration of action is long and renarcotization may develop 2 to 24 hours after administration of antagonists, a study was designed to compare the physiologic effects of opioid antagonists, using domestic horses chemically restrained with xylazine hydrochloride and carfentanil. The study was terminated after the initial 3 horses developed severe tachycardia and hypertension, which resulted in the death of 1 horse from pulmonary edema. Although it was possible that the clinical findings in these horses may have resulted from use of an inadequate dosage of carfentanil or xylazine, or both, analysis of the results more likely indicated that domestic and exotic horses may respond differently to carfentanil, and domestic horses may not be a good model for use in studies of carfentanil.