IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT The Case Against the Use of Acepromazine in Male Horses Ann E. Wagner, DVM, MS, Diplomate ACVA, ACVP Author’s address: Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523; e-mail:
[email protected]. © 2009 AAEP. 1. Introduction or priapism on a subsequent administration of Acepromazine is known to cause penile prolapse in acepromazine. The exact mechanism for this com- male horses, which is why it is frequently adminis- plication is unknown, although it has been specu- tered to facilitate cleaning the sheath. A study of lated that ␣-adrenergic nerve fibers supply motor acepromazine in geldings showed that the degree innervation to the retractor penis muscle, and there- and duration of penile protrusion is dose dependent, fore, the ␣-adrenergic blockade induced by 5 with 0.01 mg/kg, IV (ϳ5 mg in a 500-kg horse) caus- acepromazine blocks this motor function. ing 40% protrusion of the penis for Ͻ30 min, 0.04 Treatment is supportive: protecting the penis from 4,5 mg/kg, IV (ϳ20 mg in a 500-kg horse) being associ- injury and preventing or minimizing swelling. ated with 80% protrusion of the penis for ϳ1h,and There is one report of successful treatment by ad- 0.1 mg/kg, IV (ϳ50 mg for a 500-kg horse) being ministration of the anticholinergic agent benztro- associated with 90% protrusion for nearly 4 h.1 pine to a gelding with priapism in which only the Other phenothiazines such as chlorpromazine and corpus