PZP works by affecting the immune system of the mare. Most female mammals have a membrane surrounding her eggs called the zona pellucida. On the zona pellucida there are multiple glycoprotein receptors known as ZP3 receptors. Sperm normally attach themselves to these receptors, initiating fertilization. After a female horse is vaccinated with the porcine zona pellucid vaccine, her immune system is activated and starts producing antibodies from the vaccine. These antibodies then attach themselves to the ZP3 receptors, disrupting the shape of the receptor. Sperm then can no longer attach to the receptors, preventing fertilization. (Paterson and Aitkin, 1990). One advantage of the PZP vaccine is that the disruption occurs to the glycoprotein receptors on the zone pellucid. Therefore, the damages are not permanent because the glycoproteins are digested and reformed in their natural shape, eventually allowing sperm to bind and fertilization to again occur.
The vaccine is usually administered to the mares after they have reached the mature breeding age of two years old. The first year that the mares receive the vaccination, a booster is given within the first year to help establish a population of antibodies produced by the immune system. There is a 90% accuracy rate in mares that were given the booster in the same year compared to 78% accuracy in mares that only received one vaccination. (Turner et al. 2002). Although the vaccine has a high accuracy rate, immunocontraception through the PZP vaccine can take time to provide visible results, and is not a rapid method of population control. The PZP vaccine does slow reproduction, but it still takes times to change the population percentage as animals still have to die from natural causes...
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... JS Adelman 2010. National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 17 Mar. 2014.
• Paterson, M., and R. Aitkin 1990. Development of vaccines targeting the zona pellucida.Curr. Opinions Immunol. 2:743-747
• Powell, D.M. 1999. Preliminary Evaluation of Porcine Zona Pellucida (PZP) Immunocontraception for Behavioral Effects in Feral Horses (Equus caballus). Journal of Applied Animal Welfare Science, 2, 321-335.
• Gorey, Tom, 2014.Wild Horse and Burro Quick Facts. blm.gov
• Turner, J.W., Jr,; Kirkpatrick, J.F. 1982. Steroids, behavior and fertility control in feral stallions in the field. Journal of Reproduction and Fertility(Suppl.32):79-87.
• Turner A, and JF Kirkpatrick 2002. Effects of immunocontraception on population, longevity and body condition in wild mares (Equus caballus). Reproduction (Suppl. 60): 187-195.
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