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Immunological approaches to cat and dog contraception

Standard vaccines for dogs and cats (rabies, distemper, leptospirosis, parvovirus, and feline calicivirus, among others) are designed to stimulate the body's immune system to produce antibodies to a particular virus or bacteria. Researchers have explored vaccines to suppress fertility in dogs and cats for several decades, and the basic concept behind an immunological approach to control fertility is similar to efforts to prevent disease: a vaccine stimulates dogs and cats to produce antibodies to key proteins involved in reproduction. When these antibodies are present at certain levels, an animal is infertile. As is the case with most vaccines, however, the effect is not permanent, and "booster" vaccines are necessary for lifetime infertility.

To date, researchers have studied two primary targets for immunocontraception. One is gonadotropin releasing hormone (GnRH), sometimes called the “master hormone” because it controls reproductive processes in both males and females and across mammalian species. Without GnRH stimulation, gonadal hormone production is suppressed, and gametes (eggs and sperm) do not mature. By triggering creation of antibodies against GnRH, a vaccine can prevent production of hormones necessary for reproduction—and by extension cause infertility and suppress hormone-driven sexual behaviors. One GnRH vaccine formulation, GonaCon™, has been approved by the Environmental Protection Agency (EPA) for one-year contraception of adult female white-tailed deer, wild horses, and wild burros. Booster vaccines can be given to extend the period of efficacy. GonaCon™ works in males as well, but with less duration. 

A non-cellular membrane, the zona pellucida, surrounds each of a female mammal’s unfertilized eggs. One protein within the zona pellucida is believed to be essential to allowing sperm to attach to the egg. Zona pellucida (ZP) vaccines, limited to female mammals, are designed to stimulate the body to produce antibodies that attach to the sperm receptors of the zona pellucida and block fertilization. A porcine zona pellucida (PZP) vaccine, ZonaStat-H, is EPA-approved for female wild horses and burros. It is important to note that because this approach affects reproduction at the level of the ovaries, it does not suppress estrus or natural sexual behaviors (in contrast to GnRH vaccines). 

Returning to our species of interest: dogs and cats. The use of GnRH and PZP vaccines in these species have yielded mixed results. One GonaCon™ formulation has shown promise in male and, especially, female cats; some queens remained infertile for the duration of the 5-year study, and infertility averaged 2.5 years. In dogs, however, a formulation that produced long-term infertility has also caused adverse injection site reactions. In female dogs, ZP vaccines, including but not limited to ZonaStat-H, have had inconsistent results on antibody production and have not effectively prevented fertility. They have also not been effective in female cats. In short, efforts to develop immunocontraceptive vaccines that are effective, long-lasting, and safe are ongoing.

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