Contraceptive Vaccine Use

Q. A few years ago horse owners heard about PZP being used as a contraceptive on mustang mares, which I think is good. But every now and then I ask my veterinarian about options for my mares that I don’t want to get pregnant, and there don’t seem to be many options. Can you let me know whether general horse owners are using PZP?

Michelle Myers, via e-mail

A. Porcine zona pellucida (PZP) is being used as a form of contraception in wild horse populations. The PZP vaccine is derived from pig eggs and is injected via a small intramuscular dart into targeted female horses. The vaccine stimulates an immune response that distorts the outer membrane of the mare’s eggs. This blocks fertilization and prevents pregnancy in approximately 90% of mares. Successfully vaccinated mares continue to cycle but are unable to conceive and carry a pregnancy for up to a year. Veterinarians can administer an -annual booster vaccine to extend PZP’s contraceptive effect. The contraceptive vaccine is being utilized in such species as feral horses, deer, elk, llamas, bear, and elephants for reversible population control.

The PZP vaccine is not commercially available nor is it FDA-approved for public use. While it’s an invaluable tool for controlling wild horse populations, I can’t see a use for it in the public horse market because while it prevents pregnancy, it doesn’t prevent cycling and estrous behavior. The easiest methods of pregnancy prevention in privately owned horses are castration of stallions and segregation of mares from stallions. Alternative sources of pregnancy prevention and behavioral modification, such as synthetic progestin (altrenogest, marketed as Regu-Mate or Altresyn) administration or ovariectomies (surgical removal of the ovaries), involve inhibiting a mare from cycling and ovulating.

About the Author

Jeffrey Cook, DVM

Jeffrey Cook, DVM, is an ambulatory veterinarian with Rood and Riddle Equine Hospital in Lexington, Ky.

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