Acepromazine for Tranquilizing Male Horses: Pros and Cons

Acepromazine, often called simply Ace, is commonly used to tranquilize horses for veterinary procedures. However, its use in male horses can cause penile prolapse, or an inability to retract the penis back into the sheath. This effect is desired in some instances, such as when acepromazine helps the horse "let down" for sheath cleaning.

In rare cases, however, this prolapse can be permanent, requiring amputation of part or all of the penis due to persistent swelling, hematomas, and/or injury.

At the 2009 American Association of Equine Practitioners convention, held Dec. 5-9 in Las Vegas, Nev., two veterinarians presented evidence on the pros and cons of using acepromazine in stallions and geldings. Nora Matthews, DVM, Dipl. ACVA, professor of anesthesia at Texas A&M University, discussed the benefits of using acepromazine, while Ann Wagner, DVM, MS, Dipl. ACVA, ACVP, professor of veterinary clinical sciences at Colorado State University, presented the downsides.

"In Dr. Hubbell's survey, about two-thirds of the veterinarians surveyed said they use acepromazine on male horses," noted Matthews. She added that a survey of American College of Veterinary Anesthesiologists members found that the majority use acepromazine in male horses, and 5% recalled at least one case with transient penile prolapse lasting more than 12 hours.

Reasons to Use Ace

"Acepromazine is probably the longest-acting tranquilizer available ... making it very useful for shipping horses by plane or long distances," began Matthews. She listed several additional benefits of acepromazine as follows:

  • It causes less ataxia (incoordination) than some sedatives, making it useful for procedures in which it's important that the horse stay still (such as clipping, shoeing, and diagnostic imaging).
  • It causes less gastrointestinal stasis than some other tranquilizers, meaning there is less risk of the sedation contributing to colic in a susceptible horse.
  • Acepromazine has been found to help protect against death associated with general anesthesia.
  • It might help protect against hypoxemia (deficient oxygenation of blood) when used to "premedicate" before general anesthesia.
  • Some studies have suggested that acepromazine might have value in the treatment of laminitis and myopathy.
  • Cases of permanent penile prolapse, also called paraphimosis, have been rare in the literature.

"Although this author cannot deny that acepromazine can have adverse consequences, the adverse effects seem to be dose-dependent," Matthews noted (meaning that effects will be more severe at higher doses). "The dose of acepromazine administered should be kept to a minimum. Potential advantages of its use may outweigh the small likelihood of adverse effects in most male horses."

Reasons Not to Use Ace

"In rare cases, penile prolapse can result in permanent paraphimosis, particularly if the penis is allowed to become edematous or congested or develops a hematoma," said Wagner. "This condition can lead to serious and permanent consequences, particularly in breeding stallions. Immediate treatment is imperative.

"I have had some negative experiences with acepromazine in male horses," she reported. "I have seen two cases in intact stallions. Both had received about 15mg of acepromazine intravenously before elective surgeries. Both got paraphimosis at sedation that continued for a day or so." One recovered with supportive treatment (protecting the penis from injury and reducing swelling). The other did not and partial amputation was required.

Geldings aren't immune; Wagner recalled a gelding that also required partial amputation following acepromazine tranquilization for routine tooth floating. "So now I'm also reluctant to use acepromazine in geldings," she commented.

"The bottom line is that the safest option is to avoid ace in males," she recommended. "Low doses are less likely to cause problems, but even 15mg can do it." A 4-mg dose might be acceptable as this dose seems too low to cause significant penile protrusion, she noted.

"Although the incidence of paraphimosis after acepromazine has been estimated to be less than 1 in 10,000, the consequences, particularly for a breeding stallion, can be devastating," Wagner concluded. "There are many other alternative tranquilizers and sedatives."

Matthews concluded: "As with all other drugs, benefits must be weighed against possible side effects for each case."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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