Phage-GnRH Constructs for Population Control of Feral Animals: Evaluation in Cats

Abstract

The overpopulation of cats is a problem in the United States and worldwide due to public health concerns and their role as a predator of wildlife species. The focus of this research is the development of anti-fertility vaccines composed of whole phage particles carrying peptides with contraceptive properties for use in feral animals. The vaccines are designed to trigger antibody production against gonadotropin releasing hormone (GnRH). The antibodies inactivate GnRH, causing reduced release of gonadotropic hormones and gonadal atrophy. Phage-GnRH constructs with potential contraceptive properties were generated via selection from a phage display library using cat and dog GnRH antibodies as selection targets, allowing identification of phages displaying GnRH-like peptides. When tested in mice, these constructs invoked the production of antibodies against GnRH and suppressed serum testosterone. The goal of this study was to evaluate the potential of these vaccines in cats. Sexually mature male cats were characterized as to their reproductive parameters and injected with a phage-GnRH vaccine according to the following treatment groups: single phage-GnRH vaccine with adjuvant (group 1, n=5), a phage-GnRH vaccine without adjuvant and a booster one month later (group 2, n=5), or a phage-GnRH vaccine with adjuvant and two boosters three months apart (group 3, n=5). Anti-GnRH antibodies and testosterone in serum, testicular volume by ultrasound, and quality and quantity of sperm were evaluated monthly for 7-9 months following immunization. All cats developed anti-GnRH antibodies of varying levels following immunization and all but two cats still had detectable antibody levels at the conclusion on the study. Serum antibody levels increased after booster immunization in groups 2 and 3 and the highest antibody levels were reached by the cats in group 3 that received three total vaccinations. When the cats were evaluated as treatment groups as a whole, mean serum testosterone was not significantly reduced except at eight months after primary immunization for four cats in Group 3. The total testicular volume (TTV) decreased in four cats in Group 1 by a range of 24-42% and all five cats in Group 3 by a range of 15-36%, indicating potential gonadal atrophy in these treatment groups. All cats still produced sperm at the conclusion of the study, but the majority of the immunized cats had a decrease in the percentage of morphologically normal sperm cells compared to pre-immunization and this change was significant at multiple post-immunization time points for each group. On histological examination, treatment resulted in vacuolation of the corpus of the epididymis. One of the adjuvants used in this study caused unacceptable injection-site reactions. This study demonstrated the potential of phage-GnRH vaccines for immunocontraception of cats, but further refinement, including optimization of phage dose, adjuvant used, age of immunization, and immunization regimen, must be performed to see if sterility can be induced.