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 cavernosum penis, not the glans penis, was propriopromazine have also been reported to cause engorged.6 In severe cases where the penis re- penile paralysis.2 Unfortunately, in rare cases, pe- mains prolapsed, amputation may be required.3 nile prolapse can result in permanent paraphimosis, In past years, the author administered relatively particularly if the penis is allowed to become edem- low doses of acepromazine (0.03 mg/kg, IV) as a atous or congested or develops a hematoma.3 This pre-anesthetic to numerous male horses (colts, stal- condition can lead to serious and permanent conse- lions, and geldings) without complication. How- quences, most particularly in breeding stallions. ever, two intact males developed paraphimosis. It has been speculated that the presence of circulat- One of the cases resolved with symptomatic treat- ing testosterone at the time of acepromazine admin- ment, but the other, a cryptorchid presented for istration might significantly increase the risk of castration, did not respond to medical therapy and priapism.4 Even stallions that have been previ- eventually required a partial penile amputation. ously tranquilized with acepromazine without prob- This unfortunate outcome suggested use of lem may develop either flaccid prolapse of the penis acepromazine as a pre-anesthetic is contraindicated NOTES 20 2009 ր Vol. 55 ր AAEP PROCEEDINGS IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT in intact colts and stallions but perhaps not contra- kg) in both stallions and geldings, this author would indicated in geldings. not recommend acepromazine (or other phenothia- zines) in intact male horses. Acepromazine may be 2. Case Report administered to geldings before anesthesia recovery, In September 2008, a 12-yr-old, 500-kg Quarter but never exceeding 4 mg (ϳ0.008 mg/kg), because horse gelding was referred to Colorado State Uni- this dose seems to be too low to cause penile protru- versity’s (CSU) Veterinary Teaching Hospital for pe- sion.1 Although the incidence of paraphimosis af- nile paralysis. The history stated that, 2 wk ter acepromazine has been estimated to be Ͻ1in earlier, the referring veterinarian had administered 10,000,7 the consequences, particularly for a breed- acepromazine (15 mg or 0.03 mg/kg, IV) to float the ing stallion, can be devastating. There are many teeth. The day after sedation, the horse’s penis was other alternative tranquilizers and sedatives, such dropped (it was unclear whether it had been pro- as ␣-2 agonists, with or without butorphanol, that lapsed continuously since the procedure or not). can be used to sedate male horses. Two days after sedation, the veterinarian was called back, and treated the horse for paraphimosis with dimethylsulfoxide (DMSO), flunixin meglumine, References isoxsuprine, and trimethoprim sulfadiazine. The 1. Ballard S, Shults T, Kownacki AA, et al. The pharmacoki- corpus cavernosum was drained and flushed, and netics, pharmacological responses and behavioral effects of the penis was wrapped for 2 h/day. After 2 wk of acepromazine in the horse. J Vet Pharmacol Ther 1982;5: 21–31. treatment, the horse still could not retract his penis, 2. Jones RS. Penile paralysis in stallions (letter). JAmVet although he was able to urinate. At that time he Med Assoc 1966;149:124. was referred to CSU, and the penis was amputated. 3. Boyer K, Jann HW. Penile hematoma in a stallion resulting in proximal penile amputation. Equine Pract 1995;17:8–11. 3. Conclusion 4. Gerring EL. Priapism after ACP in the horse (letter). Vet Rec 1981;109:64. In the author’s personal experience, acepromazine 5. Nie GJ, Pope KC. Persistent penile prolapse associated with can be a useful tranquilizer in certain horses and acute blood loss and acepromazine maleate administration in can be safely administered in very small doses (3 or a horse. J Am Vet Med Assoc 1997;211:587–589. 4 mg total) to females or castrated males near the 6. Sharrock AG. Reversal of drug-induced priapism in a geld- end of inhalation anesthesia, before they are moved ing by medication (letter). Aust Vet J 1982;58:39–40. 7. Muir WW. Anxiolytics, nonopioid sedative-analgesics, and opi- to the recovery stall. However, after observing oid analgesics. In: Muir WW, Hubbell JAE, eds. Equine an- three cases of paraphimosis or persistent penile pro- esthesia, monitoring and emergency therapy, 2nd ed. St. Louis, lapse after acepromazine administration (ϳ0.03 mg/ MO: Saunders Elsevier, 2009;190. AAEP PROCEEDINGS ր Vol. 55 ր 2009 21.