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

The study sought to compare the physiological effects of opioid antagonists on domestic horses sedated with xylazine hydrochloride and carfentanil, an opioid used for immobilizing large exotic animals. However, complications arose – severe tachycardia and hypertension – that resulted in the death of one horse and consequently brought the study to an early close.

Objective of the Study

• The research aimed to examine the effects of opioid antagonists on domestic horses that had been chemically restrained using xylazine hydrochloride and carfentanil citrate.

Use of Carfentanil Citrate

• Carfentanil citrate is currently the single approved opioid for immobilizing large exotic animals, such as wild horses and Prezwalski’s horses, in the United States.
• The application of this drug has been escalating due to its long duration of action and the potential development of renarcotization, which usually occurs 2 to 24 hours after administration of antagonists.

Problems with the Research

• The study had to be halted prematurely when the first three horses used for the experiment developed severe tachycardia (a heart rate that exceeds the normal resting rate) and hypertension (high blood pressure).
• One of these horses died due to pulmonary edema, a condition characterized by excess fluid in the lungs that can lead to respiratory failure.

Potential Causes of the Incident

• Although it was considered a possibility that the severe reactions could have resulted from an insufficient dosage of carfentanil or xylazine, it is more plausible that the results were indicative of varied responses to carfentanil between domestic and exotic horses.
• These results suggested that domestic horses may not be the best candidates for research studies involving carfentanil.